Tuesday, January 25, 2022


Xenology ♦ Introduction

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

© 1979 Robert A. Freitas Jr.

All Rights Reserved

About Xenology (the field)    

Xenology may be defined as the scientific study of all aspects of extraterrestrial life, intelligence, and civilization. Similarly, xenobiology refers to the study of the biology of extraterrestrial lifeforms not native to Earth, xenopsychology refers to the higher mental processes of such lifeforms if they are intelligent, and so forth.

The xeno-based terminology was first coined for this usage by the renowned science fiction author Robert A. Heinlein (starting in The Star Beast, Scribner, New York, 1954 HTML commentary), though the first use of the related word "xenologist" is apparently attributable to L. Sprague de Camp ("The Animal-Cracker Plot," Astounding Science Fiction 69(July 1949); "The Hand of Zei," 1950).

This usage was subsequently defended by Heinlein and Harold A. Wooster in a 1961 article published in the journal Science (R.A. Heinlein, H. Wooster, "Xenobiology," Science 34(21 July 1961):223-225 PDF) and by Robert Freitas (CV) in a 1983 article published in the journal Nature (R.A. Freitas Jr., "Naming extraterrestrial life," Nature 301(13 January 1983):106 HTML HTML). The latter article drew a complaint ("Xenology disputed," Nature 302(10 March 1983):102) from four specialist researchers claiming to represent "20 research groups in at least eight countries" who preferred to retain use of "xenology" for the study of xenon concentrations in meteorites (an argument that would not apply to other uses of the xeno- prefix) but their plea has largely failed. By December 2008, Google listed 20,600 entries for "xenology" of which only 1140 referred to xenon and most of the rest referred to the extraterrestrial usage. Online dictionaries (e.g., Webster's New Millennium Dictionary of English, 2003-2008) now typically define "xenology" as "the scientific study of extraterrestrials, esp. their biology."


In the spirit of preserving great books, this edition of Xenology is dedicated to Ray Bradbury and his classic novel, Fahrenheit 451.

Originally published in 2013 with tabbed-pages at GaianCorps.com (version 1). This 2018 edition is an upgrade — template, tabs, images and text/layout

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization was privately published and circulated in hardcopy form during its writing in 1975-1979 and after its completion in 1979.

Additional information on the original First Edition of this book is available here, and the full Table of Contents (and free access to the entire text online) is available here.

Capsule Summary of Xenology


An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

— First Edition,  Xenology Research Institute,  1975-1979,  2008

Topics include:

  • History of the idea of extraterrestrial life
  • Comparative planetology
  • Stars, and galaxies
  • Interstellar communication techniques
  • Sociology and legal issues pertaining to first contact
  • Appropriate interaction protocols pertaining to first contact


  • Definition / origin of life
  • Exotic biochemistries
  • Possible alien bioenergetics    
  • Biomechanics
  • Sensations
  • Reproduction
  • Intelligence

Extraterrestrial Civilizations:

  • Energy sources
  • Biotechnology
  • Interstellar travel
  • Alien weapons
  • Planetary and stellar engineering
  • Xenosociology
  • Extraterrestrial governments
  • Extraterrestrial culture

word cloud

Reviews of Xenology found on the Internet
This Scientist Wrote the Ultimate Guide to Alien Weapons, Music, and Sex
Robert Freitas shares the story behind Xenology.

From: Inverse Science By Graham Templeton on May 10, 2017

Robert Freitas was still in college when he started his now-legendary handbook to alien life. Published in 1979, Xenology offered some of the first — and still among the only — serious academic discussion of potential extraterrestrial biology, culture, and more, including, yes, ray guns and orgasms.

It wasn’t just errant musings. Freitas, who would later make his name as an emerging tech researcher, winning the 2009 Feynman Prize for his work in nanotechnology, included more than 4,000 scientific references and laid the groundwork for a quietly expanding field.

Xenology is “the most comprehensive and systematic study of extraterrestrial life, intelligence, and civilization I am aware of,” philosopher Clement Vidal wrote in The Beginning and the End: The Meaning of Life in a Cosmological Perspective. “I consider it a rare scientific masterpiece.”

In an email with Inverse, Freitas took credit for laying out the first coherent discussions on various topics.

“For example, my discussion of possible alien blood chemistries was entirely novel, and I was the first to describe the possibility of coboglobin-based blood. I did the first technical discussion of thalassogens, though Asimov had coined the term a few years earlier. I invented the Sentience Quotient, a scale of brain power large enough to encompass intelligences 40 orders of magnitude superior to humans. I offered the first coherent discussions of alien weapons technologies, possible planetary sky colors, alien skeletons, alien locomotion, alien sex, the number of legs or fingers an alien might possess, possibilities for alien psychology, alien political systems, alien music, and many other specific topics. Lots of ‘firsts’ in this book!”

In case you’re wondering about alien orgasms, Freitas says it’s hard to know. Although orgasms may be seen as an evolved mechanism for promoting mating, they are absent in many organisms, including some mammals. “For this reason, xenologists remain extremely cautious in extending this extraordinarily satisfying response to all bisexual aliens,” Freitas writes in Xenology.

You never forget your “first time”.

Xenologist Robert Freitas put the USS Enterprise from Star Trek on the cover of his book.

We reached out to the researcher to get a better understanding of this singular work. How did such a foundational text come from a college student? And what does its creator think the future holds for the field(s) he helped to define?

How did you first come to this field and get educated in it?

I can clearly recall the first time I was exposed to the idea of alien life. I think I was in 7th grade, wandering around in the school library and randomly picked out an interesting-looking book that turned out to be my first exposure to science fiction. It was about some colonists on the planet Mercury who encountered an alien intelligence that was in the shape of ball lightning. I’d long forgotten the title, but with the advent of the Internet, a few years ago I tracked it down online (Battle on Mercury by Erik Van Lhin), purchased a copy, and re-read it with great fondness. As they say, you never forget your “first time”.

At college in the early 1970s, I read a lot of Larry Niven’s work, including his short stories and most memorably his Ringworld classic. As a physics major, I recall trying to work out the physics of ringworld-like structures around stars, the gravitational fields to be expected around hypothetical toroidal planets, and the physics of transcendental tachyons (which travel at infinite speed at zero energy) and rotating black holes. I also became a long-time subscriber to Analog Science Fiction magazine.

The Ph.D. track in physics looked unappealing for various reasons, so I entered law school at the University of Santa Clara. My first two published articles, in 1977, concerned the legal rights of extraterrestrials if they landed on Earth. Obviously, I was not interested in the usual topics that captivate most law students like contracts, torts, and corporate law! I did take patent law, and international law, and also did a special research project on “Survival Homicide in Space.” I knew by then that I didn’t fit the usual mold and didn’t want to practice law, but I’ve never been a quitter. So I finished my Juris Doctor degree, even though I never took the bar exam.

By late 1974, I’d already begun working on what would become my first “magnum opus” type book, to be called Xenology. It was my first book project, and it exemplifies what has become a hallmark of all my books: an encyclopedic collection of information that effectively defines a new field by creating a framework that describes all of the component elements of the new field, then describes each of the components in sufficient detail to create a convincing, comprehensive, and heavily referenced conceptual foundation that can easily be built upon by others to extend the field.

Existing knowledge and ideas are used when such exist, and where they don’t, I fill the gaps with new ideas of my own or new approaches that are often inspired by information in related or analogous fields. At the time, the only book that remotely approached what I was trying to accomplish with Xenology was Shklovskii and Sagan’s masterful 509-page tome Intelligent Life in the Universe (1966), but even that one was missing most xenological topics of interest.

During the late 1970s and early 1980s, I basically read every book and every paper I could find on the subject of extraterrestrial life, intelligence, technology, and communication — literally four or five thousands of items including popular articles, technical papers, books, NASA documents, Russian translations, etc. I took copious notes and slowly began organizing the information into a coherent whole.

How did the book itself come about?

The writing of the book was a labor of love.

It was done part-time over about five years, during and after my time in law school. As noted in the Preface of the book itself, the research was done pre-Internet, so all the references had to be located in hardcover printed versions in book-sized volumes on dusty library shelves, then carried to the xerox machine and photocopied for a nickel a page. If you stacked it up, I’d probably have 30 linear feet of material shoved in filing cabinets from this time period. Also, there were no computers with word processors, so everything had to be typed on an electric IBM Selectric typewriter (a relic I still possess, BTW), on sheets of paper, with illustrations literally pasted onto the typed pages. Copies of chapters for review had to be printed off at the copy shop, then mailed to the recipient in a large envelope via snail-mail.

It was another two decades before the entire work could be scanned into electronic form by a generous colleague, and then it was a few years after that before I could find the time to edit and clean up the electronic version sufficiently to make me comfortable with putting it online for general public access

What was the reaction at the time?

Most of my scientific reviewers were supportive, perhaps because most of them were sent only one or two chapters related to their known areas of interest or knowledge. Some were skeptical — especially a few of the radio SETI people like the late Barney Oliver — but these were offset by others like the late Ronald Bracewell, who strongly approved of my conclusions regarding probe SETI and with whom I had several discussions during my telescopic searches for ET probes.

I also got a signed postcard from the late SF writer Robert Heinlein, saying that he approved of my use of the word “xenology”.

In the late 1980s and 1990s, the book was not widely circulated, so its impact at that time was very low. The text has only been generally available for the last 10-15 years in electronic form. During that time, its influence appears to be growing via mimetic diffusion, but relatively slowly because I’ve not been promoting the book since my nanotechnology work fully occupies my time.

What has been the book’s legacy as time has gone on?

While I didn’t coin the term “xenology,” I was certainly one of the first to recommend its general usage to describe the field, in a very brief item published in Nature in 1983. Up until then, people were calling the field “exobiology” or the even more etymologically defective “astrobiology”, and in some contexts even “SETI”, “extraterrestrial communication”, “life in the universe”, and other phrases that were sometimes used to discuss broader aspects of the field.

To some extent this confusion still exists today, though the term “astrobiology” seems to have caught on to refer to the subfields described in Chapters 4-8 of my book. But xenology as a comprehensive term for the entire field of “alien studies” has not yet caught on in the mainstream scientific community, perhaps in part because I didn’t attend conferences and pursue high visibility in the 1970s and 1980s, and perhaps in part because so much of the material in my book is commonly deemed too “speculative” for serious scientific discourse. (After the book was written, Titan was discovered to have open oceans, after which my discussion of thalassogens may have seemed a little less “speculative”.) The widest usage at present of the word “xenology” in the manner I use it may be in the science fiction community.

I’ve been out of the field for a long time, so I haven’t read the recent literature and thus may be a poor judge of the book’s legacy. However, I’ve been noticing the work getting cited more and more often as time goes on. Every month or two, someone contacts me by email about the book, out of the blue. A while back, one fellow put up a mirror with my permission, and another fellow laboriously converted the entire book into a different format that he likes better, on his own time.

With exo-planetology ramping up in recent years, what future do you see for this area of thinking and research?

With a thousand extrasolar planets now known, several of them Earth-sized, theoretical planetology is experiencing a huge rebirth. This could lead to a corresponding rebirth in the entire field of xenology.

However, I would caution that the full import of the emerging technologies of AI and nanotechnology have not been sufficiently factored into everyone’s assessment of the possibilities. Given the speed at which these two “exponential” technologies are emerging in human civilization, one must assume that other intelligent species on other worlds would have experienced similar exponential technological growth. This has major implications both for what we might find out there, and for what we might not find out there.

The Reason for Xenology
fantastic world

From: Fantastic Worlds

© 2013 by Jordan S. Bassior

Xenology – the scientific study of alien life and civilizations – is a science unique in that we haven’t yet found any alien life or civilizations to study. Why, then, does the discipline exist? After all, there are no real sciences of, say, demonology or unicornology, because we’ve never discovered any real demons or unicorns. (Mystics and fantasists compile lists of imaginary demons, and fantastists and fangirls lists of imaginary unicorns, but this is not the same as “scientific study” of a subject).

The difference is that we have a very strong suspicion that alien life and civilizations do exist, for the very good reason that we exist, and the same forces which caused life and intelligence on Earth have probably caused life and intelligence on at least some other planets. We bother to discusss the issue scientifically, even though we haven’t found any such life and civilizations yet, because for various reasons such alien life and civilizations, if and when discovered, are bound to be of great significance to both the study and the destiny of the life and civilization which has originated on Earth.

The Universe is very large. As we learn more about its structure it becomes apparent to us that the natural forces which generated terrestrial planets around Sol have also generated terrestrial planets around other stars, and what we know of chemistry and paleontology make it very likely that these forces have also generated ecosystems on at least some of those worlds. Terrestrial planets seem to be common enough that it is very likely that there are alien ecosystems in some of the nearby star systems – say, within 100 or so LY of the Earth.

Such ecosystems would be important to us because they would give us a wider informational base from which to study our own ecosystem. As long as we have only one example of an evolved ecosystem (Earth’s) to go by, we cannot tell which aspects of that ecosystem are essential to being an ecosystem, and which are chance and incidental features of our particular  ecosystem. Also, since any ecosystem is essentially a colossal natural experiment, taking place over a whole planetary surface and lasting billions of years, it would be rather surprising if we didn’t find some unique and useful results from any particular new ecosystem we studied.

Paleontology tells us that it takes a planet merely a few hundred million years to generate an ecosystem, but billions of years to create sapient life. Consequently, sapience should be much rarer than life. It would be surprising if there was no alien life within 100 LY of Earth (indeed, it wouldn’t be particularly  surprising if some existed in our own star system); it would not be all that surprising if there were no alien sapients  within that radius. Furthermore, since civilization (agriculture plus writing) occurred fairly late in the history of the ape family, and spacefaring fairly late in the history of civilization, we might expect to find many savage for each civilized sapient race, and many planetbound for each spacefaring civilization, unless of course existing spacefaring civilizations have already colonized many nearby star systems.

Everything I’ve said about alien life applies to alien sapience, civilizations and spacefaring. Alien sapients would represent different experiments in being smart; alien civilizations in being civilized; alien spacefarers in being scientific. We would learn through the study of such beings just which aspects of our current sapience, civilization and science are essential, and which accidental. Additionally, we should be aware that alien civilizations, especially spacefaring ones, might pose a threat to us – it is obviously theoretically possible for such civilizations to attack us, and if they exist they might. So from purely selfish, even insular motives, we should locate any which happen to be in our vicinity, and be on our guard against them.

Do we know for certain that any of this exists? No, not yet, and that’s why this is a curious science, for it is studying something of whose reality we cannot be certain. What is certain is that the more we study the Universe beyond our lonely planet, the wider the base of information we gain for an estimation of the frequency of alien life, sapience and civilization, and hence the more solidly-grounded becomes xenology.

It is dangerous to attempt to walk through our existence as a species with our eyes squeezed firmly shut – better to open them wide to the wonders of the Universe. And, while we’re dong so, keep a lookout for the tigers.

"Xenology" - A Sci-Fi Writer's 101

From: LiveJournal

Hi, I thought this might be useful for people:

Xenology - An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization  is the almost complete, free online version of a comprehensive resource book for sci-fi writers and other interested people.

It's from the 1970s, so the science isn't entirely accurate anymore (especially fields like biochemistry and exobiology were still in their infancy back then), but I think it's still very useful and interesting.

It's written by a guy who now researches nanotechnology and who was involved with SETI and political advocacy for space exploration, so I'm fairly confident he researched this as well as he could back then.

I haven't read all of it yet, but judging from the fields that aren't my speciality, it seems understandable enough for laypeople.

Xenology, Metalaw and Thermoethics

From: Portal to the Universe – 3 Dec 2010

In 1979, the scientist, inventor (and then-newly minted lawyer) Robert A. Freitas, Jr. published the fascinating book Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization.

Freitas has now graciously published the entire book on the web for free.

It has a good summary of Andrew Haley's and Ernst Fasan's work on Metalaw, and another section detailing Fasan's elaboration of Haley's Metalaw accompanied by a useful table containing dozens of attempts by other authors to articulate metalegal concepts.

What caught my attention, however, was yet another section that contains some valuable criticism of Metalaw, criticism which obviously predates my own paper critical of Metalaw for its failure to contemplate the likely machine nature of ETI.

Freitas is skeptical of Metalaw' reliance on Kant's Categorical Imperative (and by implication, Metalaw's reliance on the natural law theory of jurisprudence).

Freitas points out that Kant ignores "the possible existence of a sentience of a qualitatively higher order than that possessed by humanity."

Freitas suggests that Ernst Fasan "falls into the same anthropocentric trap" by regarding "human-style intelligence as 'the highest possible level of life.' " Pointing out that multiple orders of higher sentience are possible (and quite likely given the likely ...

What is Metalaw?

According to Dr. Ernst Fasan, Metalaw is “the entire sum of legal rules regulating relationships between different races in the universe.” Metalaw is the “first and basic ‘law’ between races” providing the ground rules for a relationship if and when we establish communication with or encounter another intelligent race in the universe. Dr. Fasan envisioned these rules as governing both human conduct and that of extraterrestrial races so as to avoid mutually harmful activities.

Attorney Adam Korbitz presents a guide to exploring the relationship between the pioneering metalegal work of Andrew G. Haley and Dr. Ernst Fasan, and the scientific Search for Extraterrestrial Intelligence (SETI)

Dragons (Part 1): The Bloodline

From:  Thoughts of a Taoist Babe

They are not, and have never been, simple characters in children’s books. They were keepers and teachers of ancient secrets, rulers and caretakers of vast stretches of Earth land, and they came from a distant land beyond the visible star-dome of the night sky. Their presence is felt far and wide in graven images and statues of stone, their influence resonating clear to this very day.

Dragons show up everywhere, ubiquitously powerful, undeniably otherworldly, and infinitely wise. Ancient mythology is repleted with it from every corner of the world. Archaeology and palaeontology offer tantalizing clues about the dragons that roamed the lands in ancient times. And now, they are showing up in areas once thought free of mythical beings — that of genetics, biology, chemistry, astronomy, and xenology, which is the scientific study of all aspects of extraterrestrial life, intelligence, and civilization.

For more information about Xenology, click on the image of the book or follow this link here for a free online copy of the 1979 book entitled Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intellignece, and Civilization, by Robert A. Freitas Jr. This book is rather dated, but it details the very first written document about the brand new science field which is still in its infancy due to the nature of the subject matter.

For a different extrapolation of the subject matter, Dr. David Brin talks about Xenology here, in his article published in 1983, entitled Xenology: The Science of Asking Who’s Out There.

To be perfectly honest, if I had been given the chance and the choice (and the funds needed) I would have happily followed in this line of research during my years at the University, if there was ever such a thing available to be studied. But you see, there is hardly anything out there openly that can be studied. What available material is locked down so tight, it would be just about impossible to sneak a peek, let alone do a serious graduate-level scientific study on it.

And this is such a crying shame that we are not given access to study about this — most especially because we are living descendants of this ancient legacy.

But there is hope.

The great thing about living in this day and age is the crazy awesome access we all have to information about anything we ever wish to study. As Donny Miller so wisely said, "In the age of information, ignorance is a choice." And so I dig and dig and dig, and what I find is a treasure trove of knowledge out there, dug up in bits and pieces by very smart folks — folks like Dr. Joe Lewels who wrote, in his article for FATE magazine titled Humanity’s Historical Link to the Serpent Race:

As long as humanity has kept records of its existence, legends of a serpent race have persisted. These myths tell of a mysterious race of superhuman reptilian beings who descended from the heavens to participate in creating humankind and to teach the sciences, impart forbidden knowledge, impose social order, breed with us, and watch over our development. The serpent like beings were not alone, but were part of a retinue of super beings thought to be gods by the ancients.

This is by no means new information. It is as old as dirt. Clay tablets taken from Sumeria said the exact same thing, only more belabored and far far more colorful. Go to other corners of the world and the story is the same, only the names and places have been changed.

The idea of a reptilian race does not fill me with great dread, or fear, or horror, or shock, or revulsion. It does none of those things because I grew up hearing about my ancient ancestors and their deep family ties with dragons. The legend speaks of Lạc Long Quân whose maternal grandfather was a dragon living under a lake, and Âu Cơ, his wife, who gave birth to my ancient ancestors.

Dragons are not just associated with good luck, good fortune, and wisdom, they were also one of my ancestors!

Please allow me to introduce you to Dracorex. He looks just like a dragon doesn’t he?

Look at the bony protrusions! Look at the horns, the snout, look at the eye sockets! He’s a dragon straight out of mythological legends! Yet, he is as real as can be.

Dracorex is a 66-million-year-old dinosaur that was found in the continent of North America. To-date, there is only one fossil of Dracorex found, but that does not mean that only one existed. I am not saying that Dracorex is a member of the serpent-like beings who were such a huge part of our culture. I am simply saying that the existence of Dracorex is an established fact, but other than the one specimen found, there has been no other. In other words, absence of evidence is not evidence of absence.

This opens up the high probability that there are dragon bones out there…we just haven’t been able to find them yet…or even more likely, we haven’t been able to identify them as such for some inexplicable reason.

No matter.

We only need to look within to find that missing evidence. In my next posting, I will discuss further, the biological link between us modern humans and our ancient ancestors, the serpent beings.

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intellignece, and Civilization. Robert A. Freitas Jr., J.D.

Xenolgoy: The Science of Asking Who’s Out There. David Brin, Ph.D.

Humanity’s Historical Link to the Serpent Race. Joe Lewels, Ph.D.

(continue to Dragons (Part 2): The Genetics)

Table of Contents

Chapter 2

ET Life: The History of an Idea

2.1 Ancient Beginnings

2.2 The Long Interregnum

2.3 Plurality of Worlds and Divine Purpose

2.4 Science and Science Fiction

Chapter 3

The Aliens Among Us

3.1 Xenoarchaeology

3.1.1 Extraterrestrial Intervention in

Biological Evolution

3.1.2 Extraterrestrial Cultural


3.1.3 Extraterrestrial Artifacts

and Manifestations

3.2 Ufology

3.2.1 Why Believe in UFOs?

3.2.2 The Evidence for UFOs

3.2.3 The UFO Game

3.3 The Resident Aliens

Chapter 4

Xenology: The Context of the Universe

4.1 The Universe

4.2 Galaxies

4.3 The Milky Way Galaxy

4.4 The Stars

Chapter 5

General and Comparative Planetology

5.1 Planetary Evolution

5.2 Thalassogens

5.3 Planetary Atmospheres

5.4 Planetary Meteorology &


5.4.1 Climate and Weather

5.4.2 Sky Colors

5.4.3 Astrogeology

5.5 Planetary Habitability

Chapter 6

A Definition of Life

6.1 Chronology

6.2 What Is Life?

6.2.1 The Traditional Answer

6.2.22 Organization

6.2.3 Towards a Definition of Life

Chapter 7

The Origin of Life

7.1 Historical Views on the Origin of Life

7.2 Cosmochemical Evolution

7.3 Early Chemical Evolution on Earth

7.3.1 Prebiotic Synthesis

7.4 Proteins and Cells

7.5 Nucleic Acids and DNA

7.6 Early Biological Systems

Chapter 8

Exotic Biochemistries

8.1 The Argument for Diversity

8.1.1 Temperature Chauvinism

8.2 Alternative Biochemistries

8.2.1 The Limits of Carbon Aqueous

8.2.2 Alternatives to Water

8.2.3 Alternatives to Carbon

8.3 Exotic Lifeforms

Chapter 10

Alien Bioenergetics

10.1 Finding the Energy to Live

10.2 Photosynthesis

10.3 Animal Metabolism and Respiration

10.4 Alien Blood

10.5 Thermoregulation

Chapter 11

Extraterrestrial Biomechanics

11.1 Specialization and Symmetry

11.2 Xenobiomechanics

11.2.1 The Challenge of Gravity

11.2.2 Meeting the Challenge: Skeletons

11.3 Alien Locomotion

11.3.1 Aquatic Locomotion

11.3.2 Travel by Land

11.3.3 Avian Propulsion

Chapter 12

Alien Sex

12.1 Is Sex Necessary?

12.2 The Bisexual Universe

12.2.1 Intersexuality

12.2.2 Optional Sex

12.3 Alien Sex Practices

12.3.1 Alien Orgasms

12.4 Xenogamy

Chapter 13


13.1 Tactile Senses

13.2 Olfaction

13.3 Acoustical Senses

13.3.1 Two-Dimensional Sound

13.3.2 Three-Dimensional Sound

13.4 Electrical and Magnetic Senses

13.5 Vision

13.5.1 Visible Vision

13.5.2 Infrared Vision

13.5.3 Radio Vision

13.6 Alien Senses

Chapter 14

Extraterrestrial Intelligence

14.1 Evolution of Intelligence

14.1.1 In the Beginning

14.1.2 The Triune Brain

14.2 Juvenile Extraterrestrial Intelligences

14.2.1 Genetic Sentience

14.2.2 Brain Sentience

14.2.3 Communal Sentience

14.3 Alien Consciousness / Sentience


Chapter 15

Energy and Culture

15.1 Type I Civilizations: Planetary Cultures

15.2 Type II Civilizations: Stellar Cultures

15.3 Type III Civilizations: Galactic Cultures

15.4 Type IV Civilizations: Universal


Chapter 16


16.1 Bioneering

16.1.1 Intelligence Amplification

16.1.2 Genetic Surgery

16.1.3 Genetic Hybrids / Synthetic Genes

16.1.4 Ectogenesis and Cloning

16.2 Immortality

16.2.1 Xenogerontology

16.2.2 The Limits of Immortality

16.3 Androids and Cyborgs

16.3.1 Androids and Organleggers

16.3.2 The Bionic Alien

16.3.3 Enter the Robot? (aka. Uploading)

16.4 Machine Life

16.4.1 Artificial Intelligence

16.4.2 Robots and Robotics

16.4.3 Machine Evolution

Chapter 17

Interstellar Voyaging

17.1 Communication vs. Transportation

17.2 Relativistic Starflight

17.3 Conventional Interstellar Propulsion


17.3.1 Nuclear Pulse Propulsion

17.3.2 Controlled Fusion Rocket

17.3.3 Interstellar Ramjet

17.3.4 Beamed Power Laser Propulsion

17.3.5 Total Conversion Drives

17.4 Exotic Propulsion Systems

17.4.1 Gravity Catapults

17.4.2 Antigravity / Reactionless Field


17.4.3 Tachyon Starships

17.4.4 Momentum Interconversion


17.4.5 Statistical Transport

17.4.6 Black Holes and Space Warps

17.4.7 Teleportation / Transporter


17.5 Time Travel

17.6 Interstellar Navigation

17.7 Generation Ships / Suspended


Chapter 18

Alien Weapons

18.1 Chemical, Biochemical,

and Biological Weaponry

18.2 Bionic Weaponry

18.3 Sonic Weapons

18.4 Photonic Radiative Weaponry

18.5 Particulate Radiative Weaponry

18.6 Nuclear Explosives

18.7 Climate Modification and

High Technology Weapons

18.8 The Ultimate Weapon

Chapter 19

Planetary Engineering and GHT

19.1 Alien Materials Technology

19.1.1 New Forms of Matter

19.1.2 Energy Storage / Mining


19.2 Extraterrestrial Habitat Engineering

19.2.1 Terraforminge

19.2.2 Space Habitats

19.2.3 Planet Moving and Star Mining

19.2.4 Large Scale Biospheric


19.2.5 Galactic Megastructures

Chapter 20


20.1 Biological Evolution

20.1.1 Evolution Rates

20.2 Xenopsychology

20.2.1 Energy Ecology

20.2.2 Competition and Aggression

20.2.3 Universal Emotions

20.2.4 Xenophobia

20.3 Early Technological Civilizations

20.3.1 Telluric Civilizations

20.3.2 Aquatic Civilizations

20.3.3 Avian Civilizations

20.4 Alien Social Systems

20.4.1 Models for Extraterrestrial


Chapter 21

Extraterrestrial Governments

21.1 Dimensions of Extraterrestrial


21.1.1 Governance Scales

21.2 Alien Political Organizations:

Xenopolitical Factors

21.2.1 Sentience

21.2.2 Dispersion

21.2.3 Size

21.2.4 Heritage

21.2.5 Xenopolitics: Tentative


21.3 Extraterrestrial Organizational


21.3.1 System Complexity

21.3.2 System Structure

21.3.3 System Stability

21.4 Strategic Galactography

21.4.1 The Economic Viability of

Interstellar Cargo Transport

21.4.2 Galactic Trade Routes

21.4.3 Interstellar War

Chapter 22

Extraterrestrial Cultures

22.1 Alien Religion

22.2 Alien Ritual

22.2.1 Religious Rites

22.2.2 Extraterrestrial Cults

22.3 Ethics and Law

22.3.1 Extraterrestrial Ethics

22.3.2 Legal Universals

22.3.3 Xenopenology

22.4 Philosophy and Knowledge

22.4.1 Alien Logic

22.4.2 Time, Language, and Space

22.4.3 Science and Paradigmology

22.4.4 Xenoeschatology

22.5 Extraterrestrial Aesthetics

22.5.1 Xenomusicology

22.5.2 Alien Painting and Surface Arts

22.5.3 Dance and Sports

22.5.4 Alien Sculpture and Architecture

Chapter 23

Abodes of Life: The Search Begins

23.1 Theoretical Galactic Demography

23.1.1 The Drake Equation

23.2 Observational Galactic Demography

23.2.1 Direct Observation of Alien


Chapter 24

Interstellar Communication Techniques

24.1 The Cosmic Miracle

24.1.1 Eavesdropping

24.2 Extraterrestrial Signaling

24.2.1 Alternative Channels: Neutrinos,

HEPs, Gravitons and Tachyons

24.2.2 Electromagnetic Waves and

Frequency Selection

24.2.3 Acquisition and Artificiality


24.2.4 Alien Message Contents

24.2.5 SETI: Yesterday and Today

24.3 Extraterrestrial Starprobes /


24.3.1 Why Probes are Better

24.3.2 Mission Profile

24.3.3 The Nature of Alien Artifacts

24.3.4 Project Daedalus

Chapter 25

Theory and Practice of First Contact

25.1 First Contact and Metalaw

25.1.1 Basic Metalaw

25.1.2 Fasan's Metalaws

25.1.3 Universal Thermoethical Principles

of First Contact

25.2 The Character of First Contact

25.2.1 Mass-Energy Scales of Contact

25.2.2 Information-Rate Scales of


25.2.3 Generalized First Contact


25.3 First Contact Protocols and

Elementary Astropolitics

25.3.1 Encounters Between Equals:

The 0/0 Contact

25.3.2 Gods and Primitives: 11/0 Contact

25.3.3 Trees and Humans: 0/10 Contact

25.3.4 Higher-Order Contacts

Chapter 26

First Contact and the Human Response

26.1 Military and Political Response

26.1.1 Remote Contact

26.1.2 Direct Contact

26.1.3 Surprise Contact

26.2 Public Reaction and the Press

26.2.1 Rumor and Credibility

26.2.2 Panic and Mass Hysteria

26.3 Legal Issues of First Contact

26.3.1 Alien Animals

26.3.2 Legal Standards of Personhood

26.3.3 Extraterrestrial Persons

26.3.4 Aliens and American Law

26.4 Human Sociocultural Response

26.4.1 The Acculturation of Humanity

26.4.2 Social Impact of First Contact

26.4.3 The Religious Response

26.4.4 Impact on Science and


Preface and Acknowledgements for the First Edition
The Field of Xenology

What, exactly, is “xenology”? As described by the subtitle of this book, xenology may be defined as the scientific study of all aspects of extraterrestrial life, intelligence, and civilization. Similarly, xenobiology refers to the study of the biology of extraterrestrial lifeforms not native to Earth, xenopsychology refers to the higher mental processes of such lifeforms if they are intelligent, xenotechnology refers to the technologies they might possess, and so forth.

I was among the first to attempt to popularize the “xeno-“ prefix in association with the general study of extraterrestrial life (e.g., see my letter to Nature, below). However, credit for coining the xeno-based terminology in this usage is generally given to the renowned science fiction author Robert A. Heinlein (starting in The Star Beast, Scribner, New York, 1954 HTML commentary), though the first use of the related word "xenologist" is apparently attributable to L. Sprague de Camp ("The Animal-Cracker Plot," Astounding Science Fiction 69(July 1949); "The Hand of Zei," 1950).

The scientific usage of the xeno- terminology was subsequently defended in the mainstream scientific literature by Heinlein and Harold A. Wooster in a 1961 article published in the journalScience (R.A. Heinlein, H. Wooster, "Xenobiology," Science 134(21 July 1961):223-225 PDF) and subsequently by myself in a 1983 article published in the journal Nature (R.A. Freitas Jr.(CV), "Naming extraterrestrial life," Nature 301(13 January 1983):106 HTML HTML). (Heinlein had confirmed to me, by personal correspondence in August 1980, that he still regarded his coinage as both valuable and correct.)

My article in Nature drew a complaint ("Xenology disputed," Nature 302(10 March 1983):102) from four specialist researchers claiming to represent "20 research groups in at least eight countries" who preferred to retain use of "xenology" for the study of xenon concentrations in meteorites (an argument that would not apply to other uses of the xeno- prefix) but their plea has largely failed. By December 2008, Google listed 20,600 entries for "xenology" of which only 1140 referred to xenon and most of the rest referred to the extraterrestrial usage. Online dictionaries (e.g., Webster's New Millennium Dictionary of English, 2003-2008) now typically define "xenology" as "the scientific study of extraterrestrials, esp. their biology." So far, the mainstream field seems to have settled on the name “astrobiology” (the biology of stars?), but I still harbor hope that the more etymologically correct name, xenology, can be applied to the more general field of study that I tried to help define, so long ago, with my book – titled Xenology (~500,000 words, ~150 illustrations, 4000+ references), First Edition.

Why Publish the First Edition?

Reading again the text that I first wrote 30 years ago, it feels as though this book has fallen through a time warp or a crack in time, or has just been removed from a time capsule. But while some of the material seems dated, much of it still appears fresh and new, and the synthesis of the field (of xenology) is still relevant and unique. The main purpose of this book was to help create a coherent new field of study called “xenology”.

As you read this book, please bear in mind that it was written before Sagan’s “Cosmos” TV series and predated the internet, the personal computer, the cell phone, most of genetic engineering, Ronald Reagan, all but the first few Space Shuttle launches, electronic word processors and spell checkers, and Google and online reference sourcing. It was written before the sulfur volcanoes of Io or the liquid seas of Titan had been discovered, before extrasolar planets had been observed, and before my own optical and radio telescope SETI searches and other writings on replicating systems and nanotechnology (and several years before nanotechnology had even been invented, via the 1981 PNAS paper and 1986 book Engines of Creation by K. Eric Drexler). Xenology predates the first engineering study of self-replicating systems by NASA in 1980, almost all of the important work on interstellar probe SETI, and the development of the entire field of molecular nanotechnology and medical nanorobotics. In the fictional sphere, Xenology also predates all the Star Trek and all but one of the Star Wars movies, and its writing began just 6 years after the theatrical release of the classic 2001: A Space Odyssey.

If this book is so ancient, why bother to publish it now? There are several reasons.

First, I have an emotional attachment to it, having spent so many years (5) of my life writing it, back in the late 1970s. Indeed, I wrote it during my time in law school, a very trying experience for someone accustomed to scientific thought processes. Writing this book helped keep me sane during those years. (The whole thing was typed on my trusty blue IBM Selectric typewriter, and the graphics were hand-drawn or paste-ups, which explains in part why it has taken so long to get this up into "print".)

Second, Xenology was my first major effort at bookwriting. It taught me how to research, organize and write a reasonably coherent and lengthy single-topic work. It was excellent training and taught me valuable lessons in scientific writing that I’ve put to good use in my subsequent work. Anyone who is familiar with my later work will recognize the early manifestations of my characteristic proclivity to organize information in a comprehensive, almost encyclopedic manner, imposing some coherence on the information to help create a foundation for a more rigorous discipline someday to come.

Third, the work contains many thousands of literature references – a style of writing that has also become my trademark. Please bear in mind that back in the late 1970s, all of these references had to be assembled “the hard way”. In those antediluvian days, you had to look things up in a hardbound citation index and then walk the stairs and aisles of a real bricks-and-mortar library to find the right shelf containing the exact volume that you needed, then photocopy the papers for a nickel a page. Xenology was completed more than 20 years before the advent of the World Wide Web made online literature searches and pdf document retrievals a snap.

Fourth, while this book is not as technically rigorous as my later books, there is enough good material here that I thought it deserved to see the light of day. It is also reasonably well written, and contains some unique and valuable insights that I’ve not seen published elsewhere in the last 30 years. So I think it still has a valuable contribution to make to the field.

Fifth, as far as I know there is still no single text that attempts to integrate the entire field, as Xenology does. The only book that comes close is Intelligent Life in the Universe by I.S. Shklovskii and Carl Sagan, but that was published in 1966.

History of the Book - Part I

I first got interested in the study of possible extraterrestrial life through the works of Carl Sagan in the science area and Larry Niven in the science fiction area, in the early 1970s. Also, my favorite physics professor at Harvey Mudd College, Thomas Helliwell, indulged my budding freshman curiosity about rotating black holes, tachyons, calculations on the gravitational stability of toroidal planets and the dynamical stability of ringworlds around stars. At HMC, freshman were required to conduct a full-time 1-month engineering project. For my project, I chaired a 7-man team to create a design for a fusion-powered manned interstellar spaceship ("Project MISEV").

I began accumulating materials for Xenology in 1974, and began the actual writing in 1975, finally completing the last chapter, Chapter 26, in early 1979. There were 27 chapters originally planned. I never got around to writing the introductory (Chapter 1) or concluding (Chapter 27) chapters, nor one other chapter in the middle (Chapter 9) that was intended to be a summary of the unmanned interplanetary spacecraft that had been sent to other planets as part of the actual “experimental” search for life in our solar system, with a particular focus on the Viking landers on Mars that conducted the first biochemical searches for life on another planet via direct sampling. The book contains some pretty speculative material in a few places, including material from speculative fact and science fiction writers when appropriate. But generally the text tries to stick to concepts and arguments that are grounded in some kind of precedent either in biology, technology, or the social sciences and the arts.

Xenology was privately circulated while it was being written in the late 1970s. The book was reviewed by 40 notable scientists (see below), who were first contacted by letter, then mailed one or more chapters, after which these reviewers generously offered constructive comments leading to revisions. I then attempted to find a mainstream publisher, but collected only rejection slips. Finally, a science fiction writer friend (James Hogan) recommended his book agent, Ashley Grayson, who, upon reading the entire manuscript, became very enthusiastic about its prospects. Ashley kindly spent a couple of years shopping it around to the general run of speculative science and science fiction publishers. We got a few nibbles, but in the end all the editors and publishers who reviewed it concluded that the book was too lengthy (hence necessarily would have to be too highly-priced per copy) to be a commercial success. The book continued to be privately circulated to a select few others, most notably some science fiction writers and editors of my agent’s acquaintance, throughout the 1980s. The full book was never published in print (hardcopy) form or offered for sale commercially.

During the late 1970s and early 1980s, I carved out about a dozen “science fact” articles from the book materials, which were published in Analog magazine and a number of other venues. Around this same time I became one of the principal advocates for interstellar communication via material probes rather than radio waves and published a number of technical papers on this subject. I also conducted the first SETI searches for possible orbiting alien artifacts in Earth-Moon orbits using optical telescopes, published the first engineering scaling study of a self-replicating interstellar probe, performed the first radio SETI search at the tritium hyperfine line (which, if detected, would have been unambiguously artificial), and participated in the first engineering design of a self-replicating lunar factory for NASA. These activities thoroughly distracted me from further pursuing publication of Xenology in book form.

History of the Book - Part II

By 1994, I’d begun my current career in nanotechnology, starting the research that would eventually lead to my first published book in the field, the first volume in the Nanomedicine series, and beyond. At a nanotechnology conference in May 1998 I met Robert Bradbury, who had a company doing life extension research but was also writing in the area of SETI and astroengineering topics. Bradbury expressed interest in my unpublished book, and after reading some of it, offered to scan it and place it online alongside his existing collection of SETI-related works. From mid-1999 to mid-2000, I xeroxed Xenology and snailmailed it to him, chapter by chapter, which he scanned in and formatted. He also paid a Russian colleague to manually type the first 4300+ references (about half of my accumulation, but including most of the references used in the book) since these were all handwritten in a notebook.

Because of the imperfect nature of the scanning process, a large number of typos crept into the text that had to be caught and manually corrected. Bradbury did a lot of this but could not catch everything. This was a job only the author could do. Also, the last two chapters included a lot of handwritten insertions into the typed text that could not be scanned, so this material (the two longest chapters in the book) was unusually heavily laden with typos, dropped sentences, missing fragments, and the like. My personal attention was required, but by this point I was employed full time as a nanotechnology Research Scientist at Zyvex, so I couldn’t spare any cycles for the necessary corrections – and again, progress on the book languished.

While I could not consent to Bradbury placing the uncorrected manuscript online for general access in its initial unedited rough form, I also could not find time to correct it. As a compromise, I agreed that individuals upon special request could view the materials, which were placed online at Bradbury’s private Aeiveos Corp. website. This at least afforded Bradbury and a few selected SETI researchers ready access to the materials during 2000-2008 on an invitation-only basis.

During the 2000s the number of requests for access to the manuscript continued to grow. So for the last few years I’ve been slowly working, in spare moments, to clean up the text, reformat the material to be consistent with my other online books, then put the book up for free public access at my own xenology.info website that I reserved in 2002 for just this purpose. I’ve largely resisted the urge to change much, making just minor editorial corrections where appropriate, adding Section numbers, renumbering Figures and Tables, and correcting typos, but generally avoiding bringing the book up to date which should be the job of the Second Edition (if one is ever written). Such updating and correction is desperately needed, but must await a proper thoroughgoing editorial process that will be undertaken (most likely) by others.

The First Edition was originally written in the style of Scientific American (e.g., pitched to a scientific layperson reader), and it maintains this non-academic style throughout. There are only a few mathematical equations in this book. The work is heavily referenced to the primary nontechnical literature on extraterrestrial life (and related material), and is well referenced to the primary technical literature in many specialized areas but not uniformly throughout.

Xenology was current as of 1979, but the field has made 30 years of progress since then. The reader will find numerous omissions of facts and valuable references that have been published in the intervening years, and probably even a fair number of outright errors which were unknown at the time of writing. I’ve resisted the urge to rework problems and present new views. Missing also are my own three SETI studies and a couple of dozen papers I wrote in the 1980s. Many concepts that are widely discussed today were relatively unknown back then; many others have found their way into science fiction during the intervening years. For the most part, the material has held up reasonably well. The first contact protocols, scenarios and taxonomy in Chapter 25 are still relevant today – and perhaps even more so, since they obviously also apply to artificial intelligences which are now much closer to fruition than they were thirty years ago. The governance scales in Chapter 21 can be used to generate thousands of different possible governmental forms; the study of interstellar governance complexity and stability has been only lightly studied academically to this day. My discussion of coboglobin-based blood (original to me) in Section 10.4 has not been replicated elsewhere. And so forth.

Most significantly, the First Edition of Xenology was written entirely in the “pre-nanotechnology” era, thus largely ignores this all-important coming development. Even so, I anticipated this field in a small way in Section 16.4.1 when I wrote: “If alien electronic artificial intellect is possible, how physically small might it be? The theoretical lower limit of cell size is about 400 Angstrom, a bit smaller than the tiniest known living organism (the PPLO). A brain with 1010 neurons of this size would neatly fill a minute cube one-tenth millimeter on a side. But artificially designed alien microbrains theoretically could be vastly smaller still. Using molecular electronics with components on the order of 10 Angstrom in size, 1010 microneurons could be packed into a space of a few microns. This is small enough to hide inside a bacterium, a fact which may have several very interesting consequences.” It remained for other authors (including myself, in later decades) to more fully explore those "interesting consequences".


I wish to sincerely thank the aforementioned Robert J. Bradbury for his constant encouragement and enthusiasm about this book, and for laboriously scanning my typewritten pages and converting them to an initial html form over a period of about 12 months during 1999-2000. Robert also painstakingly coded into html format all of the Tables and Figures, some of them very lengthy and very complex, by hand. Without Robert’s truly Herculean initial efforts on my behalf, I could not have found the personal time or energy to carry these materials across the finish line to completion. I also thank Robert for scanning in the images for numerous figures. Some of these images have poor legibility, but this is my fault, not Robert’s. These images were scanned from my copies of library originals some of which were in turn reproduced using an ancient wet xerographic process, causing them to become heavily grayed out with time. I also regret that the text is not more heavily linked. However, each paragraph and illustration in the book is tagged with an anchor point to facilitate direct URL citation.

Please note that the official version of the book, as corrected, restored, and formatted by the author, is now formally published at the http://www.xenology.info website. No other version should be cited as authoritative or regarded as authentic.

I also wish to belatedly thank my original reviewers who read parts of the manuscript and provided critical comments. This includes: R. McNeill Alexander, Norman J. Berrill, David C. Black, Jonathan Boswell, Ronald N. Bracewell, A.G.W. Cameron, J. Desmond Clark, Mary Connors, John D. Currey, Karl W. Deutsch, Stephen H. Dole, Frank D. Drake, Freeman J. Dyson, John F. Eisenberg, Francis R. Flaim, Robert L. Forward, Sidney W. Fox, Arthur Harkins, Thomas M. Helliwell, Sol Kramer, Paul Kurtz, Paul D. MacLean, Magoroh Maruyama, Stanley L. Miller, Marvin Minsky, Peter M. Molton, Barney M. Oliver, Leslie E. Orgel, George C. Pimentel, Cyril Ponnamperuma, William K. Purves, Tim Quilici, S. Ichtiaque Rasool, Jack D. Salmon, Charles L. Seeger, Mark Stull, Jill Tarter, Francisco Valdes, Gerard de Vaucouleurs, David H. White, and Edward O. Wilson. Most of their comments were integrated into the text but a few corrective items might have been missed. As a result I must apologize in advance for any errors in the original work that may have survived. All such errors should be attributed, and reported by email, solely to the author. I also thank Ashley Grayson for his efforts on my behalf.

Finally, I must thank my wife, Nancy Ann Freitas, for her patience and support during the writing of this book, more than three decades ago near the start of our married life together. Without her help and faith in me, this book simply could not have been written.

Robert A. Freitas Jr. (CV)
Senior Research Fellow
Institute for Molecular Manufacturing
6 December 2008

Part I ♦ Perspectives

Chapter 2 ♦ Extraterrestrial Life: The History of an Idea
2.0 Extraterrestrial Life: The History of an Idea

teng muThe idea that intelligent but nonhuman living beings might exist somewhere has tantalized the minds of men since the dawn of recorded history.  Virtually every civilization or major culture on Earth has entertained some such speculation, whether in its mythology, its religious or scientific writings, or in its philosophy of nature.

The sophisticated concept of aliens indigenous to planets circling faraway stars did not blossom into existence overnight, however.  The theme of extraterrestrial life has slowly evolved over the course of many millennia of pensive human contemplation.  Before it was accepted that Earth was a mere planet and that many others could exist, intelligent nonhuman beings were commonly viewed in a mythological context.  But as man learned to appreciate the vast scale of the universe, the idea of life in the physical cosmos matured and gained wider currency.*

* There are many good historical introductions to both the scientific747, 1754, 1769, 1872 and the fictional1896, 1897, 1872 literature.

2.1 Ancient Beginnings

richard blackwell

ETs in Sumeria

While it is often pointed out that aliens appear in the most ancient of human records, the true antiquity of the idea is rarely appreciated.  An excellent example comes from the Sumerian civilization, which flourished more than five thousand years ago (and may well be the most distant ancestor of Western culture).  According to Sumer legends which have survived, codes of law, science, art, architecture and the essentials of proper social behavior all were given to the humans by alien teachers — amphibian intelligent animals with fishy heads and torsos and human feet. These creatures are never described as gods;  there is little doubt the Sumerians presumed them to be as mortal as their human students.20

Usually, though, ancient gods were seen as superior beings with celestial abodes.  The Babylonians, successors to the Sumerian civilization, held that the moving points of light in the sky which are the planets were the homes of their gods.45  Other cultures such as the Eskimos believed that the Moon and other heavenly objects were themselves gods.1872

Supposing the world to be flat and subscribing to the nontheistic Confucian philosophy, the venerable Chinese had no conception of or need for life in the firmament — although dragons and other monsters appeared frequently in the literature.  The holy books of Buddhism, on the other hand, appear to accept the plurality of worlds in countless numbers, complete with indigenous alien plant and animal lifeforms.1898

ETs in Asia

The ancient Vedda culture, which prospered on Ceylon prior to the Hindu invasion in the 6th century B.C., held that after death souls migrated to the Sun, Moon, and the stars before reaching Nirvana (the ultimate state of perfection).  The beliefs of the Hindus are also closely associated with the idea of a plurality of worlds.  The Indian philosophy, in fact, "explicitly assumes the existence of extraterrestrial intelligences."1899  In one myth, as told in the Brahmavaivartcz Purana of the god Indra, we find:

Hold!  I have spoken only of those worlds within this universe.  But consider the myriads of universes that coexist side by side, each with its Indra and Brahma, and each with its evolving and dissolving worlds … Can you presume to know them, count them, or fathom the reaches of all those universes with their multitude of worlds, each with its legions of transmigrating inhabitants?1901

The Old Testament is filled with strange events which some have argued may be linked with extraterrestrial visitations — such as the visions of Ezekiel.1058  And in the New Testament appear such positive statements as: "In my Father's house there are many mansions" (John 14:2), and so forth.

ETs and the Greek and Roman cultures

But by far the most important early contributors to the advancement of the idea of ETs were the Greek and Roman cultures.  To the Homeric Greeks, the Moon was an inhabited world separate from Earth, the dwelling place of protean gods and the spirits of departed humans.1753  Traditional Grecian mythology held that the universe created the gods, a view more consistent with the concept of mortal, fallible aliens than the usual creator-deity of other religions.

The Greek culture inherited considerable astronomical knowledge from the Egyptians and Babylonians upon which much speculation could be based.  Thales of Miletus (6th century B.C.) was a philosopher who had guessed that heavenly bodies might have a material composition similar to that of the Earth.  Around this time Pythagoras (well-known for his contributions to geometry) and others were beginning to think of Earth as a globe in space — a sharp break from the flat-world concepts of earlier thinkers.

Since other earthlike worlds might therefore exist, Xenophanes of Colophon — a contemporary of Pythagoras — populated the Moon with inhabitants, cities and mountains.602  Another Greek philosopher named Anaximenes evidently also believed in a multitude of celestial habitats, because he had the audacity to tell Alexander the Great that the Macedonian king had conquered "only one of many worlds."702

In the 5th century B.C. Democritus taught the concepts of infinite space and numerous worlds.747  One of his pupils, Metrodorus of Chios, later wrote that "to consider the Earth the only populated world in infinite space is as absurd as to assert that in an entire field sown with millet only one grain will grow."20  Anaxagoras too embraced the plurality of worlds:  "The Sun, the Moon, and all the stars are stones on fire. The Sun is a red-hot mass, or a stone, on fire. The Moon is of earthy nature … an incandescent solid, having in it plains, thountains, and ravines!" 1872

Another 5th century mathematician of the Pythagorean school stated his views on extraterrestrial most forthrightly:

The Moon has an earthy appearance because, like our Earth, it is inhabited throughout by animals and plants, only larger and more beautiful than ours:  for the animals on it are fifteen times stronger than those on the Earth …  and the day in the Moon is correspondingly longer... 1872

And from The Travels of the Young Anacharsis in Greece, written sometime during the 4th century B.C., we have:

As nature is even richer by the variety than by the number of the species, I spread in the various planets … peoples who have one, two, three, or four senses in supplement.  I then compare their geniuses with those Greece has produced, and I must confess that Homer and Pythagoras inspire my pity." 362

About this time the first "Moon romance" was written by Antonius Diogenes.  His Of the Wonderful Things beyond Thule  included a visit to the Moon;  unfortunately, the original text has not survived.1872

The Roman poet and philosopher Lucretius firmly believed in a host of inhabited worlds. As he wrote in De Rerum Natura:

Why then you must confess that other worlds exist in other regions of the sky, and different tribes of men, kinds of wild beasts.... Nothing in nature is produced alone, nothing is born unique, or grows unique, alone. Each thing is always specimen — of race or class, and many specimens belong to each.... That sky and Earth and Sun and all that comes to be are not unique but rather countless examples of a class." 733

Unfortunately for xenology, the Earth-centered (geocentric) cosmologies sponsored by Plato and Aristotle held sway.  Both philosophers were firmly opposed to the concept of a plurality of worlds. Aristotle asserted that all matter was contained in this world, thus leaving no room for any others.  The unchangeability of the heavens was cited as additional proof of this.45  These teachings were later picked up by the Christian Church and enforced as law.  It was then denied that any knowledge could exist that Aristotle had not known.

2.2 The Long Interregnum

christian huygensDespite the powerful forces arrayed behind the Aristotelian world view, it took time to halt the intellectual momentum in favor of habitable worlds. The famous Roman poet Cicero was interested in the possibility of living beings on the Moon, and his Somnium Scipionis may have inspired Plutarch (46 A.D. - 120 A.D.) to write his account of a visit to the Moon. In Facies in Orbe Lunare, after dealing with various problems involved in reaching the Moon, the Greek historian endorsed the Pythagoreans thus: "They affirm that the Moon is terrestrial and inhabited like the Earth, peopled with the greatest living creatures and the fairest plants..."1753 He continues:

It is possible that some inhabitants exist on the Moon; and those who claim that these beings must need everything that is necessary to us, have never considered the variety that nature offers so that animals differ amongst themselves more than they differ from inanimate life.

Only forty years after the death of Plutarch, the Greek satirist Lucian of Samosata (125 A.D. - 190 A.D.) wrote the first interplanetary romance that has survived the ravages of time.1872 In his elaborate True History Lucian and his fellow travelers are carried by whirlwind to the Moon, found to be inhabited by a race of men who ride on the backs of three headed birds. The adventurers have arrived at a most inopportune moment, as the Lunarians are in the middle of a war with the inhabitants of the Sun to settle a dispute over the colonization of Venus.1753 The space troops include such marvelous creatures as "Horse-vultures," "Salad-wings," and "Flea-archers" (archers astride giant lunar fleas).742 The story is reminiscent of the "space opera" of the 1930's and 1940's.

But after Lucian there was no further debate of the possibility of visiting other worlds and meeting the indigenous lifeforms there — for more than a thousand years! This may probably be attributed to the pervasiveness of the Church philosophy and its rigid opposition to the idea of the plurality of worlds. The pronouncement of Franciscus Gratianus, Bishop of Chiusi, in 1145 A.D. was perhaps typical: The belief in many worlds was to be condemned as heresy.

ETs and the Church

Of course, there was a serious logical flaw in this stance. If God really was all-powerful, why was he only able to create one world? Conversely, if only one world existed how could God possibly be truly infinite and omnipotent? The theologian Thomas Aquinas (1225 - 1274) came up with a "solution" to the problem: God had the power to create infinite worlds, but all the matter in the universe had been used to construct Earth!372

Despite the obvious holes in this reasoning, the Church subsequently partially reversed its extreme position. In 1277, under the authority of the Pope, the Bishop of Paris decried as new heresy the belief that a plurality of worlds was impossible!45 This did not, of course, mean that the Church began to teach the plurality of worlds. According to the physics of Aristotle, still in vogue until the 16th century, if any other worlds did exist they would have to gravitate to the center of the universe (where Earth was). But it became wrong to suggest that God could not create many worlds if He wished.747

The debate was far from ended. In 1410 the Jewish philosopher Crescas wrote: "Everything said in negation of the possibility of many worlds is vanity and a striving after wind." Still, he was unwilling to stick out his neck very far:

… yet we are unable by means of mere speculation to ascertain the true nature of what is outside this world; our sages, peace be on them, have seen fit to warn against searching and inquiring into what is above and what is below, what is before and what is behind...747

The first really explicit deviation from orthodoxy occurred during the Inquisition in Europe in the mid-fifteenth century. Cardinal Nicolas of Cusa, Bishop of Brixen and Christian philosopher, wrote a book called Of Learned Ignorance (1440) in which he stated:

Rather than think so many stars and parts of the heavens are uninhabited, and that this Earth or ours alone is peopled … we will suppose that in every region there are inhabitants, differing in nature by rank and all owing their origin to God.747

Considering how little we know about other animals here on Earth, he claims, "of the inhabitants … of worlds other than our own we can know still less, having no standards by which to appraise them."747 It is said that Cusa escaped the Inquisitional wrath only by virtue of his special protection and friendship with Pope Eugene IV.1753

As astronomical observations became more accurate, the geocentric Aristotelian/Ptolemaic world view began to generate problems that were difficult to resolve. Calculated positions of the planets, for instance, were invariably in error. This necessitated the concoction of elaborate "explanations" based on a kind of astronomical fudge factor.

During this time the first tale of interplanetary travel since Lucian (thirteen centuries earlier) was published. Ludovico Athsto's (1474-1533) Orlando Furioso tells of a trip to the Moon using a chariot driven by Saint John. The vehicle is drawn by flaming horses, who leap from the summit of a high mountain. The Moon, it turns out, is littered with cities and townships. The heavy theological flavor of the story may have helped save Ariosto from persecution.

A mere eleven years later the first edition of Copernicus‘ renowned De Revolutionibus Orbium Caelestium appeared, proposing the modern Sun-centered (heliocentric) solar system. If the Holy See was enraged at this they could do nothing, for the Polish astronomer died the year his book came out — 1543.

Giordano Bruno

Others were not so lucky. Forty one years after the death of Copernicus a Dominican monk by the name of Giordano Bruno (1547-1600) wrote his controversial On the Infinite Universe and Worlds. Among other things, the Italian philosopher advanced the following heterodoxies: "Innumerable suns exist; innumerable earths revolve about these suns in a manner similar to the way planets revolve around our sun. Living beings inhabit these worlds."20

Although Bruno was visiting in relatively tolerant Great Britain at the time his book was published,747 as soon as he set foot on Italian soil he was promptly arrested by the Church and incarcerated without trial for seven years.45 He was then convicted of heresy by a tribunal of the Holy See and sentenced to death. Bruno was burned at the stake in the Campo de' Fiori in Rome on February 17, 1600.

With the improvement of the telescope by Galileo (1564-1642) and the subsequent observations of the mountainous terrain of the lunar surface, it became clear that the Moon was quite similar to the Earth in many ways. His discovery of the four largest Jovian satellites confirmed the existence of many worlds. For his part in advancing the heliocentric Copernican astronomy and the hypothesis of the plurality of worlds, Galileo was arrested by the Inquisition and forced to recant his heresies. Luckily, he was not executed.

Johannes Kepler (1571-1630) further refined the Sun-centered cosmology by suggesting that planets move in ellipses rather than perfect circles. He also authored an engrossing fictional account of a trip to the Moon, published four years after his death, entitled Somnium. Lunar biology is described in some detail, including several forms of vegetation and serpentlike grotesque monsters.742

The first narrative of a trip to the Moon written in English was penned by Bishop Francis Godwin in 1638. In The Man in the Moone the main character, Domingo Gonsales, uses a team of trained geese under harness to carry him to the Moon whereupon:

Suddenly I saw myself environed with a kind of people most strange, both for their feature, demeanure, and apparel. Their stature was most diverse, but for the most part twice the height of ours; their color and countenance most pleasing, and their habit such as I know not how to express....1872

By 1640 another book was out, a two-volume set by fellow English Bishop John Wilkins, entitled The Discovery of a World in the Moone. Wilkins asserted his straightforward belief "that it is possible for some of our posterity to find out a conveyance to this other world, and if there be inhabitants there, to have commerce with them."747

The roadblocks to the idea of intelligent alien life on other worlds were rapidly disintegrating.

2.3 Plurality of Worlds and Divine Purpose

carl saganBy the early and mid-1600's the utilization of the Moon and other planets as abodes for extraterrestrial life had become an accepted theme, certainly in fiction but also increasingly in scientific writings of the time. In the 17th century — the century of great discoveries, scientific breakthroughs and grand geographical voyages around the world — more than 200 accounts of trips to the Moon appeared in print.1896

In 1656 the Jesuit Athanasius Kircher sent his hero touring the heavens with an angel as his guide. In the course of these journeys, the Moon was found to be quite habitable, including mountains, oceans, lakes, islands and rivers.1872

Life on the Moon

About a decade later in Milton's well-known Paradise Lost, the angel Raphael discusses the possibility of life on the Moon and other planets. Says he of the Moon:

Could not there be
Fields and inhabitants? Her spots thou seest
As clouds, and clouds may rain, and rain produce
Fruits in her softened soil, for some to eat
Allotted there; and other Suns, perhaps,
With their attendant Moons…

But Adam is cautioned that it is dangerous to cogitate such matters, as they are best left to the Almighty: "Dream not of other worlds, what creatures there live, in what state, condition or degree."702

David Russen in A Voyage to the Moon (1703) allowed that there might be inhabitants on the Moon, but that traveling there would be difficult because of the lack of air between worlds.742 In Robert Paltock's John Daniel (1751), a survivor of a shipwreck constructs a flying machine to escape his island prison but winds up escaping the Earth instead! On the Moon he finds copper-skinned humanoids who live in caves and worship the Sun.742 And in 1775, a Frenchman named Louis-Guillaume de la Follie published an account of the doings of beings on Mercury. In Philosophy Without Pretension, a brilliant Mercurian inventor-scientist constructs a flying machine which carries a skeptical fellow scientist to Earth and maroons him here.45

But the fictional treatments of extraterrestrial life in the late 17th and 18th centuries were executed with a growing eye to satire and witty criticism of the foibles of modern civilization. Despite the increasing interest among the scientific community in alien life, fictional tales remained remarkably free of science and technical accuracy.

For example, two of the best-known early adventure stories were Cyrano de Bergerac's (1620-1655) Voyage to the Moon (1657) and History of the States and Empires of the Sun (1662) (which was uncompleted at his death) . In the first of these tales, the narrator wears bottles filled with morning dew which are attracted to the Sun — everyone knows dew rises! — and eventually transport him to the Moon. There he meets Domingo Gonsales and his trained geese, and the lunar queen and her court are a cruel mockery of the monarchy of contemporary England.1872

Gabriel Daniel's novel A Voyage to the World of Descartes (1694) is a satire on the dualist philosophy of Descartes. Daniel's travelers found the Moon to be inhabited only by spirits.742 Voltaire's characters in Micromegas (1752) are extraterrestrials: One is a dwarf from Saturn with 72 different senses, and the other is a giant eight leagues tall from the Sirius star system possessing more than a thousand different senses. The story is a satire on the supposed intelligence of mankind, as it might be evaluated by objective aliens.742 And Aratus' narrator in his A Voyage to the Moon (1793) treks to Luna by hot air balloon, landing on an island peopled with lipedal snake-like organisms that speak English. The book caricatures British social and political life by describing the civilization of the man-snakes in a most derogatory fashion.742

Another main thrust during this era of development was along religious lines. As the astronomers during the 1600's came to accept the plurality of worlds, an assumption arose that God would never knowingly "waste" a world.747 This view, which persisted well into the 19th and even 20th centuries,95, 103, 117, 206, 599 held that if worlds did exist in space their only real purpose could be to harbor manlike beings.1902

In this vein, Ralph Cudworth wrote in The True Intellectual System of the Universe (1678): "It is not reasonable to think that all this immense vastness should lie waste[d], desert[ed], and uninhabited, and have nothing in it that could praise the Creator thereof, save only this one small spot of Earth."747 The Anglican theologian Thomas Burnet followed suit six years later in a book called The Sacred Theory of the Earth, wherein he asked:

God himself formed the Earth … he formed it to be inhabited. This is true, both of the Earth and of every habitable World whatsoever. For to what purpose is it made habitable, if not to be inhabited? We do not build houses that they should stand empty, but look out for Tenants as fast as we can.747

In a sermon preached by a young English clergyman named Richard Bentley in 1692, we find still more evidence of the new viewpoint that swept over Christianity in only a century: "It remains, therefore, that all bodies were formed for the sake of intelligent minds … each for their own inhabitants which have life and understanding."747 William Derham, another minister and author of the popular work Astrotheology (1715), was of the same opinion.

ETs and colonial America

Nor was colonial America immune to these new exotheological conceptions. Cotton Mather (1663-1728), a Puritan minister who wrote a book called The Christian Philosopher, had this to say: "Great God, what a Variety of Worlds hast thou created! How stupendous are the Displays of thy Greatness … in the Creatures with which thou hast replenished those Worlds!"*747 During this entire period of literary and theological development, scientific speculation on the nature of extraterrestrial life was on the upswing. Spaceflight to other worlds was no longer viewed as wholly impractical; when Peter Heylyn compiled his World Geography, the Moon was described along with such other "imaginary" lands as Australia, New Guinea, and the Solomon Islands.1872

Bernard de Fontenelle's Conversations about the Plurality of Worlds came out in 1686 and was an instant success. Not only did de Fontenelle conclude that intelligent beings must exist on worlds other than Earth, but he advanced the progressive notion that such beings would have those characteristics consistent with the environment of the world in which they lived.

Mercurians, therefore, were all hotheads in temperament. The inhabitants of Venus, the next planet out from the Sun, "resemble the Moors of Granada, a small, black people, burned by the Sun, full of wit and fire, always in love, writing verse, fond of music, arranging festivals, dances and tournaments every day." Jupiterians rarely encountered each other, since their planet was so large, and the extreme coldness of Saturn rendered the creatures there dull, torpid and sluggish in mind and body. It was suggested that the Moon might not be inhabited at all, because of the thinness of the atmosphere.1950

ETs and scientific discovery

The first full-length scientific book to deal seriously and specifically with the problem of extraterrestrial life was authored by the Dutch physicist and astronomer Christian Huygens. Entitled The Celestial Worlds Discover'd (1698), it contained many detailed theories and pursued with greater diligence the conform-to-the-environment theme de Fontenelle had also wrestled with.

Of the planet Mars, for instance, Huygens cautiously states: "His Light and Heat is twice, and sometimes three times less than ours, to which I suppose the Constitution of his Inhabitants is answerable." As for populating the Sun,** Huygens (unlike William Herschel more than a century later) is very pessimistic:

That the Sun is extremely hot and fiery, is beyond all dispute, and such Bodies as ours could not live one moment in such a Furnace. We must make a new fort of Animals then, such as we have no Idea or Likeness of among us, such as we can neither imagine nor conceive: which is as much to say, that truly we have nothing at all to say.602

Many 18th century notables freely gave their views on alien life. Emanuel Swedenborg (1688-1772), a Swedish scientist, mystic philosopher and theologian, fancied that Venus was inhabited by two distinct species of giants — one the gentle, religious, human herdsmen, and the other the cruel, savage plunderers "whose favorite sport is eating what has been stolen."43

In a more serious temper, in 1728 Benjamin Franklin wrote: "I believe that Man is not the most perfect Being but One; rather that as there are many degrees of Beings his Inferiors, so there are many degrees of Beings superior to him."747 The so-called father of Russian science, Mikhail Vasilievich Lomonosov (1711-1765), published many poems of both satirical and scientific bent to communicate his belief in life on other worlds to his countrymen. And John Adams, who became the second President of the United States, made the following entry in his personal diary on April 24, 1756: "… all the unnumbered Worlds that revolve round the fixt Stars are inhabited, as well as this Globe of Earth."

Legal philosophers likewise expressed interest in xenology at an early date. For example, Immanuel Kant's Universal Natural History and Theory of the Heavens (1755) set forth the then unorthodox proposition that while many worlds may be inhabited, not all planets will bear life. Furthermore, Kant felt it likely that "celestial bodies which are not yet inhabited will be hereafter, when their development has reached a later stage."

The great French philosopher Montesquieu (1689-1755) may be credited with the anticipation of modern metalegal concepts (see Chapter 25). He envisioned the possibility of humans having some form of legal relations with intelligent ETs. In his De l‘espirit des lois (1748) he stated:

Laws in the broadest sense imply relationship. That necessarily follows from the nature of things. In that sense all beings have their laws… Laws are relationships which exist between it and the different beings as well as the relations between these beings themselves.372

And back across the English channel, the British political leader Lord Bolingbroke (1678-1751) wrote that ours may not be the highest intellect in the universe. In fact, said he, "we may well suspect that ours is the lowest, in this respect, of all mundane systems."747

* A curious book was written in 1757 by Dr. Swinden, a British clergyman, called Researches on the Nature of the Fire of Hell and the Place Where It Is Situated. According to Swindon's detailed calculations, the interior volume of Earth is far too small to hold the multitude of angels that fell from Heaven after the Great Battle. Hence, the Sun is the only possible abode of the devil, it being a well-sustained fire and having plenty of room for Satanic inhabitants.

** The theory of solar-dwellers was once used as evidence at a murder trial. One Dr. Elliot, accused of the murder of Miss Boydell in 1787, stated in open court his opinion that the Sun was endowed with intelligent inhabitants. His friends asserted insanity as his defense, citing as clear proof his beliefs regarding life in the Sun.43

2.4 Science and Science Fiction

In 1929 Hugo Gernsback coined
the term "science fiction"

In the early 19th century it was still maintained by many that the Moon must be inhabited, or else God's work would be wasted. Thomas Dick carried this idea to its absurd but logical conclusion in his nonfiction theological work entitled Celestial Scenery (1838). First, he noted that the rings of Saturn contain an area of more than thirty million square miles. "It is not likely," he went on, "that the Creator would leave a space equal to nearly six hundred times the habitable parts of our globe, as a desolate waste, without nay tribes of either sensitive or intelligent existence …"1872

By the mid-nineteenth century this view remained virtually unchanged. Father Angelo Secehi, a Jesuit astronomer, asserted of the planets: "These worlds are bound to be populated by creatures capable of recognizing, honoring and loving their Creator."1905

But as the last century drew to a close, the English poetess Alice Meynell (1849-1922) sounded in verse what was to become the less-chauvinistic modern perspective:

Doubtless we shall compare together, hear
A million alien Gospels …
O, be prepared, my soul!
To read the inevitable, to scan
The million forms of God those stars unroll
When, in our turn, we show to them a Man.702

This idea that ETs will have their own religions and their own gods has replaced "waste" as the central topic of exotheological debate.

Scientific speculations were often grossly unreliable and misleading, as evidenced by the "Moon Hoax" fiasco. In July of 1822 a German astronomer by the name of Franz von Paula Guithuisen had first reported observing a great walled city on the Moon, near the crater Schröter on the lunar equator. This caused quite a flap, and the stage was set.

Great Astronomical Discoveries of 1835

The famous British astronomer Sir John Herschel (1792-1871) traveled to the Royal Observatory at Capetown, South Africa in 1834 to commence a full sky survey of the Southern hemisphere. The project was well known throughout the educated world at the time and, added to Gruithuisen's wild claims, may explain the widespread acceptance of Richard Adams Locke's concocted front page story on Sir Herschel's "amazing discovery" of inhabitants on the Moon.

Published in The New York Sun during the first week of September, 1835, the report (called "Great Astronomical Discoveries") claimed that Herschel had turned a powerful new telescope towards the Moon and had observed life there, including forests, bison-like animals, blue unicorns and finally, winged men and women:

We counted three parties of these creatures, of twelve, nine, and fifteen in each, walking erect towards a small wood near the base of the eastern precipices. Certainly they were like human beings, for their wings had now disappeared, and their attitude in walking was both erect and dignified.1872

The entire first reprinting of 20,000 copies was completely sold out on the day of publication.

During the second half of the 19th century the French scientist and popularizer Camille Flammarion wrote many discourses on the subject of extraterrestrial life. His On the Plurality of Habitable Worlds was a much-read general treatment of the subject. Another work, entitled Imaginary Worlds and Real Worlds, was a review of all previous writings on the habitability of worlds and the possibility of interplanetary communication.45 In still another volume, Lands in the Sky, Flaminarion stated with conviction:

The Humanities of the heavens are no longer a myth. Already the telescope brings us in touch with their countries; already the spectroscope enables us to analyze the air they breathe… From the bottom of our abyss we can visualize these far-away nations, these unknown cities, these extraterrestrial people!733

The publication of Darwin's Origin of Species (1859) was soon followed by the development of an idea advanced by Swedish chemist Svante Arrhenius: Life may be ubiquitous thoughout the cosmos, carried from planet to planet by tiny space-spores (panspermia).1906

Modern era of scientific Xenology

The modern era of scientific xenology was ushered in with Henderson's important little book The Fitness of the Environment (1913), in which the Harvard biochemist attempted to demonstrate that both water and carbon are necessary in any living system on any planet in the universe.879 Superior astronomical data was becoming available, providing a still more accurate view of our solar system and galaxy.

The father of Russian astronautics, Konstantin E. Tsiolkovskii, wrote extensively on spaceflight and the possibility of ETs colonizing the Galaxy ahead of us. Alien civilizations, he pointed out, might well exist at many different levels of technological development.20 In 1925, Tsiolkovskii summarized by noting the distinct probability that "perfection and dominance of the mind" have been spreading throughout the cosmos.702

Fictional treatments of extraterrestrial life proliferated. During the late 19th and 20th centuries the use of aliens became a vehicle for both romantic and far-flung scientific speculative statement.

Achille Eyraud's Voyage to Venus (1865) was the first fictional visit to that planet at a time when the idea of an inhabited Moon was virtually a dead letter.45 In another trip to Venus, Garret Putnam Serviss's A Columbus of Space (1909), we find ape-like cave dwellers and beautiful telepathic humanoids.742 John Munro painted a most delightful picture of life on Venus in A Trip to Venus (1897);1872 Edgar Rice Burroughs also took us to Venus and Mars, as did C. S. Lewis in his well-known trilogy.364348 And Jupiter's steaming jungles, replete with dinosaurs and pterodactyls, appeared in John Jacob Astor's A Journey in Other Worlds (1894) — along with a brief excursion to Saturn.742

Voyages to other stars began to be written. With the French author Charles Ischir Defontenay we are transported to the star system of Psi Cassiopeia, in his 1854 novel of the same name, for a quick dose of haunting space opera.564 David Lindsay's A Voyage to Arcturus (1920) likewise is a romance, describing a visit to the extrasolar planet Tormance by spaceship and various adventures with the inhabitants there.1872

Mars Fever

But it was certainly "Mars Fever" that inaugurated the present epoch of science in science fiction. In 1877, under unusual conditions of good seeing, the Italian astronomer Giovanni Schiaparelli observed what appeared to be "channels" crisscrossing the Martian surface. Schiaparelli never maintained that his "channels" were relics of alien technology. Yet the word gained something in the translation into English: "Channels" became "canals," with the connotation of intelligent engineering efforts.

This was snapped up by Percival Lowell, an American astronomer who became so devoted to the search for life on Mars that he established an observatory in Flagstaff, Arizona for the sole purpose of studying the Red Planet.2009 His two books were widely read. Mars (1896) may have served as the scientific background for H. G. Wells' famous novel The War of the Worlds (1898) in which Earth suffers an invasion by Martians. Mars and its Canals (1911) might well have served in the same capacity for Burroughs when he wrote A Princess of Mars (1917),The Gods of Mars (1918), and classical sequels through 1940.

Mark Wicks' To Mars via the Moon (1911) is another enthusiastic depiction of Lowellian Mars: The telepathic Martians are found to have advanced canal-building technology and a Utopian socialist system of government. Lieutenant Gullivar Jones: His Vacation (1905) by Edwin Lester Arnold is a fantasy in which we meet carefree, friendly, gracious, but apparently purposeless Martians possessing an ideal political system. The Martians in Hugh MacCoil's Mr. Stranger's Sealed Packet (1889) have voice-recording devices and electric lighting but are otherwise technologically inferior to earthlings. And in Robert Croniie's A Plunge into Space (1890) we again find the frustrated hopes of finite humans projected onto more advanced aliens: Zero population growth has been achieved, workdays are only two hours in length, and the government is so perfect that there is no need for politicians!1872

Two novels served as a bridge from romantic visits to alien planets in the 19th century to the modern era of science fiction. The first of these, Kurd Lasswitz's Concerning Two Planets (1897), is a fascinating tale of Martians who differ little from men physically but are comparatively advanced in ethics, social and physical sciences. Since they are more advanced, Lasswitz reasoned, they will be the first to visit us and not vice versa.1038

The second important work is the aforementioned War of the Worlds by H. G. Wells.1951 The interaction of man and alien is explored realistically for the first time. Man must realize that he may someday face enemies with "minds that are to our minds as ours are to the beasts in the jungle."1951 We must also learn humility, we discover: In the end the invaders are destroyed, not by Earth's pitiful military might, but by the lowly bacteria of our planet against which the aliens have no immunological defenses.

After the early 1900s the number of scientific and fictional investigations of the problems and benefits presented by intelligent extraterrestrial races rises almost exponentially. In 1929 Hugo Gernsback coined the term "science fiction",1896 and the cheap pulps of the 1920s gave way to the technological space opera of the 1930s and 1940s. Still more recently both science and science fiction have become remarkably sophisticated, dealing in detail with interstellar travel, extrasolar alien life, reasonable planetary environments conducive to the evolution of such life, and various particulars of possible alien physiology, sociology, and philosophy.

Xenology, the study of life on other worlds, is indeed "an idea whose time has come."

Chapter 3 ♦ The Aliens Among Us
mens room wall 3.0 The Aliens Among Us

Closely paralleling the historical development of xenology has been the widespread but unverified conviction that aliens already are, or have been, visitors to Earth.

Despite the pseudoscience cults and charlatans frequently associated with "ancient astronaut" and "flying saucer" theories, the hypothesis that intelligent extraterrestrials might have played some role in the evolution or emergence of human civilization is fascinating and certainly warrants critical study.

3.1 Xenoarchaeology
3.1 Xenoarchaeology
A confused study
ronald story 360

It has recently become fashionable to postulate that ETs landed on our planet ages ago, whether to influence our biological or social evolution, to collect zoo specimens, or to make anthropological surveys.1215,1221,1326,1327,1328 Extravagant speculations abound:  One book attributes to alien benevolence the discoveries of subatomic physics, general relativity, and the double helix!1880

Xenoarchaeology — the search for evidence of ancient visitation by interstellar travelers — is at best a difficult and confused field of study. Precisely because hopes and expectations are so high, it is often harder to maintain a strong, healthy skepticism. Hence, in the words of astronomer Carl Sagan, "we must accept arguments for extraterrestrial visitations to earth only when the evidence is compelling."1870

At present, an exhaustive survey of all pertinent literature and other evidence fails to uncover a single incontrovertible case of past alien presence on our planet.

Research areas

The Hypothesis of Mediocrity allows that,
as a general rule, when one goes exploring
one finds inferior things.
Hypothesis of Mediocrity

Of course there is nothing a priori absurd about the basic idea of ancient astronauts. The Hypothesis of Mediocrity allows that, as a general rule, when one goes exploring one finds inferior things.1040 For example, Columbus discovered America because European transportation technology was well advanced beyond that of the native Americans. Were this not the case, the natives would have discovered Europe!  It is plausible to conclude that if the Galaxy is teeming with life, a superior intelligence from another star system could have visited Earth for any of a myriad of good reasons.

Speculation has centered on three specific areas. First, there is the possibility that aliens arrived geological timescales (millions or billions of years) ago. Biochemical and genetic evidence has been marshaled in an attempt to demonstrate that our natural biological evolution may have been adjusted, enhanced or tampered with in some manner.

Xenoarchaeological research areas

A second proposition is that extraterrestrial "gods" and "saviors" have materially affected the development of human society and culture. The most common evidence advanced in favor of this is the virtual ubiquity of legends describing visitors from the sky. Usually these yarns seem to suggest either that the human race was exported to Earth from other worlds, or that ETs came among men and helped launch human civilization.

Naturally, a mere account of strange beings who live in the heavens and perform miracles is not compelling proof. (Where else might gods reside but in the skies?554) And the clear correlation between the movement of stars and constellations in the celestial vault and the changing seasons has probably been recognized at least since the emergence of Cro-Magnon man. Primitive belief systems often attribute divinity and magical forces to such regular features in the environment.

Yet how easy it would be for an alien humanoid to "curse" hapless natives with a portable x-ray machine!  It may be that all interstellar First Contact operations include a Thaumaturgy Division, whose duty it is to create miracles and god-myths to keep the curious at a safe distance in the unlikely event of an emergency landing. These thaumaturgists could create spectacular displays to awe primitive onlookers, such as transmutation of drinking fluids, variable-rigidity lances, and advanced force-field technology (which might perhaps be demonstrated on local bodies of water).

The third specific area of xenoarchaeological research is the quest for alien artifacts and other physical manifestations of their presence. This is of great importance, because it is often urged that in such important matters "the only acceptable evidence would be hardware."373 For instance, the discovery of a piece of advanced electronics embedded in a coal seam millions of years old, accompanied by indisputably nonhuman skeletons, might be acceptable as convincing evidence of past visitation. Another common suggestion is that the aliens might have left a durable marker of some kind, such as the black monolith depicted in the science fiction movie 2001: A Space Odessey. In fact the Moon would be an ideal location:  The artifact would last millions of years without disturbance and could only be detected by a reasonably advanced spacefaring civilization.

But we must beware of technological chauvinism in assessing possible artifacts. We can interpret certain objects as airfields, nails, or evidence of the use of nuclear explosives, but this is because we have just acquired this technology ourselves. Two hundred years ago the interpretation would have been much different; two centuries hence, it will again radically change. The fact is that the technology of space visitors will most likely be highly non-contemporary with our own.

Problems discerning ancient ET contact

The problems involved in tracking down evidence of extraterrestrial contacts in ancient times are vastly different from those of conventional archaeology and anthropology. It would not be remarkable if a few brief visits by ETs to limited areas of this planet have left no traces. Continental drift and tectonic shuffling, ice ages, volcanic activity and sedimentation will have taken their toll.

Archaeologists generally search for human settlements. Yet the chances of, say, a spacecraft crash landing near one of these is extremely small. It is highly unlikely that we could ever detect anything less than widespread, intensive alien involvement.

Let us assume arguendo that ten aerial vehicles crashed somewhere on Earth in ancient times, spewing their debris over a swath covering 10,000 m2. If the affected stratum is one meter deep and lies under an average of 10 meters of sediment, this leaves about 1015 cubic meters of soil and rock to be examined.

If we then assume that all crashes occurred only over that 10% of the Earth’s land area which is "interesting" to the ETs, and that there are ten archaeologists with suitable engineering and technical credentials searching full time for the sites (excavating an average of 10 m3 each day per investigator), it would take roughly 15,000 years just to have a 50/50 chance of finding a crash site. Even then, and assuming favorable corrosion conditions, the chances of spotting recognizable remnants of an accident would still be miniscule.

For these reasons and others, many have renewed the hunt for reconstructable contact legends passed down from early human civilizations. There is some reason to cautiously assert the validity of this technique, because we know that historical events have occasionally been faithfully recorded in myth and folklore.

Perhaps the best-known of these was the first meeting between the Tlingit people on the northeast coast of North America and a European expedition in 1786 led by the French explorer Jean La Pérouse. The oral native account of the incident remained true to the original a century later, although some of the descriptions of advanced European technology (e.g., giant sailing ships) had acquired a distinct mythological flavor over the years.554 And many other accounts of such phenomena as supernovae1557 and great floods862 have likewise survived through centuries of verbal narration.

Sagan's "Three Factors" to be authenticated

One useful test of the validity of legendary encounters with ETs might be whether or not information is contained in the tale which couldn’t possibly have been generated by the primitive civilization itself.15 For example, an ancient manuscript containing modern circuit diagrams or a "holy number" worshipped throughout the ages (which turned out to be the transcendental e or the nuclear fine structure constant) might be sufficient if it could be independently authenticated.

Carl Sagan has articulated three factors which maximize the probability that an historical encounter with aliens would be recorded in a reconstructable manner:

1. The account must be committed to writing soon after the event;

2. The contacted society undergoes a major change because of the contact;  

3. The aliens make no attempt to disguise their exogenous nature.554

If these stringent requirements can be satisfied in even a single instance, xenoarchaeologists may be able to secure proof that Earth has been visited by intelligent ETs.

3.1.1 Extraterrestrial Intervention in Biological Evolution

chapter 03 fermi einstein 239
Interplanetary adultery

The evidence that man’s biogenetic evolution has been interfered with by aliens is scanty and highly questionable. Perhaps one of the earliest mythological accounts of possible biological experimentation on apes is mentioned in the Ramayama, the second of the great Indian epic poems. Hanuman the monkey god was supposedly conceived when Shivar (a dweller in the heavens) gave Anjana (an Earth ape) a sacred cake to eat. The monkey god thus born was super-strong and highly intelligent.310 But despite the fact that Hanuman was followed by legions of other ape-heroes (Sugriva, Brahaspati, Bali, Tara and Gandha, among others), there was never any suggestion that these were the biological precursors of men.

Greek mythology is full of tales of "interplanetary adultery." Zeus, king of the gods, had scores of human concubines and was reportedly responsible for many rapes of human females. Apollo, Aphrodite, Hermes and Ares all had affairs with mere mortals. Yet most biologists today agree that a successful sexual mating between two species from different planets is improbable at best. Although lions and tigers have been crossbred in captivity (to make "ligers"), such is not the rule. Even Cro-Magnon and Neanderthal man, two species of humans, are not believed to have been interfertile.

Benevolent ETs would probably have come to Earth, not to hybridize or perpetuate their own genome, but to improve ours. This could easily be done using advanced genetic engineering to accelerate the normal evolutionary processes. The native myths of the Marquesas Islands, Hawaii, Indonesia and Tahiti all tell that the first men on Earth were given birth to by a celestial couple.310 If one wanted to do this sort of thing and a humanoid was the desired end-product, it might make sense to modify some of the local primate stock. Marmosets and many other monkeys have the same number of chromosomes as man; gorillas, chimps and orangutans have only two extra.

Erich von Daniken has suggested something along these lines, although his factual support is notoriously weak. He claims in his several books that man is an artificial mutation, separated from the ape stock long ago by alien intervention.1221 In Chariots of the Gods we find:

Dim ages ago an unknown spaceship discovered our planet. The crew of the ship soon found out that the earth had all the prerequisites for intelligent life to develop. The spacemen artificially fertilized some human female members of {an advanced primate species}

They repeated their breeding experiment several times until they produced a creature intelligent enough to have the rules of society imparted to it. The space travelers destroyed the unsuccessful specimens, {fearing that men} might retrogress and mate with animals again.1326

"Directed Panspermia,"
… space probes bearing … assorted microorganisms
capable of infecting a sterile host planet.

Unfortunately, no solid verifiable facts are adduced in favor of the hypothesis.

This area of xenoarchaeology has been severely handicapped by a dearth of qualified researchers and an excessive quantity of unusually poor scholarship.1948 A case in point is Mankind — Child of the Stars by Max H. Flindt and Otto O. Binder.1215 Their proposal, simply stated, is that we are the hybridized descendants of intelligent extraterrestrials. Apparently following Larry Niven's excellent science fiction novel Protector first published seven years earlier,1909 Flindt and Binder assert that the human race is merely a colony founded and maintained — and later abandoned — by beings from another world. Decades of detailed paleontological and evolutionary data are casually swept aside: We are asked to believe that man could not have evolved fast enough on Earth. Hence the "starmen" must be responsible.

Supposedly, humans are sexier than other animals because the ETs were downright lecherous. Not only did the starmen bring their own genes to Earth for our benefit, but "the primate line was imported"1215 as well. As if this were not enough, the authors of Mankind attribute the evolution of hundreds of species of food animals and other extinct creatures to the aliens’ kindly influence. Again, factual support is totally nonexistent.

But serious xenoarchaeological theories are being pursued by competent scientists in spite of the deluge of popularized pseudoscience on the subject. Ronald Bracewell, a respected Stanford University radioastronomer, has proposed that it would be a fine gesture for a passing extraterrestrial to have seeded our then-sterile planet, billions of years ago, with the first microorganisms that would later lead to the evolution of intelligent life.80

A less glamorous version of this conception of the origin of life is widely known as the Gold Garbage Theory. According to Dr. Thomas Gold of the Center for Radiophysics and Space Research at Cornell University, life here might have spread from a pile of waste products accidentally dumped on a barren Earth long ago.2,1910 A. G. Cairns-Smith, a well-known biochemist at the University of Glasgow in Great Britain, suggests that our original ancestors might have had alien biochemistries and has presented some (as yet nonconclusive) evidence to support this possibility.1460

Crick and Orgel's directed panspermia

But the best-known of the "earth-seeding" ideas has come from two of the world’s most eminent molecular biologists: Francis Crick at Cambridge, England and Leslie Orgel at the Salk Institute in San Diego, California. According to their theory, first presented in 1971 at the joint Soviet-American Byurakan CETI conference, organisms may have been directly transmitted to the Earth by intelligent space beings — deliberately.1283 This "directed panspermia," as they call it, could be accomplished simply by sending out unmanned space probes bearing a ton or so of assorted microorganisms capable of infecting a sterile host planet.

Crick and Orgel cite as evidence the inordinately large role of the element molybdenum in terrestrial biochemistry, peculiar because it is such a rare substance. Chromium and nickel, which are 10 and 100 times more abundant in the environment, respectively, are relatively unimportant in biochemistry. The theory has been debated extensively in the literature without conclusion.1294,1295,1296,1911,2100

3.1.2 Extraterrestrial Cultural Intervention

Early primates may have been set on the path of sociocultural development because of alien intervention, as portrayed in the popular production 2001: A Space Odyssey.1912 But there is no need to resort to fiction. Human folklore is replete with tales of interactions with strange beings from the skies.

Among the lesser-known myths is that of the Eskimos. Eskimo legends tell of being transported to the frozen northern lands in "giant metal birds". According to Pauwels and Bergier, attention has been drawn to curious cultural parallels between various archaeological sites located in Greenland, Siberia and Ceylon.1913 But apparently the claim cannot be authenticated.1001

The Gilgamesh Epic

One case which most nearly meets Sagan’s three stringent criteria (see above) is the ancient Sumerian civilization.20,554 The Sumerians were profoundly affected by the Apkallu (possible representatives of an advanced, nonhuman, amphibious extraterrestrial society), who taught them laws, science and architecture. No attempt was made by the aliens to conceal their nature. However, the first requirement — that there be a contemporary written account — is partially lacking. The only description that has survived appears in the Babylonian Gilgamesh Epic (ca. 2000 B.C.), one of the oldest existing written texts in the world today. But second-hand reports are just not good enough.

The Sumer legend is interesting because the creatures are always spoken of as "beings," "endowed with reason," and "personages" — but never as "gods"!  Were it not for the unusual subject matter the account would doubtless be considered an ordinary historical event, as there are no mystical or super natural overtones in the writing.

Most other legends don’t appear to represent a radical alteration of any culture. The 3,500-year-old Egyptian bible called the Book of the Dead  speaks of "those who with their knowledge reach the vault of the sky" and mentions "those who live among the stars".1914 Although the work purports to describe the life of Thoth, a god from the sky alleged to have given the people of the Nile the beginnings of science, literature and medicine, the Book of the Dead  is laced with mythological serpents, devils and demons.

In India, the Mahabharata is one of two beloved epic poems. The twenty-volume work, written several thousand years ago, is a history of Indian religion and mythology. The poem speaks of "vimans" that fly through the air bearing gods. In another section, two legendary characters battle each other with incredible weaponry that causes the winds to blow … meteors lashing down from the firmament … a thick gloom … the sun no longer gave any heat … clouds roared … The elephants and other creatures of the land, scorched by the energy of that weapon, ran in fright. The very waters heated, the creatures residing in that element … seemed to burn. The forms of the slain could not be distinguished.746

The Dogon of Mali in Africa worship a pyramid with a square, flat top, upon which it is said the "sky gods" landed during their visits in ancient times. Such beings supposedly taught the natives the essentials of surveying and agricultural techniques, but are always referred to as gods.310 The tale, however, appears to be purely allegorical.*

Legend of Quetzalcoatl

About the time the Toltec and Mayan cultures were beginning to intermingle (ca. 900 A.D.) there arose the legend of Quetzalcoatl, a bearded, light-skinned man who flew down from the sky to teach men law, astronomy, math, art, and the cultivation of corn and cotton. The feathered serpent was his symbol, and the pyramid built in his honor is the largest in the world (it has a volume nearly 30% greater than the largest Egyptian structure). When Quetzalcoatl’s mission to Earth was completed he returned to the morning star, promising to return someday.

The Mayans themselves are also fascinating because of the extreme accuracy of their calendar system. Furthermore, the units of time in the Mayan system included the alautun, a period of roughly 63,000,000 years!  One inscription describes events that occurred 90 million years ago, and another makes mention of a date 400 million years in the past.1848 But without more, unfortunately, a long time-sense alone cannot be considered compelling proof.

For those who wish to find evidence for extraterrestrials, the Christian Bible is chock-full of marvelous possibilities. The prophet Elijah, for instance, was protected by a fire that came down from heaven and destroyed 100 soldiers and their captains (IV Kings 1:9-12). Soon thereafter he was abducted by a "fiery chariot," and "Elijah went up by a whirlwind into heaven." (IV Kings 2:11). Similarly, Enoch is reported shanghaied by God (Genesis 5:24), although his tour of the "seven heavens" and subsequent return to Earth is published elsewhere (in The Book of the Secrets of Enoch).

Jacob wrestles with an angel until dawn and finally overpowers it (Genesis 32:22-33). After forcing the angel to bless him, Jacob releases it, exclaiming in relief: "I have seen a heavenly being face to face, yet my life has been spared."**  Daniel encountered a being on a "throne like flames of fire." (Daniel 7:9). In Revelations 4:1-6, Saint John observed "a door standing open in heaven" and then a throne "from which proceeded flashes of lightning, rumblings, and peals of thunder … and before the throne was a sea of glass like unto crystal." Seated on the throne is a humanoid, surrounded by twenty-four others (the "elders"). The list of biblical tales is virtually endless:

The God to whom Moses frequently speaks appears to lack that strength of resolve we might expect from an omniscient deity. For example, when God is about to destroy Moses’ people the prophet manages to talk the Lord out of it!  (Exodus 32:7-14)  Furthermore, Moses communicates with the being upon demand in a specially constructed Meeting Tent:  "As Moses entered the Tent, the column of cloud would come down and stand at its entrance while the Lord spoke with Moses."  (Exodus 33:9)  And God seems strangely concerned with promulgating an ethical rule that prohibits maltreatment of foreign-looking humanoids:  "When an alien resides with you in your land, do not molest him." (Leviticus 19:33)

Dr. Vyacheslav Zaitzev746 and Alexander Kazentsev981 have theorized that both Jesus Christ and the biblical angels might have been ETs. (It is interesting to note that the births of both John the Baptist and Jesus were announced to the respective mothers by angels long before they themselves knew they were pregnant, and that both mothers were barren or virgin at the time.)

The Genesis Plurals

Then we have the problem of the Genesis Plurals. There are many of them, but two are of special concern here. The first is as follows: "And God said, Let us make man in our image, after our likeness."  (Genesis 1:26)  The fact that the plurals "us" and "our" are used gives rise to the speculation that many gods are involved, that is, extraterrestrials. But it is generally accepted that these particular plurals are a veiled reference to the existence of more than one person in God (i.e., the Trinity).

The second Genesis Plural is rather harder to interpret:  "And it came to pass … that the sons of God saw the daughters of men … and they took them wives of all which they chose.  … When the sons of God came in unto the daughters of men they bore children to them." (Genesis 6:1-4)  Who are these "sons of God"?  More extraterrestrials?1845 One common explanation is that they are the descendants of Seth and Enos. Ronald Story has suggested that they were "divine beings who belonged to the heavenly court."1870 The issue remains unresolved.

One of the most controversial "contact events" in the Bible may be found in the Book of Ezekiel. To pick one passage of many:

Now it came to pass in the thirtieth year, in the fourth month, on the fifth day of the month, when I was in the midst of the captives by the river Chobar, the heavens were opened, and I saw visions of God. And I saw, and beheld a whirlwind come out of the north, and a great cloud, and a fire enfolding it, and brightness was about it, and out of the midst thereof … was the likeness of four living creatures; and this was their appearance; there was the likeness of a man in them. Every one had four faces, and every one four wings. Their feet were straight feet, and the sole of their foot … sparkled like the appearance of glowing brass. And they had the hands of a man under their wings on their four sides;  and they had faces, and wings on the four sides, and the wings of one were joined to the wings of another.

After this "landing," Ezekiel continues:

This was the vision running to and fro in the midst of the living creatures, a bright fire and lightning going forth from the fire. And the living creatures ran and returned like flashes of lightning. Now as I beheld … there appeared upon the earth by the living creatures one wheel with four faces … a wheel within a wheel. When they went they went by their four parts, and they turned not when they went … And over the heads of the living creatures was the likeness of the firmament, as the appearance of crystal, terrible to behold, and stretched out over their heads above … And I heard the noise of their wings, like the noise of many waters … and when they stood, their wings were let down. For when a voice came from above the firmament that was over their heads, they stood and let down their wings. And above the firmament was the likeness of a throne, as the appearance of the sapphire stone, and upon the throne was the appearance of a man above upon it. (Ezekiel 1:1-26)

Figure 3.1 The spaceship seen from a distance

of about 190 feet(from Blumrich1058)

figure 03 1 spaceships of ezekiel 120

An example of the depiction of the traditional interpretation.

The spacecraft began its flight to the earth with the separation from the mothership at an altitude of probably about 220 nautical miles. During the flight through the atmosphere, its speed was reduced by aerodynamic drag until eventually, at low altitudes, a brief firing of the rocket engine reduced the speed enough so that the spaceship could use its helicopters for the rest of the descent. This last phase of the flight, which begins with the brief firing of the rocket engine, was witnessed and described by Ezekiel.

Later he observes the spacecraft as it hovers a few feet above the ground in search of a suitable landing site. The brief bursts of the control rockets occur in a sequence seen as irregular by Ezekiel who construes them as lightning flickering in the space that separates the living beings. This diverts his attention from the fascinating beings to the area between them, and thus he now sees the radiator of the reactor glowing like smoldering coals.

The spacecraft has landed.

Wheels, which were housed in the lower portion of the helicopter units during the flight, have now been deployed. The straight legs with their round feet no longer touch the ground.


Figure 3.2 Helicopter Unit seen from a distance

of about 25 feet(from Blumrich1058)

figure 03 2 helicopter unit 175
Spaceships of Ezekiel

According to the late Josef Blumrich, former chief of the systems layout branch at the Marshall Spaceflight Center of NASA, Ezekiel was confronted with an "Earth Excursion Module" (Figure 3.1) manned by an alien pilot.1058 In Spaceships of Ezekiel, Blumrich presents detailed engineering analyses of a plug-nozzle planetary landing vehicle that has been seriously considered by aeronautical designers at NASA1977 and elsewhere.1001 Its "wings" are helicopter blades affixed to four columns supporting the rocket mechanism (Figure 3.2). The aerospace engineer concludes that his design would be optimal for the required missions, which are: (1) Earth-to-orbit, and (2) short surface-to-surface hops.

As the great archaeologist Heinrich Schliemann discovered the ancient city of Troy by accepting the Homeric epics literally, Blumrich has attempted to take Ezekiel at his word and reinterpret what the Hebrew prophet saw in terms of reasonable modern technology. Certainly it is doubtful that Ezekiel — a man of the 5th century B.C. — could have recognized the form or function of a bonafide spacecraft if he had seen one.

Unfortunately, most biblical reconstructions such as the above fall short of the three stringent requirements demanded by Sagan. Although the events described in the Bible clearly had an enormous effect on many cultures, the translated and retranslated record of whatever did happen 2000 years ago is now a hopelessly confused jumble of conflicting testimony. (The two accounts of Creation in Genesis, for instance, explicitly contradict each other!)

Besides the incorrectness of the astronomy and celestial mechanics in most biblical (and other) tales, the evidence here also fails because any hypothetical extraterrestrials apparently took great pains to generate a god-myth and conceal their exogenous nature. Unlike the Sumerian legends discussed earlier, the Bible is loaded with spiritual, mystical overtones which render virtually impossible the conclusive extraction of any historical visitation events that may be hidden there.

* It is interesting that the oral tradition mentions a "dark brother" of the star, Sirius.310,2022 In modern times it has been discovered that the Dog Star does possess a dark companion star, a fact unknown until a little over a century ago. Nevertheless, this can hardly be viewed as compelling evidence of extraterrestrial visitation because it is a trivial point which could easily have been adopted at random by the Dogon.

** It is notable that until about the 6th century A.D., the Church did not accept the spiritual nature of angels but considered them to be physical beings without wings.

3.1.3 Extraterrestrial Artifacts and Manifestations

From time to time peculiar artifacts have turned up, often touted as remains of alien technology here on Earth. At best most of the finds are unauthenticated, unverifiable, and frequently irrelevant.

The Salzburg Cube

Perhaps the oldest known artifact is the so-called Salzburg Cube. This object was found in 1885 in a Tertiary Period coal seam by a Dr. Gurlt. It measured 67 × 67 × 47 millimeters (with a deep groove running around its middle), weighed about 785 grams, and was said to resemble in composition a hard nickel-carbon steel.600 However, mere steel should not have been able to survive 12-70 million years of the successive acid/alkaline reactions found in the decaying vegetation in a coal bed.1001 The Cube reposed in the Salzburg Museum in Austria until 1910, when it apparently was lost.45

Bullet holes in prehistoric bison,310 remains of screws,1327 nails,49 and sparkplugs (the "Coso Artifact")83 have been unearthed, as well as handprints310 and footprints1327 molded in solidified sandstone, instruments,1326 small gem statuettes,1269 and peculiar coins.49,1001 A diffraction grating etched on a polished copper mirror was found in an early Egyptian (3rd or 4th Dynasty) tomb.49 And about 700 strange granite disks were rumored recovered from caves in the mountains of Payenk Ara Ulaa in China, in 1938. These disks bore engraved symbols telling of creatures landing a craft and meeting the local natives.746 However, the lack of corroborating artifacts is suspicious.

The Baghdad Batteries are small ovoid jars capable of producing a weak current when filled with vinegar. About a dozen such objects turned up during heavy construction work near the capital of Iraq. Ronald Story has suggested that they might have been used for primitive electroplating of silver onto copper, certainly a far cry from advanced extraterrestrial technology.1870

The Ark of the Covenant

Another technological "gift from the gods" appears in the Bible. In Exodus 25:10-22 God tells Moses how to erect the Ark of the Covenant, which serves as a transceiver to heaven. The construction details of the Ark are such that when completed, Moses should have had a giant capacitor charged to a hundred volts or so.1915,778,1326 While it is true that an arch of acacia wood with gold leaf trimmings can hardly be considered advanced technology,1870 the ability of ovens, cars and other metallic objects to audibly receive modulated radio broadcasts (on rare occasions) is a documented fact. If laboratory tests with models of the Ark can demonstrate this ability, a good case could be made in favor of alien influence: The ETs would simply have been ordering the manufacture of the simplest radio device manageable with the limited tools available to humans millennia ago.

Another Biblical tale often attributed to extraterrestrial activities is the "nuclear explosion" that destroyed Sodom and Gomorrah around 2000 B.C.1915,1326 As related in Genesis 19:24-28, "the Lord poured down on Sodom and Gomorrah sulphur and fire from out of heaven." Later that morning there was "smoke rising from the earth as though from a furnace."

But there are excellent grounds for believing that the cataclysm was the result of a great earthquake1918 followed by explosions of natural gas.1870

Excavations at the site in 1928 revealed large burned out regions of oil, sulphur and asphalt overlying a subterranean salt dome 50 meters thick. There is clear geological evidence that "a great rupture in the strata took place centuries ago. "1870

Countless other artifacts

There are countless other artifacts which could and have been attributed to space visitors, including the following:

  • The construction of the pyramids and mummification technology,1326,1915
  • The Baalbek terraces as launching platforms,746,1326
  • The rustproof iron pillar in India,1326
  • The Nazca desert "spaceport" in Peru,1326,1915
  • The subterranean tunnels and golden tablets of Juan Moricz in Ecuador,1916
  • The giant cement cylinders of New Caledonia,83
  • The peculiar statues on Easter Island,1326
  • The "catastrophic results of a landing attempt" in Tungus, Siberia in 1908,600,2202

Unfortunately, more prosaic explanations exist in all cases.80,1758,1870,1917,2008

3.2 UFology
3.2 Ufology

edward condon

Flying saucers and their progeny are largely a product of the Space Age. Since we now possess rudimentary spaceflight capability, people ask, could not aliens as well? This kind of reasoning has given added plausibility to the reports that Earth is now being regularly visited by ETs possessing high performance aerial vehicles with remarkable maneuverability (Mach-10 speeds with no sonic booms, right angle turns, vertical takeoff and landing, etc.)

UFOs in antiquity

This is not to suggest that the problem of UFOs ("Unidentified Flying Objects") is a new one. Humanity has been seeing strange lights in the sky for thousands of years. In 213 B.C. in Hadria, an "altar" was seen in the sky followed by the appearance of a humanoid in flowing white robes.1673 There were at least a dozen similar sightings during the next two hundred years. In 100 B.C., Pliny observed "a burning shield scattering sparks {as it} ran across the sky at sunset from east to west."720

The phenomena persisted into later times. In Nuremburg in 1561, for example, there reportedly was a mass sighting of flying balls and discs in the neighborhood of the rising sun.1920 The great astronomer Edmund Halley in 1716 apparently saw an object that illuminated the night sky so brightly that it could serve as a reading light for several hours.1673

It is easy to find thousands of "flying saucer" sightings, especially if we are willing to suspend our scholarly scepticism and uncritically accept all such accounts as being factual descriptions of aliens buzzing our planet. Most scientists would agree that there are many peculiar things to be seen in the heavens; it is the modern interpretation, by and large, with which they take issue.

UFO sightings by scientists

A recent poll of the members of the American Institute of Aeronautical and Astronautical Engineers turned up sightings from only 2% of the sample of 1,175 scientists.1919 But popular polls yield different results. In 1966 pollster Gallup found that more than five million Americans claimed to have seen what they believed was a genuine UFO.17 By November, 1973 the number had climbed to fifteen million (fully 11% of the adult population), and for the first time a majority of the American public believed that UFOs were real.1347

The literature in this field1790,1791 is extremely variable in quality, and opinions tend to be highly polarized with little rational debate.

  • Typical books written by "uncritical believers" include those by Leslie and Adamski,1787 Edwards,1639 Lorenzen,1672 Sanderson,632 Keyhoe1623 and Holzer.1858
  • (In 1974 one "ufologist," Ralph Blum, confidently predicted that "by 1975 the government will release definite proof that extraterrestrials are watching us."1347)
  • Slightly less credulous, perhaps, are Vallee,787,1189,1673 Cohen,331 Hynek,341,597 Saunders and Harkins,1789 McCampbell,1778 and Emmeneker,1640 who present facts somewhat more cautiously while maintaining their devout belief in the mysterious.
  • Finally we have the debunking books written by the "hardened skeptics," such as Menzel,1788 McCrosky and Broeschenstein,1792 Condon,17 Klass,695 and Story.1870
3.2.1 Why Believe in UFOs?

UFOs — the "new mythology"

Why is support for the Extraterrestrial Hypothesis (that UFOs are craft piloted by aliens) so widespread today? Part of the explanation must be the renewed interest in the subject of life on other planets. Ufologist B. L. Trench listed nearly 20 worldwide UFO investigative organizations;596 his favorite — Contact — had branch offices in 27 countries in 1971. And the television-viewing public eats it up. When the series "The Invaders"* was brought out about a decade ago, the American Broadcasting Company sold the show to fifteen foreign networks as well.695

But there is much more to the phenomenon than the current fascination with xenological topics. Man has always had religion, it is said, both to preserve moral values and to impart a measure of predictability and uniformity to the environment. In a world where morality seems as fluid as the winds and where total annihilation may be only 15 minutes away, traditional religions have been unable to supply the answers to many hard questions. It is this uneasiness about the future that has given rise to what Ronald Story appropriately labels the "new mythology".1870

The ancient-astronautists spawned by Erich von Daniken’s writings, and the contactee cults such as the Aetherius Society,1870 The Two,1921 and Gabriel Green’s Amalgamated Flying Saucer Clubs of America333 are extreme examples of a belief pattern suffusing our entire culture. Many people have begun to view extraterrestrial visitors not merely as friendly, but as technological angels who will guide us successfully through the uncertain years ahead.1347

Just as the biblical angels were the mythical beings proper to the age of early Christianity, UFOs and their benevolent alien occupants are the mythical beings proper to the Space Age.615 The famous psychiatrist Dr. Carl Gustav Jung did not find it at all surprising that scientific instead of religious imagery would be used by many to assimilate the accelerated pace of modern civilization.1920 Flying saucers serve as a partial substitute for God.346

Status Inconsistency Theory

A related idea is the Status Inconsistency Theory of UFO sightings proposed by Donald I. Warren, a University of Michigan sociologist.336 In this theory the belief in flying saucers is linked to the degree to which a person feels alienated from society. Persons who perceive their social status (as measured by, say, income) as different from their abilities or true worth (e.g., education, ethnicity) have been found to be more likely to report UFOs than those who do not have this internal conflict. Such inconsistency forces the individual to withdraw from society to a certain extent, and the resulting void is often filled by a belief in extraterrestrial benefactors. Another modern dilemma is the virtually universal distrust of governments and politicians, and a nostalgic yearning for the great leaders of the past. There is much evidence that the known propensity of the authorities to classify and conceal has done little to reassure the public that no pertinent information on flying saucers is being withheld from them.18,694,1347 For example, a poll taken in 1971 by the engineering periodical Industrial Research showed that 76% of the respondents believed the government was hiding some of the UFO facts. Since paranoia is self-reinforcing, the conviction that aliens are commuting to Earth has not been dampened by official proclamations to the contrary.

Finally there is the problem of boredom in daily life. With more leisure time on our hands than ever before, we seek amusement and fun. Certainly the discovery of beings from another world would be both an amusing and exciting distraction from routine. As Story points out tongue-in-cheek:  "Who knows? They might even let us ride in one of their spaceships!"1870

Carl Sagan believes that flying saucers are a kind of psychological projective test — a "cosmic Rorschach" — by which humans project their hopes, frailties and self-perceptions onto alien beings.15 As he says, "the idea of extraterrestrial visitation resonates with the spirit of the times in which we live."18

* The theme was that aliens are infiltrating our government as a prelude to conquest of Earth.

3.2.2 The Evidence for UFOs
Criteria for authenticity

While it is certainly true that hidden xenoarchaeological treasures may lie veiled forever in ancient legend and folklore, the observational data for flying saucers are frequently completely worthless. As British writer Maxwell Cade notes, "there is clear evidence of much fraud, more hysteria, and still more wishful thinking."45 Such bitter experience has taught us that when we have an emotional vested interest in a particular result and expectations run feverishly high, we must demand only the most scrupulous honesty from ourselves and refuse to accept any but the most rigorous, compelling evidence.20,562

Most researchers would be delighted to find extraterrestrial life be cause it would be such a momentous discovery. Perhaps the strictest rule of evidence in xenology is that all conclusions must be compelled by the facts. There must remain no rational alternative explanations.

Authenticity of UFO Reports

L. Sprague de Camp has set forth the following criteria by which to judge the authenticity of UFO reports:

  • 1. The report must be first-hand;
  • 2. The teller must show no obvious bias or prejudice;
  • 3. The teller must be a trained observer;
  • 4. The data must be adequate and available for checking;
  • 5. The teller must be clearly identified.1922

A case which satisfies these requirements, and which can perhaps be checked independently with a large number of witnesses, would be considered reliable by the majority of the scientific community.

But in addition to being reliable, UFO reports must also be exotic. An exotic case is one which is inexplicable in terms of common phenomena; for example, a strange moving light in the sky could be an aerial refueling operation, a satellite passing overhead, a police helicopter with a searchlight, etc.15

Sagan maintains that to date there are no reliable cases which are exotic, and no exotic ones which are reliable.18

Can flying saucers exist? Sagan himself has presented an interesting paradox which apparently rules out the possibility of ufonaut exploration of Earth. If there are many advanced civilizations in our galaxy then there is probably nothing terribly unusual about what is going on here. Hence, there is no urgent reason for aliens to go to the enormous expense of visiting us. On the other hand, if there are few technical cultures around, there won't be enough of them advanced enough to send visitors!15

UFO's size and shape is nonstandardized

Skeptics often cite the fact that observed UFOs are totally nonstandardized in size and shape — repeat visits by the same craft are rare. In general we would expect such standardization from experienced aliens, since the retention of a single configuration over a long period of time is possible only when its design has matured. Given a specific mission and a specific level of technology, an optimal definitive form can usually be found. So how can we explain the fact that UFOs appear to be shaped not only as cigars and disks, but also cubes, spheres, doughnuts, insect shapes, etc.?

Other arguments purporting to dispute the legitimacy of UFOs have been submitted by Friedman,694 Sagan,20,1317 Abell,1908 and Chiu.1311 The logic proceeds as follows:  Using the acceptable estimates that there are a million communicative extraterrestrial civilizations (in our galaxy of 200 billion stars) each having a lifetime of ten million years, then if each culture dispatches one exploratory starship per year, Earth — by random chance — should be visited only about once every 100,000 years. Of course, if the ETs discovered something interesting happening on our planet they’d come more often to keep closer tabs on us. What is not clear is whether humans are of such inordinate interest as to justify the large investment of alien time and resources that ufologists claim is being made.

Alien's shoes

Planning a first contact

When dealing with ufology the careful reader will always bear one additional question in mind:  If we put ourselves in the aliens’ shoes, what is the most rational way to go about planning a successful first contact effort with a planet like Earth?  Although this xenological problem is explored in greater depth later on, a few issues can profitably be raised now:

  1. Disturbance of the system — Since they are the ones with high technology, they will not fear us.1208 Thus the greatest danger to the enterprise is that of observer influence (a common problem in measurement science).77{?} If the mere act of observation will disrupt or destroy the system under observation, it behooves the observer to minimize that disruption. As Richard J. Rosa of Avco Everett Laboratories puts it: "A hundred years {may be} of little consequence to them. The fact that Columbus did not hesitate to talk to the Indians was not without consequences that were unfortunate for Europe and tragic for the Indians. Perhaps our interstellar visitors have learned to be more cautious — and considerate."344
  2. Minimizing the disruption — An advanced society can certainly make a planetary survey without the primitive indigenes knowing about it.377 As added security in maintaining anonymity, aliens and their artifacts could sport many clever disguises.49 Ufologist Jacques Vallee notes: "To make ultimate detection impossible, {the aliens} would have to project an image just beyond the belief structure of the target society."1189 In fact, it is rather difficult to explain why, if they wish to avoid contact, the UFOs allow themselves to be seen at all.747
  3. Standard first contact procedures — Spacefaring ETs will undoubtedly be experienced at the business of contacting other cultures. The following has been suggested by anthropologists familiar with the problem:

Exploration will proceed in a series of ordered steps. At each star the team will investigate the system and locate any planets. If they find a planet they will evaluate its habitability, physical resources and life forms. If any signs of intelligent life are discovered the survey team will have to decide whether to withdraw or attempt contact. This will involve careful observation from a distance to acquire information before actually making contact. At first, in order to gain language skills and social understanding, contact will be limited to individuals in small groups. In this way we can increase the chance of success at the official, formal meeting with ET leaders to arrange recognition and continued contact."615

Most rational observers would agree that the vast majority of sightings are the result of misidentification of familiar objects viewed under unusual conditions of lighting, weather, and so forth.18 There are also a multitude of outright hoaxes and exaggerations on record. For instance, in what Time magazine called the "gullibility experiment," three Cal Tech undergrads launched helium-filled polyethylene balloons from which were suspended metal rods with vanes and lighted railroad flares. Throughout the Los Angeles area reports came in of red, orange and green lights in the sky that moved at "fantastic speed."335 Similar deliberate hoaxes were arranged two years later near Castle Rock, Colorado.1312

Not all sightings of flying saucers can be summarily dismissed as hoax, weather balloon, or ball lightning.337,339,345 Dr. David Saunders at the University of Colorado has collected more than 70,000 unidentifieds and has placed them in a computer indexing and retrieval system.1789 There even seems to be enough data for meaningful statistics to begin to appear.

Trends in UFO sightings

According to Poher and Vallee, both computer specialists,
several trends and conclusions have already emerged:

  • The frequency of UFO reports increases with increased atmospheric visibility (which would not be the case if they were hoaxes);
  • the number of sightings is a bell-shaped distribution as a function of the logarithm of sighting duration;
  • the number of reports increases for objects farther away from the observer;
  • the data show a peak a few hours before midnight, and a smaller secondary peak a few hours after midnight.787
80 million cameras

Figure 3.3 UFO photographed

by the author on March 30, 1976

UFO photographed by the author using Pentax Spotamatic
and Kodak ASA-125 Plus-X Panchromatic.
The object appears to fly off to upper right in the frame.
See text for details.

What kind of information would be needed to really verify a UFO sighting?

  • Eyewitness accounts are unreliable, heavily dependent upon the observer’s education, health, emotional state, and predisposition to falsehood.
  • The kind of evidence that would be really compelling must be primarily physical.
  • Photographs, for instance, are generally regarded as hard evidence by scientists.
  • But pictures showing aliens, lights in the sky, or actual UFOs in flight are extremely easy to fake, as illustrated by the shots in Figure 3.3. (The author made a double exposure of a street light with a telephoto lens.)
No photographic evidence

After fifteen years of looking into the UFO phenomenon, NICAP (National Investigations Committee on Aerial Phenomena) director Stuart Nixon reported in November, 1972, the following conclusion regarding the literally thousands of photos he had received:  "NICAP has never analyzed a structured object picture that is fully consistent with the claim that an extraordinary flying device was photographed. In every case, there has been some small detail, or group of details, that raised the suspicion of a hoax or mistake."695

As Philip Klass aptly notes, there are more than 80 million cameras in the United States alone shooting roughly 5 billion still photos every year.695 Deft cameramen have managed to capture on film such rare occurrences as meteor falls, tornadoes, and plane crashes. And yet there is not a single photograph of UFOs or their occupants which can stand the strictest scrutiny and compel our acceptance of its authenticity.*

What about astronomical photography?  Each night hundreds of telescopes turn skyward to record events occurring in the heavens. Thornton Page, Chairman of the AAAS Special Committee on UFOs, states that "professional telescopes are not an efficient patrol net for extraterrestrial visitors" because they don’t see enough of the sky often enough.340 However, the Smithsonian Prairie Meteorite Network has sixteen wide-angle Schmidt telescopes covering an estimated 440,000 square miles of the Earth’s surface. Canadian and Czech meteor networks add a small additional area to the coverage.

According to Page’s calculations, assuming a 50/50 chance of a UFO being photographed by one of the networks and given that none have so far been detected, there can be no more than 690 luminous UFOs worldwide per year. If we just look at the United States, the upper limit becomes only 25 objects per year. (That is, if more than 25 UFOs were tracking across our skies each year, then the chances would be better than 50/50 that at least one of them would be spotted by a network telescope and recorded on film.)  In conclusion, observational astronomy can neither convincingly rule out nor compellingly affirm the existence of UFOs.

How about radar sightings?  Although it is true that many UFOs have been detected on radar screens in the last few decades, a radar return need not always correspond to a real physical object. For instance, it was discovered early in World War II that meteor trails could cause radar echoes.49 Birds and swarms of insects produced baffling returns until the true cause was ascertained.1773 Temperature inversions, so familiar to the inhabitants of the smog-filled Los Angeles basin, can cause radar beams to bend along a "duct," thus permitting the detection of objects much farther away than normal.1788 Radar signals can bounce off clear air turbulence or reflect back from patches of air whose temperature, humidity, or ionization differ from their surroundings.18

NORAD and Early Warning System

Naturally, the Early Warning BMEWS network would be ideal for picking up UFOs. In 1966 it was reported that more than 700 "uncorrelated targets" were being detected monthly.1189 Unfortunately for ufologists, the BMEWS, SAGE, and NORAD computers automatically discard any object that appears not to be following a ballistic trajectory or an Earth-orbital ellipse.18,597 And some really good cases of combined visual and radar tracking are probably being withheld for security reasons (e.g., "spoofing" tests, etc.). But Sagan and Page have pointed out that even a combination visual and radar sighting might not indicate a solid body — it could be an aurora, for instance, or mistaken identification.18 We see that it is only through the concatenation of many independent sources of confirmation that the authenticity of a UFO report can be compellingly demonstrated.

* The two best-known motion picture films of UFOs in flight, a total of 1425 frames shot in Utah and Montana in the early 1950s,1923 are highly questionable.695

Figure 3.4 UFO Related Objects

Fig. 3.4 UFO-Related Objects/Phenomena17,18

Meteorological – subsun, sundogs (parhelia), moondogs (paraselene), lenticular clouds, noctilucent clouds, mirages, "dust devils", St. Elmo’s fire, grindstone clouds, solar reflections on low-hanging clouds, lightning (ball, streak, chain, sheet), Brocken ghosts, green fireballs (around NaCl crystals or dust), swamp gas flickers (ignis fatuus, methane combustion,"will-o-the-wisp"), large flattened gliding hailstones, sun glint off shiny objects, rainbow-related phenomena, bolides, ducted ground light reflection, ice flakes, coronal effects, tornado lightning, volcano lightning, Earthquake-Associated Sky Luminescence (EASL), AgI used in cloud-seeding, pile d’assiettes clouds (stack of coins), ice halo, pilot’s halo

Astronomical – meteors, fireballs, satellite reentries, auroras, planets (Venus, Mars, Jupiter, Saturn), stars (Capella, Sirius), objects seen through haze/jet trails or magnified by temperature inversion, Moon, sunspots and solar flares, comets

Experimental and Technological – balloons sandwiched between dense air layers, test aircraft, unconventional aircraft, helicopters with bright lights, high-altitude projectiles, rocket launches, contrails, aircraft reflection or after burners, bomb tests, refueling operations, searchlight reflections, military flares, satellites, blimps, parachutes, radiosondes and pibals, landing lights

Physiological and Psychological – autokinesis (perceived motion of stationary objects), autostasis (perceived stopping of moving objects), "airship effect" (perceived connection of separate sources), "excitedness effect", hallucination and mass hysteria, afterimages, autosuggestion (seeing what one is looking for), entopic effects (retinal or vitreous humor defects within the eyeball), motes on the cornea (perceived as spots), astigmatism and myopia, failure to wear glasses, reflections from glasses, religious invention

Photographic – development defects, internal camera reflections lens flare, deliberate fakes (moon, street lamps, garbage can lids, phonograph records, hubcaps, lens cap suspended by thread, straw hat, Frisbee, models, window glass reflections)

Radar – temperature inversions and ducting effects, ionized gases in upper atmosphere, angels, bogies, phantoms, false returns (ice-laden clouds, birds, insects), "window" (long strips of chaff), ranging/calibration balls, hot-air bubble reflections

Biological – airborne debris (leaves, feathers, milkweed seeds), "angel hair" (gossamer spider parachutes), birds or flocks of birds, insect swarms, luminous fungi on birds, fireflies, glowing owl eyes, seagulls, moths, tumbleweeds

Industrial – detergent foam, soap bubbles, refuse from defective filter in chemical-industrial plant (milk, rayon), smoke plumes

Miscellaneous – kites, firefly trapped between window panes, radio astronomy dish, plastic bag with candles or flares, searchlight and headlight reflections off clouds, flashing ambulance light, tossed lighted cigarette, fireworks displays, reflection off building’s windows, airborne loose paper, beacon lights and lighthouses, water tanks, lightning rods, TV antennas, weathervanes, hoaxes

More exotic physical evidence

Despite prosaic explanations (Figure 3.4, Table 3.1), occasionally more exotic physical evidence will turn up. Coral Lorenzen described a detailed chemical and spectroscopic analysis of the alleged remains of a UFO which exploded off the coast of Brazil, near Ubatuba, in 1957.1672 The metal fragments were touted as magnesium metal purer than any manufacturer could have produced at the time of the catastrophe.

The case was investigated by the Air Force-sponsored Condon Committee study group in Colorado, authors of the 1000-plus page report on UFOs that came out in 1969.17 It turned out that magnesium of suitable purity had been produced, though only in relatively small batches, by one American company several years prior to the event at Ubatuba. In no case to date has any piece of an alleged alien spacecraft shown signs of other than terrestrial manufacture.1312

table-03-1 ufo sightings and uidentifieds

Other physical evidence

Other physical evidence (largely unconvincing) was also examined during the two year study at the University of Colorado, including stalled automobile engines, evidence of strong magnetic field fluences,* circular burn marks and "landing pad" depressions on the ground, broken tree limbs, and so forth — all to no avail.

Close encounters

A growing number of UFO reports in recent times involve observation of the alien occupants themselves. For example, the following articles appeared in the British Flying Saucer Review,  perhaps the oldest and most respected ufology journal in the world:

  • "Violent Humanoid Encountered in Bolivia" (1970 case — includes photograph of parked UFO and humanoid posing nearby);775
  • "The Humanoid at Kinnula" (1971 case — close encounter with genuine "little green man");777
  • "The Extraordinary Case of Rejuvenation" (1973 case — advanced medical knowledge imparted telepathically by humanoid aliens with "round ears and slit eyes," standing roughly 1.8 meters tall);780
  • "Remarkable Encounter at Draguinan" (1974 case — a group of French UFO enthusiasts are accosted by three silvery humanoids more than two meters tall);785
  • "UFO Landing and Repair by Crew" (1974 case — light-skinned, eight-foot-tall humanoids garbed in "wetsuits" are observed giving their flying saucer a tune-up in the forest, using wrenches and screwdrivers).782
Contactees and abductees

Perhaps the first alleged contactee in modern times was the medium Helen Smith. Her travels in space were published in 1900 along with a dictionary for translating Martian into French.1924 More recently, everything from sexual seduction of humans by aliens1623 to miraculous cures of myopia and rheumatism1347 has been attributed to direct contact with UFOs and their occupants. One of the more notorious contactees was the late George Adamski, who claimed to have shaken hands with visitors from Venus when they landed in the desert near his hamburger stand in the early 1950s.1193,1787 (Adamski has since been shown to be a fake by a member of the British UFO Society.289)

And then there are the persistent rumors that UFOs have crashed and their contents are being studied in secret by the government.814 One unconfirmed report states that the bodies of twelve tiny humanoids are being kept in cryogenic suspension in Hangar 17 at Wright Air Development Center near Dayton, Ohio. The alien corpses, and various parts of a flying saucer, are supposedly the remains of a UFO crash in the New Mexico desert in 1948.1672

Probably the most celebrated contactee case is that of Betty and Barney Hill, alleged to have been abducted aboard a spacecraft on September 19, 1961, and given a thorough medical examination by ETs.1795 (Betty was able to recall the incident via hypnotic investigation five years later, though Barney apparently could not.695) One of the few corroborative pieces of evidence is a star map which Betty had been shown by the alien pilot, and which she later reproduced from memory. Marjorie Fish, an Ohio schoolteacher, attempted to fit the map to the known positions of actual nearby suns in space. The fit she came up with contains fifteen Sol-like stars which all lie in a single geometric plane and center on what is presumably the extraterrestrials' home sun: Zeta Reticuli.351,1775

Carl Sagan and Steven Soter have disputed the authenticity of the Fish interpretation of the Hill map, but the case remains one of the most fascinating of its kind on record. Zeta Reticuli is a double star system, each sun believed to be suitable for the evolution of life as we know it and separated by a mere 0.05 light-years (only 1% of the distance to our nearest neighbor, Proxima Centauri). Unusually rapid technological advancement on the part of the sentient inhabitants of either of the Zeta Reticuli stellar systems might well result from the tantalizing closeness of the two stars. As the giant, luminous Moon beckoned to man throughout the centuries, perhaps the Zeta Reticulans too would find the challenge irresistible — only sooner.

* In case anyone is interested, I have in my files a circuit diagram for a most unusual piece of equipment — entitled "The Electronic UFO Detector".770

3.2.3 The UFO Game

Reward for proof

The National Enquirer is offering a reward of $50,000 to the first person to submit incontrovertible proof that UFOs are of extraterrestrial origin.1347 Entries have been submitted, but the prize has yet to be awarded.

Fighting fire with fire, Philip Klass in UFOs Explained declared he was so certain that UFOs are not piloted by aliens that he would personally refund the full price of his book to any purchaser if positive proof to the contrary ever comes to light.695 As an additional expression of confidence, Klass has extended a $10,000 bet to any and all takers that UFOs are not extraterrestrial spacecraft. The jackpot pays off if any one of the following events occurs: (l) Any crashed spacecraft or piece thereof is found that clearly has extraterrestrial design or construction, in the opinion of the U.S. National Academy of Sciences; or (2) the U.S. National Academy of Sciences reaches the same conclusion based on other pertinent evidence; or (3) "The first bona fide ET visitor, who was born on a celestial body other than the Earth, appears live before the General Assembly of the United Nations or on a national television program."695

Other explanations for UFOs

What are UFOs? Besides alien spaceships, these possibilities have been proposed: Time travelers from our future789,1189,1845 natural or artificial biological mechanisms,632 Satanic devils,562 and remote-controlled robots and androids.1623 Vallee claims that UFOs may be a purely "psychic" event akin to mass telekinesis,659 while astronomer-ufologist J. Allen Hynek warns us that "we may have to face the fact that the scientific framework, by its very internal logic, excludes certain classes of phenomena of which UFOs may be one … It should not surprise us if a phenomenon that is inaccessible to a scientific procedure appears irrational."597 However, while few serious ufologists would categorically assert that flying saucers are manifestations of extraterrestrial life, many consider it to be the leading hypothesis.1448

Project Starlight International

As for future research, Hynek has quietly organized the Center for UFO Studies in Northfield, Illinois. A toll-free hotline phone number has been distributed to law enforcement and other government agencies to make UFO reporting fast and convenient.1671 An independent UFO-watch station crammed with more than $20,000 worth of sophisticated electronic gear has been set up on a 400-acre site 20 miles northwest of Austin, Texas. The equipment at Project Starlight International (as the observatory is called) includes a 30-meter-diameter circle of sequenced spotlights and a low-power helium-neon red light laser to attract the saucer’s attention — should one be spotted nearby.1925

Although it is probably the opinion of the majority of physical scientists that no compelling evidence now exists for extraterrestrial UFOs, it would be unreasonable not to continue to pursue ufology with an open mind. Judgement cannot be passed until all the evidence is in.

3.3 The Resident Aliens
3.3 The Resident Aliens

larry niven
Indigenous aliens

The thrust of the chapter thus far has been the search for evidence of ETs on Earth in both ancient and contemporary mythology. But we must be careful not to overlook a possibly limitless source of alien intelligence indigenous to our own planet.

Until quite recently it was supposed that the basic mental capacities of thinking or reasoning — intelligence — served as a clear distinction between humans and other members of the animal world. Today we know we’re not so unique. It appears that virtually all living creatures possess at least the rudiments of intelligence; many elements of intellect appear in varying degrees across the phyla of the animal kingdom (especially the chordates and mollusks). Intelligence is therefore not a quality peculiar to humans or mammals alone but is developed and refined by all lifeforms.

Defining intelligence

The textbook definition of intelligence is:  "The capacity to utilize experience in adapting to new situations."  But what do we really mean by intelligent behavior?  Even a virus could be said to be "learning" when its DNA changes to adapt to new environments.

There are two approaches. The first is functional, keying on the important functions of intellect such as the capacity for self-awareness.

The second approach is structural:  What is the ultimate mental capacity of the neural network of a creature, viewed as a system?  The structural approach allows facility of comparison between various animal species, and the results are rather interesting. The analysis focuses on a single organ possessed by virtually every animal — the brain.

Brain size comparison

While it is widely recognized that high intelligence is the product of an elaborate brain,439,443,1000 a few qualifications are in order. First, within the normal range of variations of a species among its members, difference in brain size is unrelated to the intelligence of the individual animal.444 As much as 800 grams has separated human brains of apparently equal intelligence. And since organ proportions change during growth, only mature average organisms can be validly compared. Brain size is a valuable criterion only when we compare differences between adult members of different species of animal (Table 3.2).

Other factors

Brains of some social insects are relatively
larger than those of some vertebrates.

Further, size alone is not a sufficient determinant of the depth of intellect although it does fix the perimeters of mental complexity. Other factors such as neuronal density, complexity and design of brain tissue convolutions, size and efficiency of neurons, average number of intersynaptic connections and so forth are also important. Gross bulk, while a rough correlate of intelligence, is not a precise measure of it.565,2560

Use of language

It is difficult to say exactly where the threshold of human intelligence lies. It is known that human infants become facile with language only after their brain mass exceeds 800-1000 grams.217 Yet this is not a reliable cutoff point because, for instance, chimpanzees (brain weight 440 grams) raised in human company have acquired vocabularies of as many as 200 different word-symbols. Dogs, with smaller brains still, utilize a larger repertoire of signals than do many primates (but this may be because primates are vegetarian browsers while dogs are pack hunters requiring reliable intragroup communications1542). Conversely, the walrus (brain weight 1130 grams) is not known to have any symbolic language at all.

Brain size in relation to body mass

Besides absolute brain size, the relative size of the organ with respect to the rest of the body is also important. This ratio is representative of the investment made by the organism in intelligence as a survival mechanism. (It is known, for example, that even the brains of some social insects are relatively larger than those of some vertebrates.965)

Of course, these are only rough indicia of intellective capacity, good only for comparing the order of magnitude of a creature’s mental acuity. But it is a safe bet that, in general, a 1000 gram brain will be smarter than a 100 gram brain, and a brain which represents 10% of the total body weight will be more complex than one which only embodies 1% of the total.20,965

Were we to find on this planet other conscious minds with whom we might converse, it would be an excellent opportunity to practice our communication skills — before attempting first contact with ETs with whom we share no common biological heritage. We might also discover some problems the extraterrestrials may confront in trying to deal with us, and learn to anticipate the solutions.

Resident Aliens on Earth?

As can be seen in the last column of Table 3.2, the cetaceans (dolphins and whales) come closest to man in terms of both absolute and relative brain size.

  • Much has been written about the intelligence of cetaceans in popular fact15,1698,1699,1929 and fiction.1931
  • Their brains are highly convoluted and larger than human brains.
  • They are extremely social animals (aggregations of up to 100,000 individual saddle-backed dolphins have been observed roaming the open seas565).
  • Anecdotes of friendly and helpful attitudes towards men abound.
  • There are reports of porpoises saving persons from drowning, guiding ships through narrow, fog bound straits.
  • And even of performing psychological15 and psychophysiological217 tests on their human captors.
Conditions necessary for the elaboration of a civilized society

It is most difficult to measure dolphin intelligence and social abilities.1724
The famous undersea explorer Jacques Cousteau has pointed out four basic conditions necessary "for the elaboration of a civilized society."
These are:

  • Brain
  • Hand
  • Language
  • Longevity.1723

Porpoise and other cetaceans have brains nearly equal to our own, and possess lifespans of many decades. Whether or not they have a language remains to be proved. It is known that the humpback whale sings songs that often last more than 30 minutes and which are repeated with amazing accuracy.1931 Each season the songs are different.422 Dolphins, too, are capable of amazing mimicry of sounds and human speech. They could have a language of their own:  One anecdote tells of a porpoise held in captivity and later released which emitted a long, involved sequence of sounds in the presence of a school of dolphins it had encountered.15

Unfortunately, cetaceans do not have hands; any intelligence they may have cannot be worked out in technology. Sagan has hinted that the dolphins’ creative energies might have been diverted to social instead of material technology. Asks he: "Are whales and dolphins like human Homers before the invention of writing, telling of great deeds done in years gone by in the depths and far reaches of the sea?"15 Apparently a single whale song contains roughly the same number of bits of information as The Odyssey does!  Cetaceans may turn out to be "fluked philosophers … introverts who can think but not do."96

All this has motivated Arthur C. Clarke to proclaim:  "There seems little doubt that dolphins think and speak much more rapidly than we do … And yet after decades of dedicated research into human/dolphin communication no major breakthroughs have occurred. Either the animal is not as intelligent as we had hoped, or communication with alien minds is a far more demanding task than anticipated.

Of course, the very fact that we have a vested emotional interest in finding porpoises to be intelligent should raise a flag of caution to the xenologist. The cardinal rule of evidence in xenology is that evidence must be compelling to be convincing. And most zoologists would agree that at present no such evidence exists in favor of cetacean super-intellect.565,1723

Mimicking human behavoir

Hence, while the dolphin possesses a huge brain and an exceptional ability to mimic, this does not necessarily imply consciousness or even high intelligence. Elephants, whose brains are more than three times larger than those of cetaceans, are known with reasonable certainty to possess an intelligence far below human level.565 Mynah birds and parrots are capable of imitating human speech rather well. The much-heralded altruistic cooperative behavior of marine mammals in rescuing injured comrades is also observed in wild dogs, African elephants and baboons,565 and may even be instinctual as a result of environmental necessities. Sociobiologist E. O. Wilson claims that delphinid communication systems are no larger nor more complex that that of other mammals or birds.565 The common consensus among zoologists appears to be that the intelligence of the bottle-nosed dolphin can be ranked somewhere between the dog and the rhesus monkey.1724,1932

This should not be taken as conclusive that cetaceans are not extraordinarily intelligent; the simple fact is that we just don’t know yet one way or the other. Certainly no evidence exists that would rule out this possibility. But because of the great potential inherent in such a discovery, we owe it to ourselves both to continue delphinology research with vigor and to demand compelling evidence before accepting specific conclusions.


Zoo morphizing

As John Lilly has pointed out, there are two dangerous pitfalls to be studiously avoided during first contact. First is the danger of anthropomorphizing — of assuming that the alien creature possesses the same psychological constitution as humans. The second danger is what Lilly calls zoo morphizing, the mistake of denying the existence of high intellect in complex, large-brained creatures solely by inference from data on much smaller-brained animals.217 (Brian Aldiss addresses this very question in his science fiction satire The Dark Light Years.226)

Perhaps to truly comprehend the mind of the dolphin we shall have to learn to "live wetly."  We must be willing to climb down into a tank of water and live as the alien himself lives. Both Lilly201 and Brunner442 have suggested that this may be the only way for true interspecies understanding to occur. A kind of primal empathy must be established between the two communicators.

Despite the tremendous promise of cetacean intelligence research, hundreds of thousands of dolphins are ruthlessly slaughtered for food each year by the Japanese and Russians. Our own merchant fleets have been killing comparable numbers incidental to tuna fishing operations.*

During the 1800s whalers caught perhaps one animal per ship per month, but during the record catches of the last decade the average ship was hauling in a carcass every day.422,1928 The explosive harpoon used by whalers has caused intense pain and suffering:

All intelligent species shall have the right
not to serve as food for other races.

A 150 lb. weapon carrying an explosive head which bursts generally in the whale’s intestines, and the sight of one of these creatures pouring blood and gasping along on the surface, towing a 400-ton catching vessel by a heavy harpoon rope, is pitiful. So often an hour or more of torture is inflicted before the agony ends in death. I have experienced a case of five hours and nine harpoons needed to kill one mother blue whale.710

Although it is true that "the exploiters of the cetaceans are spoiling our relationships to them,"201 this is almost a trivial observation. There is a much larger lesson to be learned here.2036

Speciesism is a chauvinism so fundamental that its unabated continuance could wreck our relations with alien intelligences. As Peter Singer, a philosopher currently associated with La Trobe University in Australia, defines it: "… {Speciesism is} to discriminate against beings solely on account of their species, {an unethical practice} the same way that discrimination on the basis of race is immoral and indefensible."712

What's for dinner?

Most of us are devout speciesists. Each year in the United States we condone the slaughter of ten million pigs, thirty million cattle, and more than three billion poultry animals to adorn our dinner plates. Sixty million rabbits, rats, and other pain-feeling creatures are tortured annually in experiments frequently unnecessary or useless.

Singer explains the moral dilemma this way: The modern philosophy of "equality," strictly speaking, is false. There are no two humans who are strictly equal physically or mentally. The scope of equality (unless tied to self-interest) must therefore be determined by some objective criterion, some common characteristic capable of distinguishing those who are equal from those who are not. The problem is that any trait possessed by all humans will also be possessed by some nonhuman animals; if the conditions are tightened so as to eliminate these animals, some humans will then be eliminated. (Check, for instance, the criteria of pain-feeling, rational thought, memory, etc.)

Most distinctions that can be drawn between humans and other animals are not sharp and unmistakable. Zoologically, most attributes smoothly blend into a continuum among the many animal species. And yet whenever there is a clash of interests, even if it is a choice between the life of a nonhuman animal and a human palate, the interests of the nonhuman are disregarded.712 No amount of pain and suffering on the part of our fellow creatures seems too high a price to pay for the slightest whims of people.

Dangers of speciesism

This attitude is most unhealthy from the xenological point of view. If mere membership in the Homo sapiens club is sufficient to grant us ethical license to cruelly maim laboratory animals, why cannot superior, research-minded aliens pick out "mere humans" for similar honors? If we may brutally slash and torment bulls in bullfights, why might not ETs be able to similarly justify the staging of gladiatorial mortal combats between "human dumb animals"?  If we allow ourselves to eat the nonhumans who share this planet with us, what ethical barrier can stand in the way of highly-evolved, hungry aliens seeking to augment their menu with hairless primate meat?1949**  Speciesism is clearly one of our most dangerous chauvinisms.2115,2118,2136

When sentient lifeforms are found elsewhere in our galaxy, we’ll need all the help we can get from terrestrial interspecies communication research. Experience must be gained in empathizing with nonhuman bodies, minds, and environments. Such experience will give us the unique opportunity to view human culture through alien eyes, a necessary preliminary to our understanding of how extraterrestrial aliens may evaluate us. And communication with resident aliens would be a major step towards the goal of eliminating our speciesist biases.

As Carl Sagan poignantly observes:

It is not a question of whether we are emotionally prepared in the long run to confront a message from the stars. It is whether we can develop a sense that beings with quite different evolutionary histories, beings who may look far different from us, even "monstrous," may, nevertheless, be worthy of friendship and reverence, brotherhood and trust. We have far to go; while there is every sign that the human community is moving in this direction, the question is, are we moving fast enough?15

* The use of any marine mammal for food in the United States was outlawed by the Marine Mammal Protection Act of 1972. A white meat preparation known as mahi mahi (or "dolphinfish") is fish and not porpoise-flesh (which is full of hemoglobin and therefore dark red in color633) as some mistakenly believe.

** More than a decade ago, science fiction author Michael Kurland (and others) drew up a list of advantages in joining the Galactic Federation, to be presented to the United Nations should the appropriate occasion ever arise. At the top of the list was the following: All intelligent species shall have the right not to serve as food for other races.78

* Notes:
Table 3.2   

* Adapted from: Altman Dittmer,368 Spector,48 Lilly,217 Allen,309
 Portmann-Stingelin,960 and Buettner-Janusch.1927

** Normalized to 1.00 for man.


  • b is average species adult male brain weight, in grams.
  • (b/B) is fraction of body weight represented by brain.
  • The last column, b•(b/B), is the product of both these factors.
  • A large value indicates that the organism has both a high
    brain mass and a high brain-to-body weight ratio, which
    raises the presumption of higher intellective capacity.

table 3 2

Chapter 4 ♦ Xenology: The Context of the Universe
4.0 Xenology: The Context of the Universe

loren eiseley 360
The cosmic panorama

We now cast our eyes skyward to contemplate a still grander perspective than even human literature, folklore and ethics can afford — the boundless twinkling oceans of the star-dusted firmament. The one commutual aspect of existence we can be reasonably sure of is the physical universe, that breathtaking panoply of brilliant suns, blazing galaxies and luminous nebulae which human and alien astronomers alike must share.

There are countless reasons why the cosmic panorama per se is of xenological significance.

  • Ultimately, of course, the astronomical environment serves as the backdrop for all our speculations about life on other worlds.
  • If we are to successfully evaluate the ubiquity of biology in the universe, we must attempt to isolate those features which all lifeforms will find in common.
  • We must puzzle out whether humanity, life, and Earth are unique events or merely a footnote to a statistic in the Galactic Census.

Knowledge of the evolution and distribution of stars and galaxies will suggest the most profitable places to hunt for evidence of extraterrestrial civilizations. But our curiosity tugs at us more insistently.

  • Where will life be most abundant in the Milky Way Galaxy? In the central regions, the disk of the Galaxy, the spiral arms...?
  • What kinds of stars are most likely to harbor lifeforms and planetary systems?
  • How many other civilizations might there be, and what stage of development have they reached?
  • What are the general constraints on xenopolitical systems as regards size, complexity and distribution?
  • Are there any cosmological limits to high technology and galactic engineering?

We may also gain insight into the limits of alien philosophies of nature, the universe, and the very mechanism of creation itself.

  • How did the universe come to be the way it is?
  • Has it always existed? Will it ever die?
  • Are physical laws as we know them immutable, or do they vary in different parts of the cosmos or at different times?
  • Do other universes exist?
  • Is there any purpose to physical existence at all?

These fundamental questions have gnawed at the mind of man for millennia, and must also intrigue the sentients of other worlds.

The issues of xenology are intimately bound up with the features and properties of the physical universe.

4.1 The Universe
The Universe

john milton 339
Ten billion galaxies

On a dark, clear evening the human eye can distinguish several thousand distant suns, all of which lie in the Milky Way (our home galaxy). Floating freely in Earth orbit our senses would be assaulted by the light of nearly six times as many stars. The Palomar 200" optical telescope — now the second largest in the world — has the light-gathering power of a million human eye balls and extends our vision to several billion celestial objects in this galaxy alone. And about ten billion galaxies are observable with present-day astronomical equipment, the farthest (3C 123) lying eight billion light-years distant.1952

How big is the universe? An important clue was uncovered by the American astronomer Edwin Hubble back in the early 1920s, when he was measuring the atomic spectra emitted by various galaxies.

Hubble Effect and the expanding Universe

The farther away the object, he found, the more its spectral lines appeared to be displaced towards the lower frequencies of light. This curious phenomenon, which became known as the "redshift," was interpreted to be a kind of Doppler effect for photons.

  • Much the same as a receding siren seems to be putting out lower and lower pitched sounds as it passes by, so do galaxies seem to emit redder light as they travel away from us.
  • Since the most distant objects are seen to possess the greatest redshifts, the simplest explanation is that they are receding from Earth at velocities approaching the speed of light.
  • Nearby galaxies are moving at a far more leisurely pace. Conclusion: The universe is slowly expanding.

This is not to say that Earth has the extraordinary good fortune to lie at the exact geometric center of all creation, simply because most all astronomical objects appear to be heading away from us.

  • More correctly, our galaxy is like a spot of India ink on the surface of a spherical balloon.
  • We are surrounded by billions of similar spots. As the balloon inflates, every point on its surface moves away from all adjacent points.
  • From the chauvinistic viewpoint of each galaxy, all others will look like they’re flying away at various speeds depending on distance.
  • Each will view itself as the "center" of the universe! This idea that the cosmos will appear roughly the same from any position is called the Cosmological Principle.
The Hubble Constant

The latest measurements of galactic redshifts seem to indicate that the speed of recession increases about fifty-five kilometers per second for each megaparsec* of distance from Earth.1953 (This number is called the Hubble Constant.) Since the maximum velocity of recession which can be detected is the speed of light, then the outermost shell of the swelling universe should lie about eighteen billion light-years from Earth.

Of course, if the "balloon" deflates, points on its surface will rush together again. Using our value of the Hubble Constant, we can mathematically run time backwards and extrapolate to Time Zero — the creation event. The inverse of the Hubble Constant is in units of time and represents the age of the universe assuming a constant rate of expansion. This works out to a period of eighteen eons!**

But scientists believe that the expansion of the universe has not been constant; on the contrary, it has probably decreased with the passage of time because of gravity. Taking this into account, the true age of the universe will not be quite so large, and is now usually set at about sixteen billion years.1953,1983 Our estimate of the radius of the universe, likewise corrected, drops down, to sixteen billion light-years.

Any theory of cosmology that purports to explain the mechanics of the cosmos must, at the bare minimum, be able to account for Hubble’s redshift phenomenon. The one proposal which has been most successful in this regard is the Big Bang hypothesis.

* 1 megaparsec(Mpc) = 103 kiloparsecs(kpc) = 106 parsecs(pc) = 3.26 × 106 light-years(ly) = 3.07 × 1022 meters(m).

** An eon is one billion (109) years.

The Big Bang hypothesis

olaf stapledon 298

According to this leading view of cosmic evolution, the universe began as a highly compact fireball of pure energy and infinite density.

  • After perhaps a millionth of a second this density dropped off to nuclear values as the ylem or "cosmic egg" exploded outward.2062
  • The overall temperature may then still have exceeded 1013 K.1192
  • The stuff of the universe started to change from pure energy into matter, primarily neutrons.

When half an hour had passed most of the neutrons were gone, replaced by a mixture of 60% hydrogen ions and 40% helium ions (by mass), as well as a smattering of deuterium (heavy hydrogen).1813

Using this model, it can be calculated that at the one hour mark, the temperature was down to about 250 million degrees; after the passage of a quarter of a million years, it had fallen off to the present temperature at the surface of Sol. And 170 K was reached after 250 million years following the big bang.

This turned out to be a red-letter date in the evolution of the universe, because for the first time in history the density of matter became greater than the mass density of radiant energy.

  • Protons and electrons must have coalesced into de-ionized H, He, and D atoms, leaving no more than 0.1% still in the plasma state.1192
  • This signaled a dramatic change in the behavior of material. No longer was matter incessantly sloshed and stirred by the overpowering radiation field, which had kept it permanently osterized in a thin, gaseous form.
  • Once radiation refrained from dominating the scene, matter was free to gravitationally condense into relatively huge, massive aggregates — supergalaxies, galaxies, and stars.1813
Two variations


There are two variations of the Big Bang scenario upon which most discussion has focused. The first of these is known as the closed, or pulsating universe model.

  • According to this thesis the universe is a gravitationally "bound" system.
  • That is, some thirty eons or so from now the fragments from the original ylem explosion will cease their outward rushing and commence to fall back together again like the dots on the surface of a deflating balloon.
  • Cosmic evolution occurs in a series of alternate expansions and contractions.
  • At the very end, the Final Moment, everything is destroyed, the slate wiped clean in preparation for the beginning of the next eighty-billion-year cycle.

Besides giving rise to philosophical nihilism, this has interesting consequences for the development of life. During an expansion phase light is redshifted to the relatively harmless lower frequencies. However, during the contraction phase the intensity of dangerous high frequency radiation might become unbearable — due to a blueshift effect. If the pulsating model is correct, then we are lucky to be alive during the half of the cycle most likely to be hospitable to life. In the second half, the development and expansion of biology would be severely restricted. Are we, asks Carl Sagan, "trapped in a vast cycle of cosmic deaths and rebirths?"20

The second variant of the Big Bang theory is the open, or expanding universe model, which suggests that the cosmos will never stop enlarging and ultimately will disperse to infinity.

  • In this view, all matter reached the "escape velocity" of the universe at the time of the ylem explosion: The cosmic radius increases indefinitely.
  • This is in sharp contrast to the pulsating model, in which the radius oscillates between some maximum value and zero.

There is evidence to support the Big Bang theories. For instance, it will be recalled that the fireball cooled rather rapidly as it expanded. If this rate is extrapolated from the Bang to the present, sixteen eons later, the temperature should be down around a few degrees above absolute zero. This early prediction from evolutionary cosmology was verified in 1965 with the discovery of microwave radiation which fills the entire universe perfectly isotropically. The energy corresponds to a constant, uniform temperature of 2.7 K. This actual relic of the primeval ylem superexplosion strongly affirms the Big Bang theories, and appears to verify the Cosmological Principle mentioned earlier.

Open or closed

john holmes 330

How can we decide whether the universe is open or closed? It turns out that if the mean density of the cosmos is less than about 5 × 10-30 gm/cm3 (only about three atoms per cubic meter — intergalactic space is very nearly empty), then there is insufficient mass to hang onto the galaxies gravitationally. The universe would be open and must disperse to infinity.

Measurements of the actual density are difficult to make. However, based on the latest data, revised Hubble Constant, and such parameters as the observed density of galaxy-clusters locally and the abundance of deuterium in space,1959 astronomers have reached a tentative conclusion: The universe is open.1953

Another class of cosmological theories which has persisted for decades in various forms is the steady-state model, which suggests that the universe is not thinning out at all despite the apparent recession of galaxies. According to a typical model of this variety, neutrons appear suddenly out of nowhere in the interstellar void, roughly one particle per cubic meter every few eons or so. The local density is thus maintained at a constant level, the outflowing mass exactly balanced by the spontaneously generation of matter within the included volume.

Hoyle-Narlikar cosmology

Figure 4.1 The Hoyle-Narlikar cosmology

figure 4 1 500px

The most recent attempt to forge a more plausible steady-state model is the Hoyle-Narlikar cosmology (Figure 4.1).1956

This is based on the concept of a "mass field," which is such that the mass of a chunk of matter is dependent upon its spatial and temporal location in space-time.

  • The universe consists of a checkerboard pattern of normal and reversed mass fields.

While mass never becomes negative, its value does vary from zero at a boundary to some maximum value defined by the field.

  • Einstein’s relativistic cosmology is said to represent a special case,1955 valid near a boundary but not across it or very far away from it.
  • Astronomer Hoyle infers that we are close to such a boundary.
The Local Universe

Figure 4.2 The Local Universe

figure 4 2 500px

If mass has been steadily increasing since we crossed the border some sixteen billion years ago, then galaxies we passed along the way — which lie nearer the boundary — should have older, less massive atoms. If less massive atoms also have less massive electrons, these electrons should lie in larger atomic orbits and generally emit spectral lines of lower frequency. That is, the spectra should be redshifted. The observed redshift of galaxies is explained, not by their headlong flight, but because the electrons comprising their atoms are lighter in weight.1954

Hoyle also has an explanation for the 2.7 K background radiation. It is known that light is most efficiently scattered by particles of low mass. Hence, the boundary at Time Zero (where mass goes to zero) must completely scatter all radiation coming from previous cells. The background is just the smeared out starlight emitted by galaxies on the other side of the border. Hoyle calculates that such galaxies still exist prior to Time Zero as far back as 150 eons.1956

Many other unusual theories have been proposed from time to time, including:

  • The Klein-Alfvén matter-antimatter cosmology.1192
  • The universe-as-a-black-hole idea.1963 (and black holes as accretion nuclei for elliptical653 and spira1964 galaxies)
  • The Everett-Wheeler "splitting universe" scheme.1982,3683
  • And other multiple universe ("multiverse") theories.1512,1957,1958

The cosmological problem has not yet lent itself to a definitive solution. Perhaps it never will until we are able to ask ETs, situated elsewhere in distant space, for their observations and ideas.

4.2 Galaxies
The Local Supergalaxy

Figure 4.3 The Local Supergalaxy

figure 04 3 the local supergalaxy 500

If the Big Bang theories of the universe are essentially correct, then it was not long after Time Zero on the cosmic time scale that matter began to condense gravitationally. Any small nonuniformities in the density of the heretofore homogeneous gas would be aggravated, and local condensations could begin to occur.

Today we bear witness to what astronomers believe is the end product of that grand condensation process: Stars. These giant plasma balls glow by the energy of intense thermonuclear fusion reactions, at temperatures reaching many hundreds of millions of degrees in their cores. These incandescent globes are collected into great structures called galaxies, which exist in many shapes and sizes. It is now known that galaxy-clusters also exist, assemblages of a few to as many as thousands of individual galaxies. More than 80% of all nearby galaxies belong to such clusters.1974,2150 The spaces between them are virtually devoid of stars, gas, and other matter.

A few astronomers today are of the opinion that order exists in the cosmos on an even larger scale. They claim to have discovered monstrous aggregates of galaxy-clusters possessing literally millions of individual galaxies, with masses ranging from 1016-1017 Msun each.20,399,1191,1271,1974,1985,3676

More than twenty nearby "supergalaxies" have been tentatively identified, having diameters from thirty to ninety megaparsecs.1974,1985 However, less than a dozen can be identified in much detail within about 160 Mpc (~500,000,000 light-years).399 (For comparison, the radius of the entire universe in the Big Bang cosmologies is roughly 4900 Mpc.) The spaces between supergalaxies is incredibly empty, even more so than between galaxies and clusters of galaxies.

We are believed to be embedded in the Virgo Supercluster, otherwise known as the Local Supergalaxy (Figure 4.3). The Local Supergalaxy is a squat, roughly cylindrical collection of nearby galaxy-clusters with a total mass of perhaps 1015 Msun.2025 It is about forty megaparsecs in diameter and twenty megaparsecs thick.399 Its radius is thus about 0.8% that of the known universe, which is about 10-7 of the total "volume" of the cosmos.

The Supergalaxy rotates counterclockwise as viewed from the North Super-galactic Pole, about once every hundred billion years. Since the origin of the universe, it has yet to make so much as a quarter-turn!

The Local Group

Figure 4.4 The Local Group1974,2025

figure 4 4

We are situated in a galaxy-cluster, called the Local Group (Figure 4.4), some twelve megaparsecs from the center of the Supergalaxy near Virgo I.

We are rotating with the Supergalaxy at about 2.5 million kilometers per hour, roughly 0.2% the speed of light.

The drawing depicts the approximate extent of the Local Supergalaxy as it is presently understood, along with the thirty-one largest galaxy-clusters* in this hemisphere. It should be noted that our own Local Group is the smallest of these.

* Galaxy clusters seem to form "clouds" within the main corpus of supergalaxies. The Local Group is a member of the Local Cloud of clusters. The Cloud is tilted about 140° with respect to the Supergalactic Plane.1984 The Local Cloud also possesses rather large regions of high-velocity neutral hydrogen gas clouds. These intergalactic gas clouds (IGCs) mass about 108-109 Msun, and sport about 100-1000 atoms per cubic meter.1986 In the Local Cloud of galaxy clusters, there are about twenty IGCs per cubic megaparsec.1986

Local Group members

Table 4.1 The members of the Local Group1945,1974,2025

table 04 1 members of local group 500

Galaxy-clusters range from as little as fifty kiloparsecs in diameter (Stephan’s Quintet, four member galaxies) to more than eight megaparsecs (Coma cluster, several thousand members). Ours is a modest-sized cluster, with twenty-one member galaxies and a diameter of 800-900 kpc.1974,2025 The Local Group is somewhat flattened in shape, with most components (Table 4.1) in the southern hemisphere of our Milky Way Galaxy.* Eleven intergalactic tramp globular star clusters have also been spotted, the lone gypsy wanderers of the forbidding intergalactic void.1974

* The largest Local Group member — Galaxy Andromeda — has an apparent velocity towards Sol, believed due to our rotation about the center of the Milky Way.1337 Andromeda is also tilted about 150° to our angle of view.2025

Characteristics of galaxies

Table 4.2 Characteristics of galaxies1945,1974,2025

table 04 2 characteristics of galaxies 400

There are basically three kinds of galaxies: Irregulars, spirals, and ellipticals (Table 4.2). Irregulars are small, formless collections of stars, containing perhaps 109 Msun. These galaxies consist of about 10-50% neutral hydrogen gas and dust20 and have very few old reddish stars and very many young blue-white stars.1974 Much of the matter that could be utilized in the construction of stars hasn’t been used up yet.

Spiral galaxies have consumed far more of their hydrogen — only about 1% of the original amount remains, on the average. There are a fair number of both old and young stars. The typical spiral has three major components: The halo (a spheroidal volume of space with very old stars in highly elliptical orbits), the nuclear bulge or "core," and the galactic disk (which contains the spiral structure and most of the mass). Great dust lanes are usually very conspicuous throughout.20,1976

Elliptical galaxies are generally ellipsoidal in shape. Virtually all of the neutral hydrogen has been depleted, and there is little dust. Most of the building materials for stars are gone. Few suns have formed in very recent times; consequently, the stars tend to be very old.

The above taxonomy is not believed to be an evolutionary sequence, say, from youth to senility. Each of the three types of galaxy is thought to have originated in much different ways. For instance, if we start out with a very low mass protogalaxy, the hydrogen density will be low and stars cannot form very fast. An irregular galaxy is the result, such as the Large Magellanic Cloud in our own Local Group. If the mass is large but rotation is slow, then most of the hydrogen has a chance to condense into stars before the contraction causes angular momentum to rise prohibitively. The matter is consumed immediately, leaving none for later on. An elliptical galaxy is the end product of this process. Finally, if mass is high and rotation is fast, star formation will proceed with greater restraint. Stars may continue to form for many tens of billions of years. Such is the probable history of a spiral.20,1974

Abundance of galactic types

Current estimates of the abundance of galactic types run as follows: Spirals 60%, ellipticals 30%, irregulars 10%.1973,2150,2475 There are two subclasses of spirals, normal and barred. The arms of bar spirals attach to a thick girder of stars passing symmetrically across the center of the galaxy. (The Milky Way itself is believed by some to have a small football-shaped, bar-like structure at its center.1976) Normal spirals with spheroidal cores are twice as abundant as the barred variety. About a million large galaxies lie within a few hundred megaparsecs of Sol.20

About half of all galaxies are "dwarfs."1945 Dwarf ellipticals and irregulars exist; probably for dynamical reasons, there are no dwarf spiral galaxies.1945 Roughly 5% of all galaxies form physical pairs ("binary galaxies") or multiple systems, and at least 1% show some "marked visible peculiarity".1973

Heavy elements needed for life

Figure 4.5 Great Ursa Major Spiral:M 101 / NGC5457

Figure 4.5 Great Ursa Major Spiral: M 101 / NGC5457.
Nearby spiral galaxy, yet outside our Local Cluster.
(Mount Wilson and Palomar Observatories, Plate VIII from Broms1191)

Which galaxies are most likely to harbor intelligent life? One of the prerequisites for life as we know it is a planetary environment in which to flourish. Perhaps an atmosphere and oceans are also required, along with an abundance of various carbonaceous chemical substances. It appears fairly safe to conclude that "heavy elements" (carbon, oxygen, silicon, etc.) must be present if life is to arise. Primordial hydrogen and helium alone won’t do.

Scientists believe that heavies are generated as a natural product of stellar evolution. Normal thermonuclear processes in stars produce elements that run the gamut from lithium to iron, and stellar supernovae generate still heavier atoms (iron through uranium). A single, good-sized supernova explosion may inject as many as 103 Earth-masses of heavies into the interstellar medium.

Over a period of billions of years, the stuff from which stars are born has become more and more enriched with heavy elements. Ultimately, this has made possible both planets and the development of life. But where are these heavy atoms most abundant?

It is generally agreed that dwarf galaxies are extremely metal poor.1816,1818 Consequently, we may immediately eliminate about half of all galaxies from contention.

We also know that virtually all stars in elliptical galaxies were formed at least ten billion years ago, soon after the Big Bang.1974 Although there is some evidence that the heavy element deficiency is small or negligible compared to our Galaxy,1816 if the theories of stellar nucleogenesis are correct then elliptical galaxy stars appeared long before the interstellar medium was impregnated with heavies. So ellipticals probably contain fewer habitable worlds.

Spectroscopic data for irregular galaxies indicate a marked deficiency in heavy elements,20 as much as 30% less than in our Galaxy generally.1816 Irregulars are slow starters — the ambient gaseous medium probably has not been sufficiently enriched to produce as many planetary systems. Furthermore, the available mass in irregular galaxies tends to run a couple orders of magnitude less than that available for star-building in ellipticals and spirals.1945 We would therefore expect somewhat fewer sites for life than in our own Galaxy.

Biology favors spiral galaxies

It appears that the best place to look for biology is in the spiral galaxies (Figure 4.5),2032 a conclusion tentatively affirmed by our presence in one. This is indeed fortunate, since these comprise a majority of all giant galaxies.

4.3 The Milky Way Galaxy
The Milky Way

Figure 4.6 The Milky Way galaxy(schematic only)1945,1961,1976,1780,1816

Figure 4.6 Top View of the Milky Way galaxy

figure 4 6A 500px

Figure 4.6 Side View of the Milky Way galaxy

figure 4 6B

Our Galaxy is a rather typical spiral, consisting of three distinct regions (Halo, Core, and Disk) and four distinct components (stars, gas, dust and high energy particles) (Figure 4.6).

The Halo is a rather thin distribution of very old stars, spread out roughly spherically to a radius of twenty-five kiloparsecs or more from center. Probably about 5% of the entire mass of the Milky Way lies in the Halo1781 (~17% of all stars1816). The Core is several kiloparsecs in radius, and stellar densities rise to values millions of times higher than near Sol. This closely-packed nucleus of our galaxy contains perhaps 10% of all stars.57,1976 The main disk of stars is a bit more than fifteen kiloparsecs (50,000 ly) in radius and averages about one kiloparsec thick. Sol is located only ten parsecs above the Galactic Plane,20,57 and about ten kiloparsecs from the center.20,1945,1976

The gross mass of the Milky Way is about 1.5 × 1011 Msun (3 × 1041 kg), representing a total of perhaps 250 billion stars of various types (Figure 4.7). Its aggregate energy output is roughly 5 × 1037 watts, and it rotates once every 240 million years in the clockwise direction as viewed from the North Galactic Pole. The Milky Way has made some fifty revolutions since its initial condensation twelve billion years in the past, and Sol has traveled nearly twenty full circuits since the origin of the Solar System about 4.6 eons ago.

Figure 4.7 Composite picture of the Milky Way

(Lund Observatory), from Shapley 1878

figure 04 7 composite picture of the milky way

The stars to be found in each of the three regions of the Galaxy are of distinctly different character. The Halo "Population II" suns are very old, reddish stars with heavy element abundances hundreds of times less than in the vicinity of Sol and in the Disk generally.1945,2032 These stars have highly elliptical orbits around the Core, and appear to be a relic of an earlier evolutionary stage of the Milky Way. Both individual stars and giant spherical collections (called globular clusters) inhabit the Halo. Globulars usually have 105-106 old Population II stars, and run 20-100 parsecs in diameter.1556,1973

The Disk "Population I" comprise the bulk of the stars in our Galaxy. Sol and most of our stellar neighbors are members of this population, although there are certainly a few Halo stars kicking around in the Disk (only about 3-5% of all stars near Sol1816). Disk stars have nearly circular orbits about the Core, and are pretty well confined to a layer one kiloparsec from the Galactic Plane.1780

It is believed that the Core is also comprised of Disk Population I stars, but there are some peculiar differences. The Core suns tend to be very old, reddish objects much like the Halo population, and yet the abundance of heavy elements appears to be at least six or seven times higher than in the Disk, near Sol.1818

Interstellar gas

Like the stars themselves, interstellar gas is composed mainly of hydrogen (about 60% by mass) and helium (about 40% by mass). These gases, whether neutral or ionized, occur in discrete patches several parsecs wide in concentrations of more than ten atoms per cubic centimeter. A few exceptionally small, concentrated clouds exist with densities well above 1000 atoms/cm3 — as in the Orion Nebula and the Horseshoe Nebula.1816 In the Milky Way there is an estimated 6 × 107 Msun of ionized hydrogen and 1.4 × 109 Msun of neutral hydrogen, for an overall density of about 0.6 atoms/cm3.1945

Interstellar dust

Both gas and interstellar dust (dust mass ~106 Msun or less) lie flat in the Disk, confined to within two hundred parsecs of the Galactic Plane.1816 The presence of this dust (10-3-10-4 cm particles) obscures visibility along the Plane by absorption and scattering of light — which limits our view of the Galaxy in the optical spectrum to a few thousand parsecs along the line of sight.20,1972

But it is important to keep in mind how truly thin this dust is dispersed. While we find at least one atom of hydrogen in each cm3 of interstellar space, there is only one tiny dust flyspeck in a cube of space twenty kilometers on an edge.

New stars are constantly forming in these dust clouds, as well as larger grains of "dirty ice."1972 Blast waves from novae and supernovae, galactic winds and wakes,1151,1960 and density waves that may be responsible for the spiral arms all propagate in this tenuous "galactic atmosphere."

Spiral arms

Figure 4.8 The nature of the spiral arm

feature of the Milky Way Galaxy

We have until now neglected what is probably the most interesting feature of the Milky Way — the spiral arms. Contrary to common belief,607 they are not concentrated regions of stars. The brilliant arms of spiral galaxies have less than 5% more stars than interarm regions (which is where Sol is).57 The most visible among these few extra stars are the class O and class B suns, the gas-guzzling "cosmic Cadillacs" of the Galactic showroom. These showy white stars are very massive and very young, and they consume all their hydrogen fuel in a relatively brief time. The spiral arms are regions of much higher gas density, marking off the boundaries of the maternity wards of the Milky Way. All normal Disk stars, as well as O and B classes, are born there.

Were we to photograph the Disk so as to eliminate the 0.1% or so of ostentatious O and B suns, we would see an almost flat, featureless distribution of normal stars (Figure 4.8). It is only because of a very few bright stars that we have any visible spiral structure at all. Hence, the Galaxy really appears to be a solid dish of common yellow and red stars with a very light sprinkling of hot, white ones in a generally spiral pattern.

Since O and B stars have such short lifetimes, most of them die before their orbits can carry them very far from where they were born. A few do escape, however, and become mixed in with the rest of the stars. (Regulus, in the constellation Leo and about 26 parsecs from Sol, is one such escapee.) O and B suns are largely confined to within 70 parsecs of the Galactic Plane,1780 and have virtually perfect circular orbits around the Core in the Plane. These stars are sometimes referred to as "Extreme Population I."

There is one minor complication to the view of the Milky Way presented thus far. The concentration of hydrogen in the spiral arms is a stable feature of the Galaxy, and is thought to represent a wave of greatly increased gas density traveling across the Disk. Where density is highest, the hot O & B stars can be formed, trailing from what amounts to a galactic density-shockwave.

Distribution of life

Figure 4.9 Position and Orientation of Earth and Sol

in the Milky Way Galaxy

Position of Earth and Sol in the Milky Way Galaxy

image text sm
figure 4 9A 500px

Orientation of Earth and Sol in the Milky Way Galaxy

figure 4 9B 500px

We recall that Sol orbits the Core (Figure 4.9) approximately once every 240 million years. The problem is that the spiral density wave circles the Galaxy at a much slower rate, about once every 400 million years. Consequently, the bright spiral arm stars trail forward,* not backward, from the leading edge of the bow shock.1976

The distribution of life in the Milky Way is intimately connected with Galactic evolutionary history.1811 The Halo population is the oldest sub system, remnant of the first stars and star clusters formed from the original virgin hydrogen cloud — the protogalaxy — 12 eons ago. As the cloud gravitationally condensed and began to rotate faster, it flattened out and became more dense.1809 It has been estimated that about a hundred million years were required for the gas to fall ten kiloparsecs to the Galactic Plane.1827 During this time the Disk population was formed (after the Halo), which soon found itself rich in heavy elements.1807 The spiral arm population is the youngest subsystem of the Milky Way (105-109 years old), is also rich in heavies, and is closely restricted to the Plane.1945

* Sol's forward motion should carry us into the Orion Arm in 107 years or so.

Halo stars and globular clusters

Figure 4.10 Heavy element abundance and distribution

in the disk of the Milky Way Galaxy57,1972

Figure 4.10A Heavy element abundance in the disk of
the Milky Way Galaxy

figure 4 10A 500px

Figure 4.10B Heavy element distribution in the disk of
the Milky Way Galaxy

figure 4 10B 500px

The abundance of heavy elements increases markedly toward the center of our Galaxy (Figure 4.10). The concentration in the Core is an order of magnitude higher than near the rim of the Disk, and as much as three orders of magnitude greater than in the Halo. Based on the distribution of heavies, where might we expect to find planets and life?

It is difficult to avoid excluding the oldest Halo stars, orbiting high above the plane of the Galaxy.33 For the most part, these stars are extremely metal-poor. In addition, they are few in number, widely dispersed, and exceedingly dim because of their distance, Halo population II stars generally are not a good place to look for biology.312,1633

Globular clusters are conspicuous collections of hundreds of thousands of individual suns. There may be as many as 2000 such clusters in the Galaxy,1973 but at present only about 200 are known to exist for certain.1807,1945 Since the component stars are population II, they, like the lone Halo objects, are exceedingly poor in metals. This alone would be enough to rule out all but the slimmest chance of finding life,1633 but there are other problems. For instance, stars in these clusters are so tightly packed that encounters between them may become important inasmuch as the stability of planetary systems is concerned.352 Also, a large number of the stars have left the "main sequence" (see below) and have become red giants. This stage of their evolution is marked by large variations in luminosity and dramatic increases in stellar radius.20,1556,1973 It would appear that globular clusters are not fruitful places to search for intelligent lifeforms.

What about the Core?

Table 4.3. Star Densities at the Galactic Core1821

table 04 3 star densities at the galatic core 400

If not in the Halo, how about the Core? As discussed above, the central regions of the Galaxy are more metal-rich than anywhere else in the Milky Way. The potential exists, therefore, for a vast multitude of terrestrial-like planets and planetary systems. There are probably also large quantities of organic and inorganic molecules near the Core — just what’s needed to start the ball of life rolling.1816,1961

Objections to Core life

One quick objection to life at the Core might be that with such an immense concentration of stars in such a small volume (Table 4.3), the radiation flux might be too intense. However, simple order-of-magnitude calculations reveal that this is not a problem. Even in the innermost recesses of the nucleus, the total radiation received by a habitable planet will be no more than 0.06% in excess of that received from its primary. This should not be incompatible with an otherwise stable environment.

However, more serious objections to Core life may be raised. For example, we know that on the average about one supernova occurs every fifty years in a typical spiral galaxy.1962 In general, a hundred light-years is considered the distance of minimum biological effect for supernovae468,469,498 (Astrophysicists Krasovskii and Sagan have suggested that one nearby supernova event about 108 years ago may have contributed to the extinction of the dinosaurs.20) Since there are about 104 stars within 100 light-years of Sol, the mean time between catastrophic events is about two billion years, a comfortably lengthy period of time.20 On the other hand, there are more than two hundred million stars within 100 light-years of the center of the Galaxy. Assuming the same supernova rate, the mean time between damaging events would be reduced to 50,000 years. This may well prove intolerable to life.

Another argument against populating the Core is based on dynamical considerations. The grand game of stellar billiards, involving close encounters and collisions between stars once every million years or so,20 could make life in the central regions quite impossible. As Dr. R. H. Sanders and Dr. G. T. Wrixen of the National Radio Astronomy Observatory put it: "It is doubtful that there would be any life on planets in the galactic nucleus, since with such high stellar densities close encounters between stars would be so frequent that planets would be ripped out of their orbit every few hundred million years."1961 But this argument loses much of its appeal if we consider the outer Core regions (say, from 1-3 kiloparsecs out) where the star density is only an order of magnitude or so above Sol-normal.

Finally, there are indications that violent events are occurring at or near the Core. Astronomers Burbidge, Hoyle and Lequeux have hypothesized that the expanding 3-kpc arm observed near the Core could be the result of an explosion that savagely ripped through the central regions a mere twenty million years ago.1816 More recently, this theory has been refined to the following numbers: Titanic explosions may occur every 500 million years, releasing some 1053 joules of energy (equivalent to total conversion of half a million solar masses into pure energy), followed by an ejection of one billion solar masses of matter.1961 Needless to say, this would be an extremely disruptive event. [Note added in 2008: Many astrophysicists now believe there is a black hole at the center of the Milky Way Galaxy.]

The Core's aesthetic spacescape

It appears doubtful that life will have evolved in the nucleus of our Galaxy, although biology in the outer Core regions is entirely possible. Looking at it from an aesthetic point of view, the spacescape enjoyed by the in habitants must be fantastically beautiful. Hundreds of stars would appear brighter than Sirius, our brightest. Their most luminous suns would be an order of magnitude brighter than Venus is to us.1360 Starlight filtering through thick, patchy dust clouds near the nucleus would produce interesting optical effects. The evening sky at the Core would be as bright as moonlight on a clear night on Earth — total darkness might be unknown to these extra-terrestrials. But stars would still look like mere points of light. Only within one parsec of the center of the nucleus would supergiant stars appear as distinct globes to the naked human eye.

Where else can life exist in the Milky Way? Scientists believe that the most likely place for life will be in the Disk, both in the interarm and spiral arm regions. Heavy elements are plentiful there, and planets should be numerous.2032 Stars are far enough apart to preclude close encounters, and supernovae are few and far between. Finally, most of the stars in the Galaxy may be found in this region.

There is every indication that extraterrestrial life will be abundant throughout the volume of the Disk.

4.4 The Stars
Variety of stars

Figure 4.11 Stellar number density near Sol, and stellar

contraction time, as a function of stellar mass57,1808

Local Stellar Density vs. Mass57

figure 4 11A 500px

Stellar Contraction Time vs. Mass1808

figure 4 11B 500px

Stars come in many sizes, brightnesses, shapes and colors. In Orion we find the beautiful orange-red Betelgeuse keeping company with the brilliant bluish-white Rigel. In constellation Auriga lies the Sol-like familiar yellow star Capella. The brightest star in Libra, named Zubeneschamali, is naked-eye green in color and is best seen low in the midnight summer sky.1191

More than two-thirds of all stars form multiple systems — double stars, triple stars and more. With a telescope one can observe the gold and blue splendor of g Andromedae, the twin red and green suns of a Hercules, and the exquisite orange, yellow and blue of zCancri.49 The stars in eclipsing binaries are often extremely near to one another, so close that the tidal force pulls the smaller sun into an ellipsoidal shape. Gigantic beautiful whorls and ribbons of luminous matter flow from one to the other in complex patterns so faint they can only be witnessed visually by the local inhabitants of these systems. Even with our most powerful telescopes we cannot actually see these processes but must infer them from indirect evidence.20

Besides color and shape, stars differ markedly in their relative luminosity. This property varies among suns across more than eight orders of magnitude — as much as a hundred thousand times brighter, to more than a thousand times dimmer, than Sol.

Star groups

If the spectra of a large number of stars are compared, however, certain regularities immediately become apparent. All stars can be divided into relatively few groups whose spectra all look pretty much the same. These are the classes O, B, A, F, G, K, and M. (There are a few others — R, N, S — but these are of lesser importance.)*

We’ve already seen that the O and B stars are the hot, short-lived, young and massive suns of spiral arm fame. Classes A and F are less hot and have longer lifetimes. Sol is class G. But the majority of all stars fall into the two classes K and M. These are relatively feeble, undistinguished objects, yet they burn little fuel and live extremely long lives — more than ten thousand times longer than their O and B counterparts. Luminosity, then, is a rough index of both the rate of fuel consumption and the life span of a star.32

Numbers from zero to nine are used to further subdivide the spectral classes. For instance, a G0 sun is more luminous than a G5, which in turn is brighter than a G9 — the dimmest in the G class. The next-faintest star, of course, would be K0. M suns are the feeblest of all.

The brightest star on record is class O5, since objects from O0 to O4 have not been found. Stars with numbers between zero and four are often referred to as "early," while those with higher numbers are considered "late." Sol, technically a G2 sun, would thus be viewed as an "early spectral class G star."

Stellar mass, in contrast to luminosity, is restricted to within relatively narrow limits (Figure 4.11). Few stars have masses beyond an order of magnitude more or less than Sol’s. There is good reason for this.

* Traditional mneumonic: "Oh Be A Fine Girl, Kiss Me Right Now. Smack!" Suggested non-sexist mnemonic: "Out Beyond Andromeda, Fiery Gases Kindle Many Red New Stars."2111 The modern version doesn’t seem to be catching on.

Gravity vs. radiation pressure

A star in the process of formation is a battleground for two opposing forces which struggle constantly to gain the upper hand. Gravity, which tries to collapse the ball of gas into a small volume with high density, is counteracted by radiation pressure, which grows more intense as the star’s thermonuclear furnace kindles and catches. The protostar shrinks to the point where radiation and gravity exactly balance each other, and relative stability is achieved.

Below about 0.01 Msun the ball of gas just sits there, big and cold. Gravitational forces predominate. Internal pressures are just too low for nuclear fires to ignite. Dr. Hong-Yee Chiu at the NASA Institute for Space Studies calculates that stellar mass must be greater than about 0.02 Msun for fusion reactions to be initiated.1314 This prediction squares well with observations. The lightest stars known — of M9 class — are all at least 0.05 Msun or more. Jupiter, the gas giant planet and possible arrested protostar, masses only 0.001 Msun.

In the direction of higher mass, Chiu calculates that if the body exceeds about 30 Msun the radiation pressure must be so great it would literally blow the star apart. Indeed, the largest stars known mass very close to this value.1314

Hertzsprung-Russell diagram

Figure 4.12 Hertzsprung-Russell Diagram

figure 4 12 400

The Hertzsprung-Russell diagram (Figure 4.12) is a plot of luminosity as a function of stellar class. About 91% of all stars fall neatly onto a narrow strip running diagonally from top to bottom. This is known as the main sequence.

The main sequence is not an evolutionary track, and is perhaps best thought of as a "house" in which a star resides for most of its life. It is believed that the earliest stages of stellar evolution involve the condensation of a giant cloud of gas and dust many light-years in diameter and massing perhaps 1000 Msun.1945 As contraction proceeds, the material fragments into many smaller globules until only tiny pieces remain. These units contain a few Msun of matter and measure about a light-year across.

As the protostar shrinks its gravitational potential energy is converted to heat, and after millions of years the object has drawn itself together as a warm cloud about the diameter of our solar system (say, 40 AU). At this point, energy resources are shifted to ionizing instead of heating the gas. The protostar shrinks down to less than 1 AU in perhaps twenty years or so.1808

A star suddenly appears in the midst of the whirling gas. We see that the actual contraction phase is very short, lasting less than one percent of the sun’s total main sequence lifetime.57

These T Tauri stars are stellar newborns, and their luminosity fluctuates erratically with time.20 Another peculiar feature of such objects is the blowing off of prodigious quantities of matter. It has been estimated that the original protostar loses from 30-50% or more of its starting mass in this fashion.85,473,1945 Hydrogen burning begins as the T Tauri stage draws to a close, and the star enters the main sequence as a full adult.1808

Star characteristics

Table 4.4 Typical Characteristics of Stars and Stellar Types

table 4 4 600px

Naturally, not all stars of the same mass cease contraction at the same position on the H-R diagram. Those protostars which are deficient in heavy elements — such as might be the case in globular clusters — arrive at the main sequence at a considerably lower luminosity than most Disk stars. These are called the subdwarfs.20,1945

For most normal suns, however, the mass determines both the point of entry onto the main sequence and the length of time of residence there (Table 4.4). Large O and B stars enter high on the sequence, and remain only a few tens of millions of years; the bantamweight K and M suns enter near the bottom and stay for tens of eons. Luminosity on the main sequence increases only very slightly with the passage of time. Sol, for example, has grown only 20% hotter since it left the T Tauri stage five eons ago.20

Stars are evicted from the main sequence only when all or most of their hydrogen fuel in the car has been exhausted. With the sharp reduction in radiation pressure the core contracts. Hydrogen gas in the outermost shell begins to burn. Collapse of the core raises the temperature there, so that helium-, carbon-, and ultimately oxygen-burning become possible. The star thus separates into two rather distinct components — diffuse burning shell and dense, hot core.

In this "red giant" stage, the shell of hydrogen may be gradually driven outward leaving a brilliant white core behind. (Stars which have left the main sequence remain red giants for perhaps 1% of their total lifetimes.) This "white dwarf" soon finishes off the remainder of its fuel and all fusion reactions cease. A white dwarf slowly cools to become an invisible black dwarf. Life for Sol-sized stars ends as inauspiciously as it began — as cold, dark matter.

More massive suns have more spectacular deaths. Stars about 30% heavier than Sol go supernova, leaving behind a small, dense object called a neutron star — essentially a gigantic atomic nucleus, perhaps ten kilometers in diameter, spinning furiously in space.1214,1314 Densities run about 1014 times higher than that of lead. The pulsar in the Crab Nebula is one of many such objects observed by astronomers in the last decade or so.

Black holes

Suns with initial masses of 3 Msun or more also supernova, but instead of neutron stars these titanic explosions create spherical nuggets of gravitationally collapsed matter that have come to be known as black holes.* These holes in space represent such a high local mass density that light itself moves too slowly to achieve escape velocity at the surface. Observational astronomers think they’ve detected one "BH," probably a couple kilometers in diameter, located in the constellation Cygnus.1970

When a star leaves the main sequence, so much energy is released that any life present is probably destroyed. Consequently, as far as the search for extraterrestrial life is concerned, only main sequence stars need be considered as possible candidates for habitable extrasolar systems.328 T Tauri objects, giants and supergiants, white and black dwarfs all may be eliminated from consideration. Fortunately, this still leaves us with about three-quarters of all suns in the Galaxy as putative abodes for life.

Genesis time

We know that life required 4.6 eons to arrive at its present stage of development here on Earth. Even if a certain margin of variation is allowed to account for differing speeds of evolution on different planets, the first fossil records of marine invertebrates don’t appear until the opening of the Cambrian Period a mere 600 million years ago. It is plausible to conclude that at least three or four eons — the so-called "genesis time" — may be required on any planet for intelligent life to gain a foothold.214

If this is indeed the case, then life will be restricted to stars of class F5 and later.57,328 Suns of earlier classes remain on the main sequence for less than the critical genesis time of several billion years, rendering improbable the emergence of intelligence.

* The properties of black holes are fascinating, and many excellent reviews have been written, including those by Thorne,1965,1966,1967 Penrose,1968 Kaufmann,1971 Ruffini and Wheeler,1969 and Hawking.2021

Angular momentum

Figure 4.13 Stellar rotation vs. spectral class

(Main Sequence stars only)20,328

figure 4 13 500px

Another argument in favor of class F5 as the early cutoff point is based on measurements of stellar rotation among the various classes of stars. There appears to be a sharp break at F5 in the amount of angular momentum possessed by suns (Figure 4.13). This conspicuous phenomenon can reasonably be explained by invoking the presence of planets.1278

It is suspected that the birth of planetary systems is closely linked to the contraction and evolution of the primary. Approximately 98% of the angular momentum of our solar system is carried by the planets — which represent only 0.2% of the total mass!

The hotter, fast-rotating stars are thought to be devoid of planets because they still retain the high initial rotation rate caused by the condensation of the original protostar. Cooler stars, later than F5, appear to have lost this great rotation somehow. One reasonable interpretation is that, like Sol in our system, these stars invested most of their angular momentum in their planets during the process of solar system formation.328


Figure 4.14 Stellar ecospheres (habitable zones)

figure 4 14 400

What is the smallest star that can harbor life? To answer this question we must briefly consider the concept of habitable zones or stellar ecospheres (Figure 4.14). An ecosphere is that region of space surrounding a sun where the radiation is neither too strong nor too feeble to support life. Too close to a star and a planet will fry; too far away, and it will freeze. The habitable zone lies between these two extremes.

Dr. Stephen H. Dole of the Rand Corporation has defined the limits of ecospheres so as to ensure that at least 10% of the surface of a world remains habitable all the time.214 Dole estimates that to accomplish this the radiation from the primary must be within 35% of Earth-normal. (This may be too pessimistic57,600 or too optimistic1907,2031 to suit some, but it’s a good first guess.) Of course, the size of the ecosphere will vary from star to star, the less massive dim suns having much smaller zones of habitability than the more massive, brighter ones (Table 4.5). And planets must huddle closer to cooler stars to keep warm; the ecospheres of F stars will lie at considerably greater distances than the zones surrounding, say, class K suns.

Another argument frequently advanced is that since K and M stars have relatively close ecospheres, planets within these habitable zones will become partially or totally tidally locked to their primary. That is, such planets would rotate extremely slowly; worse, they might become one-face worlds, always presenting only one side to the sun for heat. This could result in the atmosphere freezing out on the cold side57,214,1908 or other environmental severities.20

Stars massing less than 0.7 Msun may have ecospheres so narrow and close as to possess no havens from such rotational arrest.214 This corresponds roughly to stellar class K3. On the other hand, K2 and earlier stars should have at least a small region within their habitable zones in which tidal braking is much less severe.

Ultraviolet radiation

Table 4.5 General Planetary Orbital Parameters

for Habitable Zone vs. Stellar Mass

table 4 5 500px

Dr. S.I. Rasool at NASA has also suggested that the atmospheric evolution of planets may be critically dependent on the amount of ultraviolet radiation emitted by the primary.376 A deficiency in the UV could mean that the hydrogen and helium in the primeval solar system might not have a chance to dissipate from even the innermost planets, which would remain large, gaseous, and quite jovian. (Also, it is believed by some that M suns may be "flare stars," which emit sudden blasts of deadly UV at random intervals.57,1775)

But there are more serious complications involved in the ultraviolet problem. The steady-state intensity of UV radiation at the surface of the primitive Earth was at least an order of magnitude greater than the next most abundant source of energy.1017 An ultraviolet deficit might greatly slow or even preclude the origin of life and early biochemical evolution.

It would appear that class K stars radiate at least an order of magnitude less UV than class G, although this has been disputed by some.57,1775 Class M suns are even more niggardly, emitting less than 1% as much UV as Sol at equivalent locations within their ecospheres. The evidence, while far from conclusive, seems to rule out stars later than early K as possible abodes for life.214,1018

Multiple star systems

Figure 4.15 Planetary surface temperature

inside habitable zones

figure 4 15 400

As a first approximation, then, we choose to limit ourselves to population I stars in classes F5 through K2 on the main sequence — perhaps 11% of all Milky Way suns (Figure 4.15).

There is one further restriction on our selection of life-supporting stellar environments.2148 About one-third of all stars occur in pairs (binary stars), and some two-thirds occur in multiples of all kinds (binaries, trinaries, hexastellar systems, etc.).20 There should be less chance of finding habitable worlds in multiple star systems because of the relatively large variations in planetary surface temperatures (due to the peculiar convoluted orbit traced by a planet circling many suns).50,1020,1053 The danger of "slingshot" ejection must also be reckoned with.

Calculations reveal that if the components of a binary star system follow relatively circular orbits and are either very close together or very far apart, stable orbits and moderate planetary temperatures are possible.214* Dr. Su-Shu Huang, formerly a physicist at NASA’s Goddard Space Flight Center in Washington, made a preliminary determination of habitable orbital configuarations near binaries whose components are roughly equivalent in mass.1020

If good planetary orbits are to exist, the two stars must lie either less than 0.4L½ AU apart or more than 13L½ AU apart, where L is solar luminosity in Solar units, Lsun.

Of course, if either component of a binary system is class F4 or earlier, then both are unlikely to have been around sufficiently long for intelligent life to have arisen (though planets and simple lifeforms are not precluded). We also must reject population II binaries, as well as those which have a red giant, white dwarf, neutron star or black hole as one member of the pair.1018

Dr. T.A. Heppenheimer at the Center for Space Science in California has completed some simple calculations on the formation of planets in binary systems.1300 His preliminary results indicate that, taking into account the typically large orbital eccentricity (e ~ 0.5) found in binary star systems, the components must actually be separated by more than 30 AU if they are to provide suitable habitats for biology. Apparently about one-third of all F5-K2 binaries within five parsecs of Earth satisfy this requirement.575,1300,2029

In conclusion, our quest for life on other worlds should be limited to perhaps 5% of all stars in the Galaxy. The basic search therefore encompasses some ten billion suns, most of which lie in the Disk and outer Core regions of the Milky Way.

* It has been suggested that the Trojan points of double stars might be a good place to look for habitable planets.607

Chapter 5 ♦ General and Comparative Planetology
5.0 General and Comparative Planetology

Table 5.1 Important Properties of the 25 Largest Bodies

in the Solar System

table 5 1 500px

Table 5.2 Important Compositional Data on the Earth367,1644

table 5 2 500px

Figure 5.1 Estimated Ranges of Some Interesting Properties

for Terrestrial-type Planets

Planetary Surface Gravity for Terrestrials

figure 5 1A 500px

Length of Day* on Terrestrial Planets (empirical relationships only)214

figure 5 1B 500px

Planetary Radius (Terrestrials)

figure 5 1C 500px

Escape Velocities for Terrestrial Planets

figure 5 1D 500px
Catastrophic theories

Historically, scientists have been willing to populate the Moon, Mars, and even Sol with a great multitude of living beings. But they often were loath to extend this cosmic fecundity to regions outside our own solar system.

  • The main hangup was that until only a few decades ago, the very idea of an abundance of planets circling other stars was scoffed at by most professional astronomers.
  • Sol’s family of worlds was believed to be an extreme rarity, if not an absolutely unique event, in the Galaxy.

The cause of this pessimism regarding possible habitats for life in the universe was due in part to the currency of the so-called “catastrophic” theories of solar system formation.

  • These held that the planets were born when a vagabond star passed too close to Sol, ripping away rather sizeable hunks of solar matter.
  • The filaments of star-stuff then condensed into solid worlds, which fortuitously assumed nicely circular orbits around the sun.

The problem with this model is that stars are very far apart in the Disk of the Galaxy, so collisions of this sort must be quite improbable. The catastrophic theories lead to the inevitable conclusion that there are less than perhaps twenty solar systems in the entire Galaxy.20 This, in turn, implies that few if any habitable worlds exist outside our own solar system.

A dramatic turnabout

In the 1930s and early 1940s a dramatic turnabout in attitude occurred.2038

  • Young stars in the process of formation were observed to be embedded in dense dust clouds lacked by older stars.
  • Young stars were also seen to possess large amounts of angular momentum which older stars don’t have.
  • Nearby suns were observed to wobble very slightly from side to side as they traveled through space, as if thrown off balance by the presence of a heavy, unseen companion.
  • These and other observations were hailed as strong evidence that many, if not all stars, are accompanied by a planetary entourage.

Today, astronomers think of solar system formation, not as an exceedingly rare event, but as a normal and common adjunct to stellar evolution.

  • With two hundred billion stars in our Milky Way Galaxy, and more than a billion galaxies in the universe at large, the number of possible habitats for life becomes truly staggering.
  • If there are 1020 planetary systems throughout the cosmos, then on the average more than a million of them are born every hour.20

The central objective of the science of general planetology is fairly straightforward:

  • To study the physical and chemical properties of all non-self-luminous material bodies, whether they occur in our own system or in orbit around some distant star.*

A planet, consequently, is defined as any aggregate of matter possessing insufficient mass to sustain spontaneous thermonuclear reactions in its interior.214

Xenology and planetology

Xenology has two questions to ask of planetology.

  • First, exactly how common are solar systems in the Galaxy? How many of them are there, under what conditions do they arise, and where are we most likely to find them?
  • Questions of planetary evolution and distribution are of immense xenological importance, both in the practical sense of knowing where to search for extraterrestrial life and in the theoretical sense of being able to assess the uniqueness of life on Earth.

The second question posed by xenologists is whether or not our solar system (Table 5.1) and home planet (Table 5.2) are "typical" ones.

  • This is basically a test of the Hypothesis of Mediocrity. Are conditions here roughly the same as on worlds circling other suns, or are things vastly different?
  • What is the allowable range of planetary characteristics such as surface temperature, pressure, gravity, atmospheric composition, lithospheric structure, meteorology, seismology, and so forth (Figure 5.1)?
  • Virtually anything we can learn about a planet enhances our understanding of the lifeforms indigenous thereto. It has been said that there is no property of a planet that is not of some xenological significance.630

* The reader is strongly advised to peruse a copy of Stephen Dole’s Habitable Planets for Man,214 which is an excellent introduction to general planetology with an eye to the specific problem of finding human-habitable worlds.

5.1 Planetary Evolution
Planetary evolution theory

To decide just how abundant planets are in the Galaxy, the most logical place to start is with planetary evolution theory. If we can specify conditions conducive to the birth and development of solar systems, we may then compare these requirements to the observed Galactic environment and form a reasonable opinion as to the likelihood and frequency of planet formation.

Unfortunately, the array of historical planetary evolution schemes20,2033,2109 and the ongoing proliferation of both mundane1278 and unusual816,1264 models in modern times are beyond the scope of this book. We will not deal with them at length here, especially since excellent and comprehensive reviews are readily available elsewhere.20,600,816,1278,2025,2033

While all conclusions regarding planetary formation even today must be viewed as tentative, it appears that accretion models suffice to account for most of the observed properties of bodies in our solar system. In one theory which is gaining wider acceptance, a large, slowly rotating cloud of interstellar gas and dust about a light-year in diameter begins to slowly shrink. As it draws itself together gravitationally over a period of perhaps ten million years,1945 it becomes denser. Were it merely a glob of ordinary neutral gas, it would end up as a small, rapidly rotating ball of hydrogen. Most of its mass would be flung away unceremoniously — and there would be no planets.1549

But radiation generated during the contraction of the hydrogen ionizes the gas, converting it into a plasma — an electrically-charged, highly conductive but tenuous fluid. The Swedish physicist Hannes Alfvén, of the Royal Institute of Technology in Stockholm, was the first to demonstrate a viable mechanism by which angular momentum could be readily transferred from the protostar (the contracting solar nebula) to the surrounding plasma medium. This was fortunate indeed, because until that time a major problem had been to figure out why the planets (with 0.2% of the solar system’s mass) should carry roughly 98% of the total angular momentum.

The magnetic coupling concept

The magnetic coupling concept announced by Alfvén, and later wielded into a classical theory by world-famous astronomer Fred Hoyle, goes something like this: As the protostar collapses, its magnetic field lines of force are dragged closer together but are held firmly in place. Since the infalling clouds are ionized, the field lines are “glued” to the incoming particles. Thus the protostar’s magnetism is coupled directly to the solar nebula; when the protostar tries to speed up as it contracts, the external medium resists the attempt and absorbs the angular momentum itself. The final result is a small, still slowly turning protostar, surrounded by a rapidly rotating disk of matter.

(This theory helps to explain the observed sudden drop-off in stellar rotation later than spectral class F5 (see Chapter 4). Massive, hot stars earlier than F5 apparently are unable to “glue” the magnetic field lines as tightly as cooler suns can. As a result, the field lines wrap themselves uselessly around these bright stars and fail to effect a momentum transfer to the solar nebula. There is no accretion, no planets form, and the protostar retains much of its original rotation. Stars earlier than F5 are thus less likely to spawn worlds than later-class suns.)

The planets themselves form in the disk of matter surrounding the protostar. This tenuous material probably consists of 98% hydrogen and helium, 2% heavier elements — much like the composition of Sol today. As the cloud becomes denser, gases and dust particles begin to adhere and condense to form tiny grains. Clumping of the grains in not unlikely, because such grains are believed to have a fluffy snowflake-like structure.2038 By the time the development of the protostar gets into full swing, these particles have become millimeter- or centimeter-sized — small cosmic pebbles which naturally tend to gravitate toward the midplane of the nebula. The time required for this downfall is no longer than 10-100 years, and the nebular disk thus created probably measures on the order of 1 AU thick and 100 AU in diameter at this point.2051

Goldreich-Ward instability mechanism

The disk material must accrete quickly into bodies large enough to avoid the pressure of the inrushing gases in the plane. Were the grains unable to pull themselves into boulder-sized chunks, most of the matter would be swept remorselessly into the yawning solar “vacuum cleaner” at the rotational center of the accretion disk.33 A means has been proposed to solve this problem, called the “Goldreich-Ward instability mechanism.” According to this theory, a powerful gravitational instability can appear in the plane of the disk provided the cosmic pebbles are not moving too fast with respect to one another.2038

Calculations show that this instability should be sufficient to cause aggregation within the thin sheet of pebbles into hundred-ton bodies with the diameters of asteroids — say, one to ten kilometers. Higher-order clustering might then ensue as these bodies begin collecting each other up by collision. This epoch of titanic surface impacts must be reflected in the cratering record we see on the Moon, Mercury, and elsewhere. In our solar system, such impacts were intense during the first 100-500 million years but rapidly tapered off to their present low level about four eons ago.225,2063

Two classes of Planets

Two general classes of planets are found forming in the accretion disk. These are jovians (Jupiter-like, gas giants, mostly hydrogen and helium) and terrestrials (Earth-like, rocky crust, dense metal core). The terrestrials tend to appear nearest to the protostar, in the hottest regions of the solar nebula. They are the result of simple mass accretion to build up small, rocky, dense bodies.


The jovians are formed far from the central regions. A small, heavy core serves as a seedling for the accumulation of vast quantities of material. The true jovians — such as Saturn and Jupiter — develop such massive central bodies that they cause the nebular gas to destabilize and condense into a thick, dense shell. This represents most of the final planetary mass. Jovians act much like miniature protostars, voraciously sweeping the nearby space clean of gas and dust.2051 The subjovians — represented by Uranus and Neptune in our system — don’t have nearly so massive a core as the jovians. Thus, they can retain only those gases normally gravitationally concentrated near the planetary centrum. Subjovians do not grow as large as jovians.

This behavior can be explained in part by the process of differentiation of chemical elements in the condensing solar nebula. According to the detailed hydrodynamic model created by A. G. W. Cameron and his colleagues at the Harvard College Observatory, subjovians tend to form in the outermost regions of the nebula where the pressures are only about 10-7 atm* and the temperatures under 100 K. Matter there consists largely of interstellar grains, mostly water-ice condensed upon a small rocky substrate.

Uranus and Neptune, then, consist mostly of ice with a little bit of rock. When sufficient mass has accreted, these bodies can gravitationally draw in some of the solar nebula for atmosphere. Hydrogen and helium will thus comprise perhaps 20% of the total mass of subjovian bodies.2051 Comets are believed to have originated under similar conditions.2038

Jovians are found closer to the swollen protostar. Most likely they occur in a region where the pressure is about 10-6 atm and temperatures are 100-200 K or more. At such high temperatures the ice evaporates, leaving only rocky materials to condense. However, due to the higher pressures there is more material around, and it turns out that accretion proceeds faster. This leads to the aforementioned instability and sudden, massive gas collection from the nebula.2051

The amounts of gas gobbled by a jovian during this period is astounding. In fact, it appears that even now, 4.6 eons later, Jupiter and Saturn are still in the process of “swallowing” their great feast of hydrogen and helium. Both worlds emit roughly three times more energy than they receive from Sol.2096,210 This heat is due to the slow collapse of the planets gravitationally.598,2032,2048,2057 (The shrinkage amounts to about 1 millimeter per year.2032)

* one atmosphere (1 atm) = sea level air pressure at Earth’s surface.


Figure 5.2 Condensation in the Primitive Solar Nebula2049,2050,2051

figure 5 2 500px

The terrestrials form closest to the protosun, where pressures range from 10-5 to 10-4 atm and the temperature climbs from 200 K to well over 1400 K.1564 It is a region of very high convection, so the matter is kept well-stirred. Only small cores with miniscule amounts of nebular gas can accrete. (The extent of this growth restriction is made more clear if we consider stripping the jovians down to their heavy elements. If we did this, we’d find both Jupiter and Saturn with 15-20 Mearth (Earth-masses) of heavies.2091,2096,2098 This is far more than Earth, the most massive terrestrial world in our system.) Total accretion time for terrestrials probably runs on the order of a thousand to a million years.2043,2044

We see that the bulk composition of planets in any single-sun system should follow a quite regular, orderly progression (Figure 5.2). The innermost worlds will consist of the most refractory matter, with the planets at progressively greater distances from the primary consisting of the less refractory materials.22

To sum up:

  • We expect that planets lying within or close to the habitable zones of stars will be generally terrestrial in character.
  • Far outside the habitable zone at great distance from the sun, jovians and subjovians put in an appearance.
  • And no planets will be found closer to a star than perhaps one-quarter of the distance to the center of the habitable zone.
  • No substance found in the solar nebula could condense in the extreme heat encountered there.
Tentative verification of accretion model

Figure 5.3 Results of Computer Simulations of Planetary Formation1258

Planetary Systems Synthesized by Computer Model

figure 5 3A 500px

Above are a few examples (among hundreds) of planetary systems synthesized by Stephen Dole’s computer model.

  • The sun is at the far left in the diagram and is omitted for clarity.
  • Planets, their orbital distances from their sun, planetary masses and orbital eccentricities all are shown.

þ Solid, filled-in circles represent terrestrial worlds;
þ Gas giants are represented by horizontal shading.

Our Own Solar System

figure 05 3b

For comparison, our own solar system is diagrammed similarly above. Note the overall similarities:

  • Terrestrials in close, jovians further out.

þ Solid, filled-in circles represent terrestrial worlds;
þ Gas giants are represented by horizontal shading.

The fundamental correctness of the accretion model has been tentatively verified by Stephen H. Dole of the Rand Corporation.1258 Dole set up a computer program to simulate the primitive solar system in the process of formation.

Accretion nuclei with random orbits are shot into a nebula surrounding a theoretical protostar of 1 Msun Nuclei aggregate dust in the nebula, assumed to be 2% of the total by mass, until a specified critical mass is reached beyond which gas can be accumulated as well.

The growing planetesimals coalesce if their orbits cross or if they come too close. Nuclei continue to be injected until all dust has been swept from the system. The model is simplistic, to be sure,2037 and yet the results are most intriguing.

Despite the fact that Dole varied the initial conditions considerably, the final products always seemed remarkably similar (Figure 5.3). After each run, the end result was a solar system which looked much like our own.

The total number of worlds formed varied from seven to thirteen, and the Titus-Bode “law”1254,1304 of planetary orbital spacing (so well-known to beginning astronomy students) seemed to hold up approximately in all cases.2054 While every such system is quite unique, the surprising thing is that each shares many features of Sol’s system and yields results consonant with accretive evolutionary theories.

Jupiter and Saturn – “failed stars”

Figure 5.4 Computer Synthesis of Multiple-Star Systems1258

Examples of binary and multiple star systems

generated by computer model

figure 5 4A 500px

Examples of binary and multiple star systems generated by Stephen Dole's computer model are shown above.

  • As the coefficient of density, A, is increased by a factor of ten, terrestrial worlds disappear and the jovians accrete into larger and larger masses, eventually becoming a few self-luminous stars.
  • (Density, A, is measured in solar masses per cubic AU.)

þ Terrestrials are represented as solid circles.
þ Jovians by horizontal shading.
þ Red dwarf stars by cross-hatching.
þ And the open circle represents a class K6 orange dwarf star.

Another set of sample solar systems

figure 05 4b

Another set of sample solar systems is included above for comparison.

Dole’s program generated another unexpected result. It has long been suspected that the processes which give rise to binary and multiple star systems may actually preclude the formation of planets.20,1300 In our Galaxy, the average separation of binary components is about 20 AU, corresponding roughly to the orbital distances of the jovian gas giants in our solar system. (Jupiter and Saturn have often been called “failed stars.”2048 In this view, we narrowly missed out on finding ourselves in the middle of a triple star system.)

By increasing the density of the initial protocloud an order of magnitude higher than before, Dole’s program generated larger and larger jovians (Figure 5.4). Eventually the threshold between planetology and astrophysics was crossed. In one high-density run, a class K6 orange dwarf star appears near Saturn’s present orbit, along with two superjovians and a faint red dwarf further sunward. No terrestrials are formed.

As Dole says, the general trend is clear. Jovians multiply at the expense of terrestrials. An increase of one critical parameter — the nebular density — may well result in the generation of binary and multiple star systems to the eventual exclusion of terrestrial worlds.1258

Both theoretical and numerical accretion models of solar system formation suggest that planets are probably the rule rather than the exception, and that terrestrials should form near most single stars in the inner regions of the solar nebula. This augurs well for the abundance of habitable worlds and extraterrestrial life in the Galaxy.

5.2 Thalassogens

Table 5.3 Cosmic Abundance of the Elements

(number of atoms)6

table 05 3 cosmic abundance of the elements 400
Water - catalyst of life

Life on Earth is dependent upon the oceans for both its origin and its evolutionary development. The early organic compounds which ultimately gave rise to living organisms were stirred and stewed in the primitive seas — our entire biological character is molded by the properties of water. Indeed, it is difficult for biochemists to imagine that life could have had its origin in any other medium. Complex chemical reactions must have a reasonable chance of occurring. A liquid medium of some kind is required, capable of dissolving salts and other compounds and then commingling them in the degree of intimacy required for the origin of life. While it is certainly more, water in this sense may be viewed as a “catalyst” of life.

But must conditions on other worlds exactly parallel those found on Earth? Is water the only possible fluid in which life may originate? We don’t really know the answer to this question (see Chapter 8). Of interest to us here, however, is whatever light can be shed on the problem by the science of planetology.

Thalassogen - coined by Asimov

Isaac Asimov has coined the term “thalassogen,” by which he refers to any substance capable of forming a planetary ocean.1399 Looking for possible thalassogens is somewhat broader than the search for liquids that can sustain life, because some of them may turn out to be anathemic to all conceivable biochemistries. But the planetologists’ quest for thalassogens is certainly an excellent starting point for our inquiry.

What substances are available for ocean-building? There are two characteristics which must be possessed by seas on any planet in our Galaxy. First of all, the very elements comprising the thalassogen molecules must be relatively abundant in the universe (Table 5.3). For instance, the element mercury is a liquid at normal temperatures and so might be considered as a thalassogen. However, its abundance cosmically is only about 0.000000001% of all atoms, which is hardly enough to cover a world the size of Earth to a depth of a millimeter or so.39,1413


Any substance capable of forming a planetary ocean.

Molecule/element must be both abundant and
simple to qualify as a thalassogen.

Must have a prominent liquid phase under
the conditions typically encountered on planets.

How about oceans of dimethyl butanol? The atoms which make up this substance — carbon, hydrogen, and oxygen — are certainly among the most plentiful in the universe. Unfortunately, the compound is subject to numerous degradations by heat and chemical interactions, and is chemically unlikely to be synthesized in oceanic quantities. So dimethyl butanol must remain relatively scarce on planetary surfaces, despite the ubiquity of its constituent elements.

Two requirements of a Thalassogen

A molecule must therefore be both abundant and simple to qualify as a thalassogen. Rare elements, and molecules which are horribly complex, have a very low likelihood of being found in the oceanic state.

Apart from availability, there is one further basic requirement: The putative thalassogen must have a prominent liquid phase under the conditions typically encountered on planets. If the environment is such that the molecule has a hard time liquefying at all, clearly it will not be present in pelagic quantities on the surfaces of worlds.

Consider Mars, for example. At the surface of the red planet the atmospheric pressure is only 1% that on Earth.2044 Under such conditions, any carbon dioxide frozen at the poles cannot melt to liquid CO2 upon heating. Quite the contrary, the “dry ice” there sublimes — that is, it passes directly from the solid to the gaseous state. This occurs even at more Earthlike pressures. Above 5.2 atm, though, CO2 is able to melt and form liquid carbon dioxide. Venus, whose atmosphere is mostly CO2 at nearly 100 atm, might have liquid carbon dioxide at its surface were it moved out to a cooler orbit and if the pressure could be maintained above 5.2 atm.

Consider the elemental abundances as noted in Table 5.3 above. Taking the cosmic values first, we see that two of the elements — the noble gases helium (He) and neon (Ne) — can be present in elemental form only. The most abundant atom, hydrogen (H), exists either in chemical combination (terrestrial worlds) or in large quantities in elemental form (as on the jovians). Oxygen (O), nitrogen (N), and sulfur each can achieve liquidity at temperatures that might be expected on planetary surfaces.

The elements silicon (Si), magnesium (Mg), and iron (Fe) unite with others on the list to form sulfides, oxides, nitrides and hydrides. The metal sulfides and oxides are extremely refractory, having melting/decomposing points above 1000 °C. They probably will not exist in liquid form on any normal planet for very long. Nitrides and hydrides of the aforementioned elements all tend to decompose either with elevated temperatures (i.e. before they have a chance to liquefy) or in the presence of water (which is likely to be ubiquitous anywhere in the universe). So none of these substances would make very good thalassogens.

Chemically hydrogenated

Compounds comprised of hydrogen, oxygen, nitrogen, carbon and sulfur must also be considered. It has been argued that in a primarily hydrogenous environment, everything will tend to become as chemically hydrogenated as possible.1399 Hence,

  • oxygen will become water (H2O)
  • nitrogen will go to ammonia (NH3)
  • carbon will become methane (CH4)
  • sulfur will react to form hydrogen sulfide (H2S)

Many other simple compounds have been discovered, floating naturally in interstellar space, by radio astronomers in the last decade.1002 These substances are observed in vast clouds, and include carbon monoxide (CO), sulfur dioxide (SO2), cyanogen (CN), hydrogen cyanide (HCN) and so forth.521 A full consideration of all interstellar molecules discovered to date, and many other possibilities not yet detected, is unfortunately beyond the scope of this book.

Of course, oceans are not found in space but on planetary surfaces. Therefore, it is also relevant to consider the elemental abundances in the crusts of planets. We look for clues to additional compounds which might be generated by chemical reactions incident to planetary heating and volcanism, and which might be able to serve as thalassogens. From Table 5-1 we find only three elements — oxygen, hydrogen, and carbon — which are useful in this regard. Carbon dioxide (CO2) and water are the most common substances formed from these elements to be found on terrestrial worlds. Other molecules which might arguably arise under various planetary conditions include nitrogen dioxide (NO2) and carbon disulfide (CS2), although there are serious objections to both of these on reaction equilibrium grounds.

Table 5.4 Melting/Boiling Points and Liquidity Ranges

for Possible Thalassogens at 1 atm Pressure*

table 5 4 500px
* Notes to Table 5.4

* At higher pressures these values become slightly higher.

Tc, the critical temperature, is the highest temperature at which
     the compound stays liquefied (at any pressure).

Pc, the critical pressure, is likewise the highest pressure for which
     the substance remains in the liquid state (at any temperature).


So much for availability. What about liquidity? Even the coldest planet in our system (Pluto) has a surface temperature of at least 43 K.2037 So the first three possibilities listed in Table 5.4 — helium, hydrogen, and neon — can be ruled out because no reasonable world could be cold enough. But most of the remaining molecules could well be available as oceans on the surfaces of planets at the proper solar distances. (This is a gross oversimplification, of course, because relative abundances should also be taken into account.)

The lower the liquidity range, the faster the world must be spinning to maintain even temperatures. Cyanogen is particularly suspect on these grounds. As a general rule, the larger the range of liquidity the higher the probability of finding a planet whose temperatures fortuitously remain within the appropriate limits.

Xenologists are primarily interested in those thalassogens which might allow life to arise naturally on a planetary surface. We know that water, with its liquidity range of 100 K, has been capable of supporting and sustaining biology. The Hypothesis of Mediocrity allows us to take this as a minimum (or reasonable) value.

Using this standard, we see that water, carbon disulfide and sulfur all have liquidity ranges equal to or greater than 100 K. Another marginal possibility is carbon dioxide, and perhaps sulfur dioxide as well.352 Ammonia is a very long shot.

For a million years, humanity has become accustomed to the shimmering blueness of the open seas. On a world with oceans of CO2, we would feel right at home. Carbon dioxide is a sparkling clear liquid slightly less dense than water. Oceans of it would possess the same evocative rich blueness as the seas of Earth. (Marine sulfur dioxide and ammonia should look similar.)

Carbon disulfide oceans would demand peculiar chemical conditions in the planetary crust to sustain them. CS2 is not believed to have existed in the primary atmospheres of any of the terrestrial worlds in our solar system. Nevertheless, as someone clever has remarked, absence of evidence is not evidence of absence. We’ve seen that the carbon disulfide molecule satisfies the most fundamental requirements of all thalassogens.

Oceans of this foul-smelling, poisonous substance would appear light-yellow in color in the shallower regions near coasts, due to the presence of colloidal sulfur particles. In deeper waters sunlight would begin to add a scattering component, causing a change of color to a peculiar shade of light-green. If there is any ammonia or hydrogen chloride around (even in trace amounts), simple chemical reactions would turn the sea a brilliant crimson.

Oceans of molten sulfur are the most fascinating of all, for they would change both color and viscosity regularly with oscillations in the planetary surface temperature. Between 386 K and about 430 K liquid sulfur is a thin, transparent, pale-yellow fluid. As the temperature increases from 430 K to 470 K, the substance becomes dark red in color and extremely thick and viscous. From 470 K to 500 °K the viscosity falls off but the color darkens from red to black. Above 500 K the sooty color remains, but the sea becomes thin and fast-flowing once again. Pelagic sulfur would make for a most interesting planetary environment indeed!

5.3 Planetary Atmospheres
Atmospheric assessment

arthur eddington

In the absence of an atmosphere, it is difficult to imagine an ocean of water or any other thalassogen being present on a world. It appears that both liquids and gases are required in the chemical interactions which lead to the origin of life. Discounting the occasional origin of life in the subsurface regime of its crust, a world probably cannot be suitable for living organisms unless it possesses some kind of atmosphere.20

While atmospheres may exist without oceans, oceans may not exist without atmospheres. More factors must be taken into account in assessing a molecule as a possible atmospheric constituent.

  • First, it must be reasonably abundant.
  • Second, it must be present in either gaseous or vapor form at reasonable planetary temperatures.
  • Third, the molecule must be neither so lightweight nor so hot as to have escaped from the world over a period of eons.
  • Fourth, effects of planetary surface chemistry become extremely important in the evolution of atmospheres — the presence of large oceans is especially significant.
  • Fifth, natural biological modification of the atmosphere must be considered.

As far as abundance is concerned, there are fewer restrictions on composition than when we were talking about thalassogens. While oceans may represent 0.01-0.1% of the total mass of a terrestrial planetary body, an atmosphere will run two or three orders of magnitude less. Consequently, Tables 5.3 and 5.4 are far from complete. Far less abundant molecules, rejected as thalassogens on grounds of scarcity, are welcome as constituents of the air.

Looking at the boiling points (and vapor pressures) of the molecules in Table 5.4, we note that virtually all have a gaseous phase at reasonable temperatures for some planets. (E.g., Pluto may have a neon atmosphere!2064) In view of the liberal temperature and abundance requirements, literally hundreds of molecules may comprise planetary atmospheres in various concentrations and pressures. An exhaustive treatment is clearly beyond the scope of this book.

The third consideration is the escape of molecules from a world by a process known as thermal evaporation. Just as rockets must achieve escape velocity to overcome Earth’s insistent gravitational tug, so must atoms. Gas molecules which are traveling fast enough and are light enough can stream off into space, leaving the planet high and dry. Higher temperature means higher energy which means higher velocity. Also, the lighter a molecule is at any given temperature, the more likely it is to escape because it needs less energy to get away. Light molecules thus leak off faster than heavy ones.

Close to the surface of a world, molecules cannot travel very far before they bump into one another. Even a particle moving at ten times the escape velocity would strike several others before it had traveled one centimeter. It would distribute its energy, slow down, and not escape.

But in the exosphere (as it is called) of a planet, molecules can fly literally kilometers before a collision occurs. Only in the upper atmosphere can gas which is hot enough to escape have a reasonable chance of making it. So it is this exosphere temperature, and not the planetary surface temperature, which is relevant to the escape of atmospheric components. Earth’s exosphere, to use an example, lies at roughly 600 kilometers and varies from about 1500-2000 K.20,214,521

Airless, light atmosphere, and heavy atmosphere

Figure 5.5 Retention of Planetary Atmospheres

as a Function of Molecular Weight(after Dole214)

figure 5 5 500px

Atmospheric constituents whose molecular weight places them above a planet are retained, those below are not. The closer a planet lies to the molecular weight (MW) = 1.0 line (corresponding to molecular hydrogen), the more massive its atmosphere is likely to be. Planets lying below this line will probably be gas giants.

Table 5.5 Potential Atmospheric Constituents(from Dole214)

table 5 5 500px

From the abundances listed in Table 5.3 we might expect planets to start out with mostly hydrogen and helium, with less than 2% other elements as impurities. Jovians are massive enough (high escape velocity) and cold enough (low velocity molecules) to hold the concentrations of these two elements to within spitting distance of their primitive solar nebula abundances. On worlds as small and hot as Earth, though, hydrogen escapes in a characteristic time of perhaps 1000 years.20 On still smaller and hotter worlds, like Mercury, the gas is retainable only for a matter of hours. (The characteristic time for hydrogen on Jupiter is estimated to be something like 10200 years.57)

On the other hand, most average-sized terrestrial planets are quite capable of hanging on to carbon dioxide, water, nitrogen and oxygen. (These are also retained by the jovians, but the proportion is vastly smaller because of all the hydrogen and helium around.) Following Dole,214 we may classify all planets into three general categories: Airless, light atmosphere, and heavy atmosphere.

Atmospheric Constituents

Airless worlds are those which lie above the molecular weight MW = 100 line on the planetary atmosphere retention graph on Figure 5.5. Mercury,1566 Luna and the asteroids in our solar system have virtually negligible gaseous envelopes. Planets which lie between this line and the MW = 5 line will have atmospheres of small mass relative to the main rocky body. Gases, if present in the first place, will be retained according to their molecular weight and the specific surface conditions they encounter. Finally, planets lying below the MW = 5 line will possess atmospheres which represent a sizeable fraction of the total mass. Such will consist primarily of hydrogen and helium, with trace impurities of methane, ammonia, and so forth (depending on temperature).

Still, we are not yet in a position to predict the atmospheric composition of terrestrial worlds. Venus and Earth, for instance, have roughly the same mass but their atmospheres are vastly different. According to the discussion above, one might have expected the Cytherian air to be less dense than our own because it’s hotter closer to Sol (and so gas should be lost more quickly). Yet the surface pressure on Venus is ~100 atm. Clearly, other forces are at work besides simple selective leakage of gases.

Element Abundances

Table 5.6. Element Abundances on Earth as Compared

to the Primitive Solar Nebula315,521

table 05 6 element abundances on earth 400

Part of the mystery may be cleared up by considering the information contained in Table 5.6. As we expect, there is a large depletion of the lighter elements — hydrogen and helium. But why are other elements so severely dissipated as well? Most peculiarly, why are argon, krypton, and xenon pretty well gone from Earth, despite the fact that the characteristic leakage times for these components should be 1070 years or more?

If we look at what the composition of Earth should be (based on thermal evaporation considerations alone) and then compare it to the actual makeup of our planet, several very striking facts emerge. Most of the solid elements that go into rocks — silicon, aluminum, magnesium, sodium — are present in just the right amounts. Most of the oxygen around was similarly tied up. However, all the gaseous components are depleted by an average of six orders of magnitude! What’s going on?

Planetologists today believe that in primitive times Earth (and the other terrestrials in this system) lost not only H and He due to thermal evaporation but most of the rest of its atmosphere as well.2031 The exact mechanism by which this cosmic dust broom operated is not clear, but it may be connected with the T Tauri gales associated with the early stages of evolution of Sol-like stars. The lack of noble gases is significant because they are the heaviest molecules present in any planetary atmosphere. If even they are gone, it’s virtually certain that all lighter components have also been scoured away.

Atmospheric mysteries of Earth and Venus

But then — how do we account for our present atmosphere? If Venus started out as an almost airless globe, where did it manage to find 100 atm worth of carbon dioxide?

The four elements common to all terrestrial environments, C, H, O, and N, are the four least depleted of all the gaseous components. Why is this so? It appears evident that compounds containing these elements were actually incorporated into the early Earth in both solid and gaseous form.33 Later, they were released from their rocky vault to take up new careers as atmosphere and ocean.

When the primitive Earth contracted and began to melt, trapped gases slowly bubbled to the surface.2042 Volcanoes today emit as much as 60% water and 20% CO2 in their eruption products,2031 and molten rock can dissolve perhaps 5% of its weight in water. Scientists suspect that by similar processes, our air and water gradually emerged from the interior of the planet.2031

The early hot crustal material may have had large amounts of free iron, which would have reduced much of the water and carbon dioxide to methane and hydrogen.57 Our secondary atmosphere thus probably began as a chemically reducing environment, rich in effluent H2, CH4, H2O, NH3, and increasing amounts of CO2 and N2.20,57,521,1293,1645

Surface chemistry

Table 5.7. Summary of Terrestrial Planetary

Atmospheric Evolution2041,2044

table 05 7 summary of terrestrial atmospheric evolution 400

We arrive at the fourth important factor relating to planetary atmospheres: Surface chemistry effects. The evolution of the air of a world is closely linked to its mass, temperature, geological activity, and oceans. Most terrestrial planets destined to have light atmospheres (Table 5.7) are expected to have gone through the same processes of outgassing as described above for the Earth — though perhaps at slightly different rates.

Rasool's Model of Planetary Atmospheric Evolution

Figure 5.6 Rasool's Model of Planetary Atmospheric Evolution

CO2/H2O Phase Diagram for Terrestrial Planets2065

figure 5 6A 350px

Atmospheric physicist S. I. Rasool assumes that atmospheres of terrestrials are the product of early “degassing” from the molten interiors of the primitive planets. Shown in the diagram at left is the triangular region of pressure and temperature in which water remains a liquid thalassogen (cross-hatched area). Also depicted are the evolutionary tracks of three typical terrestrials in our own solar system.

The theoretical development of Venus is illustrated by two curves — one for a non-rotating world (upper curve, marked “VENUS”) and one for a fast-rotating planet (curve starts at 330 K).

Tracks for Mars and Earth are also shown, and still another curve depicts the startling conclusion that Earth would have missed the water-liquidity triangle altogether had it started out a mere 5°C hotter,

Apparently our lush, verdant planet would have become a close duplicate of hellish Venus were it a mere 6-10 million kilometers closer to Sol.1907

Thermal Evaporation of Planetary Atmospheres2031

figure 5 6B 350px

The drawing at right presents the effective escape time for various gaseous components of planetary atmospheres, as a function of atomic (molecular) weight A. Various exosphere temperatures are assumed for each of the planets shown.

For a planet to be a terrestrial world capable of evolving advanced lifeforms, its retention curve must lie to the right of helium on the diagram, so that both this gas and hydrogen are lost in one “genesis time” (~5 x 109 years).

At the opposite extreme, a planet must be able to retain all other gases for at least one genesis time. The Moon fails to fulfill this requirement by several orders of magnitude.

Dr. S. Ichtiaque Rasool, Chief Scientist at the Planetary Programs Office of NASA and a specialist in planetary aeronomy, has formulated a fascinating theoretical model (Figure 5.6) for atmospheric evolutionary processes.2065 The model predicts that terrestrial worlds relatively close to their primary (like Venus) will always be too hot for water vapor to condense at the surface into oceans. With no water in pelagic quantities to dissolve it, the CO2 disgorged into the air by volcanoes must remain aloft. A dense atmosphere soon builds up. Temperatures are further elevated by the greenhouse effect*: The carbon dioxide forms a warm blanket over the entire planet, absorbing and reemitting the infrared heat radiated by the illuminated planetary surface. This effect adds only 30 K to the temperature of Earth’s atmosphere, but amounts to a whopping 500 K on Venus!

On such a hot terrestrial world, the water vapor could be split into its component atoms by the ultraviolet rays from Sol. The hydrogen would then be lost to space by thermal evaporation, and the oxygen could combine with the surface rocks and disappear from the air. The carbon dioxide level is partially buffered by chemical reactions with silicate rocks in the crust. These reactions tend to eat up CO2 and produce carbonate rocks, or limestone. Unfortunately, buffer reactions proceed at a reasonable rate only if there is plenty of water around. But as we’ve seen, there won’t be much on a hot terrestrial. The volcanoes can go on dumping carbon dioxide into the atmosphere and the crust can do little to prevent it. This process is commonly known as a “runaway greenhouse.”2037,2065,2066

On a world closer to the center of the habitable zone (like Earth), the chain of events is much different because things are cooler. The atmosphere begins to emerge at the time when the nearly airless surface has a temperature at or near the freezing point of water. As the CO2 comes out and the planet starts to greenhouse, the temperature rises slightly. Water sloshes together in liquid form and becomes ocean. The carbonate-producing buffer reactions begin in earnest, laying down gargantuan deposits of limestone and chalk as the carbon dioxide is removed from the air. The greenhouse does not run away.

We see that the surface temperature of the planet is of critical importance in determining the fate of its atmosphere. Rasool calculates that a change of perhaps 10 K (hotter) would be enough to have caused Earth to miss the liquid phase of water altogether and become a close replica of Venus.2065

The model also predicts what happens to terrestrial worlds in the outlying regions of the habitable zone (like Mars). Here again we have no oceans forming, because any water emitted by volcanoes is frozen out. Carbon dioxide may build up, free from the moderating influence of silicate buffering reactions. (But Mars is a small, cold planet, so degassing from the interior proceeds much slower than for a larger body. A 1 Mearth world at Mars’ orbit should eventually become quite Earthlike, though it will naturally take much more time.)

* Technically this is a misnomer because it’s not the way horticultural greenhouses keep warm. Rather than selective passage of visible (but not infrared) wavelengths, they work simply because a body of air is physically confined and heat cannot escape by convection. In 1908, Dr. Robert W. Wood constructed two greenhouses — one of glass and one with rock salt panes (NaCl passes infrared, unlike glass) — and both worked equally well.

Slow planetary evolution of Mars

As regards Mars: After perhaps ten eons or so of slow planetary evolution, enough carbon dioxide may accumulate to produce a respectable greenhouse effect. Since the water has not been lost but is merely stored away at the poles, oceans could develop when the temperature manages to rise above 273 K — the freezing point of H2O. In this view, Mars has never had oceans and is in an earlier stage of evolution than Earth. (There are some who would disagree with this conclusion, arguing from the riverbed-like structures observed on the Martian surface by Mariner 9 and Viking.15,2044,2074)

So the story of the gross atmospheric conditions is largely the story of water and carbon dioxide. But what about the other components of the air? Well, much of the hydrogen is lost to space by thermal evaporation from the exosphere. Nitrogen is released by volcanism and is relatively inert — it remains in the air relatively unchanged. The ammonia dissolves in the water, if there is any, or dissociates into hydrogen and nitrogen. Methane undergoes organic reactions, again, if there is an ocean. And oxygen is produced when water is split apart in the exosphere by ultraviolet radiation. O2 can reach natural concentrations of perhaps 0.1% of the air. For example, Ganymede and Callisto are believed to have thin oxygen atmospheres (~10-3 atm), which could have arisen as fast as ten thousand years in this fashion.2095

Earth, the only oxidizing planet

Figure 5.7 Biological Modulation of Planetary Atmospheres1293

Figure 5.7A Atmosphere of a Lifeless Earth

figure 5 7A 350px
This model of the Earth with no life from the start is not unlike what would be expected from interpolation to conditions lying midway between Venus and Mars.
The surface pressure is still about 1 atm, but the air is perhaps 97% nitrogen and a few percent carbon dioxide.

Figure 5.7B Atmosphere with Life Present

figure 5 7B 350
The effects of adding life to a planet’s geophysical history are striking when biological modulation of the atmosphere takes place.
While the air is still mostly nitrogen, autotrophic living organisms convert perhaps 95% of the available carbon dioxide into biomass-carbon and free oxygen — which is then utilized by animal life.

Production of oxygen is a good example of what is called a “self-limiting” process. As the concentration of O2 rises, a thin ozone (O3) shield begins to form which screens out the UV rays from the water vapor below. As ozone increases, less H2 is dissociated and less free oxygen is produced.

It seems that natural mechanisms may be able to change a reducing (hydrogenating) atmosphere into a more neutral one, but apparently simple chemistry alone is incapable of creating an oxidizing atmosphere.96 Earth is the only planet in the solar system that is oxidizing. Why?

Biological modification of the atmosphere

The answer is found in the biological modification of the air — our fifth important factor It appears that until perhaps two eons ago, the carbon dioxide in Earth’s air (say, 1%) kept the surface temperature well greenhoused to warmer levels. As the blue-green algae began to work their photosynthetic magic in our oceans, they took over from the silicate rock and carbonate buffer chemistry in the removal of CO2. After only about 500 million years, Earth’s atmosphere changed from 0.1% O2 to about 20% O2. This effectively removed about an order of magnitude of carbon dioxide from the air, reducing its concentration down to about 0.1% of the total. Instead of limestone formations, carbon began to be incorporated as biomass (Figure 5.7).

Earth as a living, complex entity

The presence of an oxidizing atmosphere is probably a good test for biology.* We know that Earth’s crust is rather underoxidized and would eat up most of the abundant O2 in our air in a relatively short time. As Carl Sagan has pointed out, “a high level of oxygen such as we have in the Earth’s atmosphere can only be accounted for by vigorous biological activity.”445 (The photosynthetic recycling time for the O2 in our atmosphere is roughly 2000 years.1945)

But scientists today argue that more than just oxygen levels are controlled by terrestrial biota. Dr. Lynn Margulis of the Boston University Department of Biology and Dr. J. E. Lovelock, an applied physicist at the University of Reading in England, believe the Earth is a complex “entity” which could almost be described as living. They present evidence that biology not only modifies our environment but modulates it as well.1293

That is, the conditions in Earth’s oceans, atmosphere, lithosphere and biosphere are all regulated by life on the surface in such a way as to maximize the growth of the biosphere. It gives one pause to consider that those same forces of natural selection responsible for the diversity, abundance, and efficacy of lifeforms on this world are also operative on the biospheric, global scale. As species evolve over time, so do complex feedback mechanisms seek and preserve planetary homeostasis — the optimum physical and chemical environment for life on Earth.

* Life is quite possible (and in fact originated) in fully reducing atmospheres. However, advanced forms of life need far more energy. Hence, they appear less likely to arise in hydrogenous environments because their metabolisms would seem to be less energy-efficient.

Atmosphere: summary of conclusions

Table 5.8 Exotic Biological Modulation Schemes: Theoretical Atmosphere/

Thalassogen Biochemical Energy Systems, Neglecting Abundance Problems (after Asimov1358)

table 05 8 535

Let us now attempt a brief summary of our conclusions regarding terrestrial planet atmospheres generally. First, abundance and gaseous state requirements are so loose that it is difficult to exclude virtually any reasonable candidate molecule on these grounds alone. As far as thermal evaporation is concerned, a planet in the habitable zone with a mass greater than perhaps 0.1 Mearth should be able to hang onto any gas already present (other than hydrogen or helium) for geological time periods.

It appears that the typical terrestrial without oceans is most likely to carry an atmosphere consisting of more than 95% carbon dioxide through out much of its evolutionary history. Planets with oceans of liquid water should develop an equivalent predominance of nitrogen in the air, because the CO2 is returned to the crust via silicate buffer reactions. (There are no precedents in our system for nonaqueous terrestrial oceans, and unfortunately the chemical surface processes have not yet been worked out in detail for alternative thalassogens.)

We see that the total surface pressures may range from less than 0.01 atm to more than 100 atm, depending primarily upon the rate of outgassing of the secondary atmosphere from the interior of the planet. Larger, more massive worlds should tend to outgas faster and build up thicker air, as a general rule.

Finally, if life is present, thermodynamically unstable components may appear in the atmosphere — such as oxygen on Earth. Of course, any other chemically active gaseous oxidant may equally well be found, depending on the particular modulating biochemistry of the life on the planet’s surface (Table 5.8).

5.4 Planetary Meteorology and Astrogeology
5.4 Planetary Meteorology and Astrogeology

lord byron 327So far we have confined ourselves to an examination of the gross, bulk properties of planets, oceans and atmospheres. But xenologists are also very much interested in somewhat “smaller scale” phenomena. What kinds of climate and weather will the aliens have? Will their world know lazy clouds, blue skies and shimmering auroras? Are their mountains tall or short (e.g., "astrogeology"2144), and how fierce are their storms and quakes? What color is their sun?

The answers to such questions, and many others like them, are extremely hard to come by in a definitive way because the causative dements are so complex and variable. Yet they are of vital importance if we hope to comprehend alien art and culture, languages, architectural forms and lifestyles, and even ET social patterns and individual psychologies.

5.4.1 Climate and Weather
Many factors for climate change

Table 5.9 Wind Speed and Planetary Surface

Conditions for Terrestrial Planets1566,2066,2087

table 05 9

We’ve already hinted at the effects of evolutionary history on a planet’s surface temperature. What else can be said about the overall climate? First of all, the thinner the atmosphere the greater will be the diurnal variations in temperature. This is because a dense, massive atmosphere has more ”thermal inertia.” Since huge amounts of heat are stored, a brief nighttime cooling-off period has very little effect. But if the air is thin and lightweight (as on Mars), very little heat is reposited. Thus, on the night side the surface and the air above it cool rapidly, leading to large swings in temperature between the two sides of the planet. This results in faster-moving winds (Table 5.9), but because the air is less dense the energy available is actually less.

Perhaps one of the most decisive factors in planetary meteorology is the rotation rate of the planet. On a planet such as Venus, where a single “day” lasts months, surface winds are believed to be no more than a few kilometers per hour, maximum.1257,2041 On worlds with intermediate rotation rates like Earth and Mars, typical wind speeds range around 50-70 km/hour.1257,2067 Fast-spinning bodies like Jupiter are known to have winds averaging 140-290 km/hour and higher near the equator.1141,1257,2045 Naturally, faster rotation and stronger winds means larger Coriolis forces, along with more violent cyclonic disturbances such as tornadoes, hurricanes, typhoons and water-spouts. Also, slow worlds tend to have greater day/night thermal differentials than faster ones because the air is not as well stirred, Surface temperatures are less uniform as a result.214

The heat capacity of the molecules in the atmosphere is also important. This may be thought of as the amount of energy which must be added to a unit of air to raise its temperature a fixed amount. It can also be conceptualized in terms of energy loss: How much heat must be lost to drop the atmospheric temperature one degree? 

An atmosphere like Earth’s in every respect but comprised of hydrogen would have nearly fifteen times the heat capacity of normal air. It would thus take fifteen tithes longer to heat up or cool down, so surface temperatures on a hydrogen-atmosphere planet should be pretty much the same every where.1257 There would be little if any “climate” as we know it on such a terrestrial.1257

The presence of oceans affects the climate in many ways. Largely pelagic worlds should experience smaller variations in surface temperature because the water acts as a giant thermal buffer.286 On dry worlds, the climate is likely to be more “continental,” or desert-like.214 With no seas, meteorology becomes more volatile — weather changes more rapidly.

Many other factors are important too. The winds are driven by the energy supplied fun a planet’s star. Worlds near the inside edge of the habitable zone should therefore have more violent weather, because more energy is available. Unfortunately, life is more complicated than this because of the vagaries of atmospheric evolution, albedo differences, and the problem of self-heating planets (like Jupiter and Saturn).

Another factor which is extremely complicated is the effect of planetary mass and surface gravity on wind and air pressure patterns. If Dole’s empirical relation between mass and angular momentum holds up,* then it is a fair guess that worlds with high mass will have higher velocity winds, in general. And there are other, more subtle problems. For instance, the winds on Mars often blow at more than half the local speed of sound. One wonders what a “transsonic meteorology” might be like.2037

* Using our own solar system as his source of data, Dole finds that angular velocity is directly proportional to the square root of planetary mass for planets which are not tidally braked or locked.214

Comparative meteorology

Figure 5.8 Different Patterns of Cyclonic Meteorology

figure 5 8A 450px
figure 5 8B line
figure 5 8B 450px

Some insight into comparative meteorology can perhaps be gained by looking at the peculiar manifestations of weather on other planets in the solar system. Mars has global-scale storms the likes of which have never been seen on Earth. Most every Martian year, dust storms enshroud the entire world in a dull-ochre blanket for months on end. Winds exceed 320 km/hr during this time — far in excess of most Earthly hurricanes. Yet Mars has roughly the same rotation rate as our planet, is colder and farther from Sol, and has a thinner and less massive atmosphere. How can such a magnificent storm develop?

A small, natural cyclonic disturbance is where it all begins (Figure 5.8). Airborne particles absorb more sunlight and heat up the surrounding gas; outside of this local turbulence the air is cooler. The temperature differential causes major winds to begin to circulate. While hurricanes on Earth are caused by water vapor condensation near the eye, Martian hurricanes get their energy directly from the sun.2044

Earth has a relatively massive atmosphere with large thermal inertia, so temperature changes occur only very slowly. Our planet thus has a long “response time” to change. Not so on Mars. The Martian air responds to changes in temperature in a matter of hours, because its thermal inertia is low. Winds can build up much faster.

The cyclonic disturbance grows larger and the winds go higher still. One planetologist has estimated that once the turbulence extends about ten kilometers vertically and perhaps 50-90 kilometers horizontally, the storm cannot be stopped,1313A kind of “runaway weather,” the Martian hurricane continues to grow until it virtually covers the globe. At this point, the thermal gradient which drives the winds lessens and finally disappears, and the storm soon begins to taper off.*

Science fiction writer Arthur C. Clarke has considered an unusual form of weather that might exist on cold terrestrials (like Titan), which are thought to possess large amounts of solid ammonia and gaseous methane. We know that the smaller the liquidity range of a thalassogen, the more volatile will be the meteorology. Sudden weather changes should be commonplace. As an example, liquid methane may be present in small pools on Titan in local cold spots on the surface. Because it has such a narrow liquidity range, the methane could abruptly flash into steam at the first gust of warmer air or if there is a momentary break in the clouds. The high winds thus generated, Clarke suggests, might be called “methane monsoons.”1947

Another hard science fictioneer, Hal Clement, has written of the peculiar behavior of weather on planets with very high surface pressures. Gases — and air — are generally at least a thousand times less dense than liquids. But what if we have an atmosphere with a base pressure from 100-1000 times Earth-normal? The air will take on liquid-like densities, becoming thick and viscous.1936

What can we say about the presence of frozen thalassogen on the planetary surface? It is well-known that for the greater part of its history, Earth was without polar icecaps. We have them now only because we are in the middle of an Ice Age. Ice Ages are believed by some to occur cyclically every 200 million years or so, triggered by small changes in Sol’s output or by orbital and rotational resonances.2068,3678

(Of course, icecaps need not form only at the poles. A tidally-locked, one-face planet might have a single icecap on the night side only. Or, peculiar resonances between planetary rotation rate and orbital eccentricity could give rise to icecaps located on either side of the equator — although this remains a strictly speculative possibility.2070)

Will all planets with open oceans have icebergs? The answer to this deceptively simple question actually has deep climatic significance. We know that the present climate of our world is in a state of very delicate balance. Surface conditions are largely dictated by the overall energy balance. The greenhouse effect acts to hold heat in and trap energy; Earth’s shiny polar caps tend toward the opposite extreme, reflecting energy back into space and cooling the planet.

* Because the Martian atmosphere is only 1% as dense as that of Earth, the wind packs only about 10% as much punch, An astronaut standing in a 320 km/hour gale on the surface of the red planet would feel the equivalent of a 32 km/hour wind on Earth.1313

Baroclinic Meteorology

Baroclinic flow:

  • Climate powered by large temperature differential between equator and poles (ΔT > 10-100°C).
  • Vertical pressure gradient minimal.
figure 5 8A

Characteristic of:

  • Planets with low pressures.
  • Planets with slower rotation.
  • Planets with negligible internal heating, or which are heated from above (e.g. an optically thick atmosphere).
  • Planets whose atmospheric constituents have relatively low heat capacity (e.g. O2, N2).
  • Planets having a solid surface.

CALMS are regions of “Coriolis pileup.” unstable with little wind, source of cyclonic disturbances (hurricanes). Cold, dry air falls, removes low altitude moisture, creating most of world’s deserts.

DOLDRUMS — moist, warm, rising air causes cloud cover “zone” at Equator ± 10° latitude.

PRESSURE REGIONS (low and high) form into localized eddies and whorls.

FEATURES persist for weeks (Earth) or for months (Mars).

Typical examples in our solar system:

  • EARTH, MARS (especially in Martian autumn and spring)
  • VENUS (single Hadley cell, “symmetric” regime circulation)
Barotropic Meteorology

Barotropic Flow:

  • Climate powered by vertical pressure gradient forces.
  • Temperature differential between equator and pole minimal (ΔT < 5°C).
figure 5 8A

Characteristic of:

  • Planets with high pressures.
  • Planets with fast rotation.
  • Planets with significant internal heating.
  • Planets whose atmospheric constituents have relatively large heat capacity (e.g. H2, He).
  • Planets with no solid surface.

ZONES contain moist, warm, rising air.

BELTS contain dry, cool, falling air.

WINDS flow around planet at zone/belt boundaries.

Low and high PRESSURE REGIONS girdle planet in a series of concentric zonal systems.

Atmospheric FEATURES can persist for centuries because there is no solid surface below the weather, and therefore any real frictional drag.

Typical example in our solar system:

Reverse greenhouse effect "Runaway icecaps" — an Ice Age

Icebergs are floating chunks of frozen thalassogen. This proves to be a destabilizing factor in Earth’s climate, because ice reflects energy away far better than the liquid water of the oceans. If there is a prolonged, unusually cold spell planetwide and abnormally great: amounts of ice are produced, more of Sol’s life-giving warmth is cast away by the highly reflective ice floating on the surface. Our planet cools because less heat is available. The icecaps spread, and Earth cools still further. The effect is the exact opposite of the runaway greenhouse discussed earlier, and might properly be termed “runaway icecaps” — an Ice Age.

On the other hand, if the solid form of the thalassogen is less reflective (i.e. darker) than the liquid, the climate should be relatively stable. Any ice formed during a sudden cold snap must subsequently absorb more energy than the surrounding liquid — and soon melt. Icecaps would be unlikely, Ice Ages practically impossible.

Similarly, if a thalassogen cannot form floating icebergs, then even if the ice is highly reflective it still will submerge below the surface of the liquid before it can give rise to thermal instability and runaway icecaps. That is, it moves itself out of the way before it can do much damage. Of course, one man’s bread is another’s poison. The lack of icebergs may promote a more stable climate, but it will also make biology much less likely.

If there are no icebergs, and frozen thalassogen sinks to the ocean bottom because it’s denser, then the sea may freeze from the bottom up and thaw only from the top down. Over the normal range of temperature variations, it is entirely possible that the whole body of liquid could freeze solid for various lengths of time. This is xenologically significant, as the viability of life in such an inimical environment must necessarily be greatly decreased.47,1551

Water is virtually unique

Water is virtually unique in this respect: The frozen form, water-ice, floats atop the liquid form. Water expands slightly when it freezes, so the ice is less dense than the fluid. (Only elemental bismuth metal and a very few other rare substances display this behavior.) Hence, where water is the thalassogen, bergs will float and life is not precluded by the threat of a planetwide oceanic freezeup during cold spells.974 (The price paid for this advantage is climatic instability — it would appear that Ice Ages are possible only if water is the thalassogen.)

Densities of Thalassogens

Table 5.10 Densities of Some Thalassogens of Interest2062,2063,2069

table 05 10 densities of some thalassogens 400

Not so with all other thalassogens of interest. As we see from Table 5.10, no other single thalassogen has the unique property of floating iceberg production. Even if we allow for a dual thalassogen system, say of ammonia and methane,1947 it is rather difficult to arrange for icebergs or floes of solid ice. Ammonia-ice not only sinks in liquid ammonia, but in liquid methane as well.*

But there are a few possibilities. Water icebergs should float on oceans of liquid oxygen, as should methane and ammonia bergs. Water-ice will also float on carbon dioxide seas at the right pressures. But sulfur, hydrogen, carbon dioxide and oxygen floes are probably out of the question on any kind of reasonable plet.

Many other specific meteorological phenomena are also of major interest to xenologists. For instance, clouds and fogs should be common in any atmosphere with reasonable pressures. Condensation nuclei will always be plentiful, and most thalassogens can condense to tiny droplets around them at moderate temperatures. Rain should likewise be a regular occurrence at the surface of worlds possessing large open bodies of liquid thalassogen. (Of course, other things may rain down — such as the periodic volcanic ash “rains” in Iceland.)

The height at which clouds form is a function of humidity, thalassogen vapor pressure, atmospheric thermal lapse rate, and a score of other interrelated factors. The suggestion that more massive worlds with higher gravity must have lower-hanging clouds2075 is simply too facile to be of much use to us.

Any planet which has clouds, rain, and sunlight reaching the surface will also have rainbows from time to time. These beautiful spectral arcs are the result of thalassogen droplets suspended in the air, acting as tiny prisms in concert to separate the incoming light into its constituent colors. The larger the droplets, the more intensely vivid the bow will appear.2149

Ignoring for the moment many other important factors, a larger planet with higher surface gravity will pull raindrops down before they have a chance to grow very large. Rainbows on larger worlds should tend to be rather dim, unimpressive affairs. On smaller worlds, where droplets can grow to larger sizes because they fall more slowly, rainbows should be impressive riots of color.2059 Furthermore, if there happens to be a very bright moon overhead or more than one sun; bows might appear in several parts of the sky at the same time.2059

* It should be noted that there are some six different allotropic forms of water-ice which form at various temperatures and pressures. Only one of these — ”natural ice” or ice I as the chemists call it — is lighter than water. Ice II through ice VII all sink if placed on the liquid.


larry niven

How about lightning discharges? Electrical storms occur because molecules are split apart in the upper atmosphere to ions, which are then carried to the ground by dust and rain. This charges up the planet to at least half a million volts from ground to top of atmosphere — a process likely on any world, save for the exact details of scale height and voltage. Planets with regular and intense sand or dust storms may generate intense electrical fields that could lead to more severe or more frequent discharges.1232

Another important factor is the breakdown voltage of the air — the voltage at which a spark will jump a gap of unit distance. A charged cloud may be 100 million volts higher than the surface below, which is high enough for the “spark” to leap to earth. The spark gap voltage for dry air (at 1 atm) is usually listed as 11,000 volts/cm, and can be corrected for variations in temperature, pressure, and humidity. Now, if the atmosphere was comprised of a more conductive gas, such as neon, the spark gap voltage would only be 800 volts/cm (at 1 atm). This means that lightning should occur more frequently in neon (hydrogen, helium, etc.) than in oxygen (nitrogen, halogens, etc.).

This prediction may perhaps claim some support from the radio observations of Jupiter in the last decade or so. Decameter radio wave outbursts lasting from seconds to hours have been detected, with an equivalent energy of trillions of terrestrial lightning strokes per event.609 Similar outbursts have been observed on Saturn.2097

Auroral displays

Will alien worlds have auroras too? Probably. These displays appear at the north and south planetary magnetic poles, and are caused by the funneling of solar wind ions in the converging magnetic field of the planet. Rapidly rotating, massive worlds should tend to have stronger magnetic fields. Also, hotter stars most likely have more vigorous solar winds. We would guess that a 4 Mearth planet with a ten-hour day circling an F5 sun will probably have far more striking auroral displays than a tidally-locked 0.4 Mearth planet orbiting a K2 star.

Mirage physics is also rather interesting. On Earth, mirages often result when there is a layer of warm air lying close to the ground. This air, being hotter, is less dense. It acts as a giant lens. Light coming from the sky near the horizon swoops down close to the ground and is refracted back up.2073 The mirage of water on an open highway is just a smeared-out image of blue sky.

Mirages on Earth generally appear about 100 meters away from the observer at ground level. On Mars, where the atmosphere is so thin the air is hardly heated by the ground at all, the refraction layer is thinner.950 The mirage backs away, out to about one kilometer. (To date, no Martian mirages have been photographed by the Viking landers, possibly due to the extreme roughness of the terrain and because the camera horizon is too close.2094)

Fishbowl effect simulation

On planets with very high density air, as on Venus, the mirage concept literally takes on new meaning. The transfer of heat from ground to near-surface air is complete, and it is believed by many that the extreme refraction near the ground will cause a kind of “fishbowl effect.”15,2060 The horizon would appear above the observer at all times,* appearing to bend upward at the sides.2034 (The idea has already been used in science fiction.2071)

Dr. Conway Snyder at the Jet Propulsion Laboratory in Pasadena, California has performed a numerical simulation of the light-bending phenomenon at the Cytherian surface.2066 Let us imagine with him, for a moment, that we are aliens on the surface of Venus. Our eyeballs can see into the microwave region of the spectrum as well as the visible. What do we see?

The horizon appears to be elevated upward, all around us, at 9.4° from the horizontal. (Only 5°, if visibility drops to 200 kilometers.) Since Venus rotates backwards, the sun rises in the west and sets in the east, creeping across the sky at an imperceptible eight minutes of arc per hour. We are standing at the equator at the time of the equinox, so Sol lies directly over head at noon, Cytherian daylight time.

As the sun slowly falls toward the horizon, its shape begins to change. Its vertical dimension commences to shrink, while the horizontal component remains unchanged. At 6 PM Cytherian time, Sol should just be setting — but it isn’t. Instead, it lies 10.4° up, but is squashed down to a quarter of its normal size. By 7 PM the squashing has become 250:1 compared with the horizontal dimension, and by 8 PM, 30,000:1.

Sometime close to 12 PM, the tiny solar sliver suddenly increases in length dramatically, and at the stroke of midnight wraps itself around the horizon in a pencil-thin ring of light. The line then breaks in the east, the sun begins to reassemble itself in the west, and sunrise begins.

If we are more than 3/8° away from the solar latitude, however, the ring of light will not appear. Instead, we see the compressed sun-image “crawl like a worm across the horizon during the night, from the point where it has set to the place where it is planning to rise.”2086

* Calculations indicate the effect would be rather small, though, perhaps a few degrees inclination at most.2068 The first pictures back from the two Russian Venera spacecraft that landed on Venus in 1975 showed no evidence of the fish bowl,2034,2079 but since the maximum range in the photos was only a few hundred meters the issue remains unresolved.

5.4.2 Sky Colors
Why the sky is blue

Figure 5.9 Scattering of Light in Planetary Atmospheres

Scattering of light by airborne particulate matter1993,1994,1995

figure 5 9A 350px

The family of curves at right represent the relative amount of light scattered away — at each particular frequency of light — by a hypothetical cloud of perfectly spherical, transparent, uniform size droplets suspended in air. However, ideal Mie monodispersions rarely occur in nature. Clouds of particles are of various shapes, textures, sizes, colors and degrees of opacity. Consequently, this graph is an abstracted, idealized version of reality and should be interpreted in that spirit.

The beginnings of Rayleigh scattering are seen in the visible portion of the spectrum, in the right-hand section of the graph, where the particle radius r falls below a tenth of a micron or so. The flat parts of the curves stretching horizontally to the left indicate uniform scattering of all frequencies of light. The wavy parts in the middle demonstrate the oscillatory nature of the preferential scattering by color.

Extinction of light by passage through a cloud of particles1995

figure 5 9B 350

The three curves at right are simply a slightly different way of looking at the data in the previous graph.

Here, we compare the relative attenuation of blue, green, and red light as it passes through the same cloud of idealized haze/fog particles we considered before.

Note that for small droplets in the air, blue is preferentially scattered giving a blue sky. At large particle sizes, no frequency is preferred and the sky washes out white. For intermediate scatterer radii, red and blue alternate in their supremacy. Note also that green, whenever it predominates, is accompanied by large amounts of the other two colors — leaving a largely white sky with per haps the faintest of greenish tinges.

Particles responsible for natural atmospheric scattering1994

figure 5 9C 350px

This table lists the approximate size (in microns) and concentration of typical scattering particles normally present in Earth’s air.

Aitken nuclei are hygroscopic (water-absorbing) microscopic condensation nuclei, the result of chimney flue gases, tobacco smoke, sulfur dioxide industrial emissions, and a host of natural sources.

What about the color of alien skies? Must they always be blue?2059 Of course, ETs will probably have different physiological seeing equipment than ours, but we shall permit ourselves the minor anthropocentric convenience of viewing their world through human eyes.

Light that reaches our eyes from the sky is merely sunlight scattered by the atmosphere. Had the Earth no air, our sky would appear quite black. This explains where the light comes from, but not why it is blue.

In 1899, a famous Englishman by the name of Lord Rayleigh devised an explanation for the color of the sky (Figure 5.9). According to his mathematical theory, scattering from very small particles (such as air molecules) increases as the fourth power of wavelength.1995 This means that blue light, which has a very small wavelength, is highly scattered, while red light, with a relatively long wavelength, is scattered much less — sixteen times less, in fact.1990,1991 So the blue light is preferentially removed from sunbeams and spread out uniformly from horizon to horizon. A little red is also present, and some yellow and green too, but blue is clearly predominant.

Scattering theory

We can correct the Rayleigh theory for differences in planetary surface pressure and temperature. It turns out that the amount of light scattered is directly proportional to the atmospheric pressure, and inversely proportional to the temperature.1994 So if we double the pressure we double the amount of light scattered in all colors — and the sky gets brighter generally. Doubling the temperature has the opposite effect: the intensity of scattering is cut in half. On the surface of a high pressure planet like Venus, the effect would be rather extreme. All colors would be so strongly scattered that the sky be comes a dim, featureless white.2059

In a perfectly clear, Earthlike atmosphere, the sky would be a rich blue hue. But we observe it to be a hazy, lighter blue. Why?

The Rayleigh theory applies only to particles which are much smaller than the wavelength of light, say, less than 10-100 Angstroms.1994 If the scatterers in the air are much larger than this (as with dust in the atmosphere), Rayleigh’s formulation breaks down and the vastly more complicated Mie theory must be used1995 — the details of which are beyond the scope of this book.

Rayleigh’s theory tells us that particles smaller than about 0.1 micron will preferentially scatter blue light. The Mie theory explains the behavior of atmospheres containing particles larger than about 4 microns. Above this critical size all frequencies of light are equally scattered, and the result is a gray or white sky. (Since there is always plenty of particulate matter, water haze and industrial pollutants floating around in the air — perhaps 100-1000 kg over each square kilometer — the sky’s sharp natural blueness is washed out unless we move to higher altitudes.)

Between 0.1 and 4 microns, the Mie theory becomes especially complex.1995 The selection by color oscillates, sometimes preferring to scatter more blue and sometimes more red.1993,1995 This effect is extremely sensitive to particle size. A uniform haze of 0.4 micron particles would scatter more blue (blue sky), but a similar cloud of 0.6 micron particles would produce more red (red sky).1993,1994

If this is true, why don’t we commonly see such vivid colors in natural Earthly hazes and fogs? The reason is the natural fogs and mists contain a mixture of all sizes of particles, from one to ten microns or larger.1995 As a result, these interesting color effects are added together randomly and average themselves out to a bland whiteness — which we do observe. If some reasonable mechanism could be proposed to get particles of a single, specific size into the atmosphere (i.e., a “monodispersion”); quite beautiful red and blue sky colors would be possible.

Barring this fascinating alternative, as particles of increasing size are added to a “Rayleigh atmosphere” the sky color will appear to change from dark blue to powdered blue, to whitish blue, and finally to grayish white.

A third factor affects sky color besides Rayleigh and Mie scattering. The color of the particles themselves is very important. A red particle, for instance, absorbs all light but red — which it reflects. Thus, it appears red in color. A green particle tend: to absorb blue and red but reflect green. (Under red or blue light such a particle would look black, but in green light it looks green.) So an atmosphere heavily laden with, say, green dust particles should also take on a distinct greenish hue.

The Red sky of Mars

We are now in a position to understand why the sky of Mars is red.1989,2035 We add up the contributions from three effects: (1) Rayleigh scattering should give blue sky light, but will only be about 1% of its intensity on Earth because the Martian air pressure is only 0.01 atm;2035 (2) Dust motes an estimated two microns in diameter1989 should produce a bright haze without color by Mie scattering; and (3) Particles in the Martian atmosphere are reddish surface dust, which reflect red light while preferentially absorbing blue and green. Hence, the sky of Mars is unusually bright, and appears a hazy “salmon pink” or “orange cream”1989 (“embarrassed brick”?2035). It is clear that many other sky colors are similarly possible, provided a planet can be found with fine surface dust of the desired color.

Examples of non-blue skies

There are other ways to get non-blue skies. For instance, we have discussed the process of frequency-selective light absorption by dust particles. Molecules of gas exhibit this property too.619,620 The sky would no longer be blue under a fluorine atmosphere, to take one example. This gas absorbs blue strongly, and appears pale yellow in color. The sky would take on this color.

Chlorine air should appear green, because it absorbs light preferentially at the blue and orange-red ends of the spectrum. Similarly, an atmosphere of nitrogen dioxide would provide an orange-brown sky. If sulfur vapor is available, the air would alter color dramatically with large temperature changes. Near the boiling point at 720 K the sulfur sky would be dark yellow; as the temperature climbed to 770 K the atmosphere would turn a deep red, returning to straw yellow at about 1120 K.

The problem with using gases such as these is that they absorb light too darned well! At one atm pressure, a few meters of pure chlorine gas would transmit no visible light.2059 This is because even though blue and red are removed preferentially, some green is also eliminated. The sulfur vapor fares no better, sadly. At 1 atm pressure, blue light is cut to below human eye visibility in less than half a meter, and the red is gone in fifty meters.

So if the partial pressures of any of the aforementioned gaseous absorbers exceeds perhaps 0.001-0.01 atm, no light of any color will be able to reach the surface of the planet from the outside. Any inhabitants there must find their way around without the assistance of eyeballs.

If we want to use gaseous absorbers, it is better to choose weak absorbers instead of strong ones. For instance, under a deep ozone atmosphere the sky would probably appear reddish, because the gas is known to slightly absorb blue, yellow, and green sunlight rather well. Methane and ammonia, weak absorbers as they are, would provide a lovely blue-green sky (because absorption is mainly in the red) assuming the atmosphere was thick enough.2059

Blackbody radiation

If the temperature at the surface is sufficiently high, another factor must be taken into account: blackbody radiation. Just as a stove’s heating element glows red when it is hot, so will the surface of a fried world like Venus. On Venus, red light emitted by the hot rocks could be orders of magnitude brighter than terrestrial moonlight — about like Earth on a dark, rainy day. In the blue the intensity would be about 100,000 times less than in the red, roughly 10% as bright as moonlight. Since red clearly predominates, reflections off the cloud base will give the appearance of a red sky, assuming fair or good visibility.

Still another trick to get colorful skies is to arrange for permanent colored cloud covers. Arthur Clarke suggests in Imperial Earth that the skies of Titan may be white with beautiful orange and red streaks and whorls, because of the presence of hydrocarbons and other organics in the atmosphere.1947 This is similar to what is believed to impart coloration both to the orange bands and the Red Spot of Jupiter. Unfortunately, 20th-century humans are unlikely to find photochemical smog a very attractive method of obtaining unusual sky colors.

Other aesthetic possibilities

More aesthetically appealing are the possibilities of continuous luminescence, phosphorescence, and fluorescence as an adjunct to sky color phenomena.1991 But perhaps the most intriguing of all is the striking sunset effect called the “green flash,” which occurs just after the sun has dropped below the horizon.1992 The red and yellow light is not refracted enough to reach the observer at this point, and the blue has all been scattered away. This leaves only green, which is experienced as a brilliant flash during optimal viewing conditions.2059

But flashes on other planets could appear vastly different. Even on Earth, blue and violet flashes have been seen at higher altitudes.1992 On low-pressure worlds, where blue is scattered less (as on Mars), blue flashes may be the rule. the planet's rotation is slow enough, the “flash” could become a “glow,” lasting for seconds or even minutes.

Rayleigh Molecular Scattering

Table 5.11 Rayleigh Molecular Scattering in Planetary

Atmospheres as a Function of Stellar Class

table 05 11

It might be supposed that by changing stars one might be able to affect the color of the sky. After all, sky light is just scattered sunlight, and a class K sun puts out a lot more red than a class F star. However, as we see in Table 5.11, the consequences of illuminating an Earthlike atmosphere under the light of different stars are not great. Blue will predominate in the Rayleigh sky color, even if light from the coolest, reddest class M sun is used. On the other hand, we note that a terrestrial planet circling an F5 star will have skies of much deeper blue than a world associated with, say, a K2 sun. Stellar class is at best a very fine adjustment to sky color, in capable of countermanding the dictates of the atmosphere.

What about the appearance of the primary itself, as viewed from the planet’s surface? If the planet is in orbit around an orange or red star, the sun would seem bigger and redder than Sol does in our sky. Colors at the surface, illuminated by sunlight, would appear slightly different — the blues darker and the reds brighter. Shadows would have blurrier outlines than those on Earth. But an F5 star might cast sharper shadows, with a slight bluish tinge.877

As far as color is concerned, if the observers are beneath an atmosphere which either scatters the blue (blue sky) or absorbs blue preferentially (red sky), then light from the star will lose blueness and appear redder.1991 This effect is most striking at sunset on Earth, when the blue in Sol's rays is so completely attenuated that fiery red alone remains.1990 Were the surface pressure perhaps five or ten times greater, Sol would appear similarly reddish at high noon and deep crimson at sunset (but much dimmer). Wispy puffs of clouds would catch the ruddy solar rays throughout the day, streaking and mottling the luminous azure sky with magnificent ever-changing patterns of coralline and cerise.

If the observers are at the bottom of an atmosphere which absorbs the red (blue sky) or scatters the red preferentially (red sky), the sun will appear bluer than normal.1993 This effect has been seen, albeit rather infrequently, on Earth from time to time. Owing to the presence of particles at high altitudes following the great volcanic eruption at Krakatoa in August, 1883, the Moon took on a distinct blue-green color. This phenomenon of “blue moon” was observed in Great Britain on September 26, 1950, due to widespread fires covering a quarter-million acres of forestland in northern British Columbia, and on other occasions elsewhere.360 Blue suns and green suns are also possible in the same manner,1993 and have been observed infrequently.2077

5.4.3 Astrogeology
Tectonic and seismic activity

bernard de fontenelle 360

While the skies and seas of alien worlds are fascinating subjects for discussion, it is mainly upon the surface of a planet (its crust, or lithosphere) that life evolves and flourishes. Scientists who study mountain-building (orogeny), tectonic and seismic activity, and the construction of worlds generally, call themselves “astrogeologists” or “astrogeophysicists.”598,2144

As Dole has pointed out, our knowledge of the forces responsible for earthquakes, volcanoes, and mountain-building is still incomplete.214 One suggestion is that quakes and volcanoes are more likely on planets with higher gravitational compression and more internal heat generation due to radioactive decay. Planets smaller than Earth would tend to have less gravitational contractive force, relatively larger surface areas (compared to total mass) across which to radiate heat off to space,1237 and relatively smaller volumes of heat-producing radioactive substances. Small worlds will thus tend to have lower internal temperatures,1237 thicker and more solid crusts, and therefore much less volcanism and seismic activity.

Larger planets have relatively great volumes of radioactive material, higher gravitational compressive energy, and comparatively smaller surface-to-volume ratios (so it’s harder to get rid of heat).1237 They should have larger molten cores, mantles that rise closer to the surface, and thinner crusts that can buckle and slip around more easily. If these suppositions are true in general for high-mass terrestrial worlds, more frequent and more severe quakes might be predicted, as well as higher levels of volcanic activity.

This theory squares with the reported characteristics of planets in our own solar system. The lightest world that has been intensively investigated is the Moon, within which only the faintest tremors have been detected deep below the surface.2056 The lunar lithosphere has solidified down to a depth of roughly 1000 kilometers.1291,2043 When the core loses heat and contracts, the mantle is so thick and rigid it cannot buckle. Consequently, there is no real geologic surface activity on the Moon.1291,2043

The towering Olympus Mons

Mercury, the next most massive world examined by astrogeologists, is believed to have no surface tectonic activity at this time — although various surface migrations and volcanism a few eons ago are evident.1565,2040 Mars apparently has seismic activity. The red planet also seems to have some lithospheric collapse due to mantle contraction, but there is no clear and convincing evidence for horizontal plate movements across the surface. It has been suggested that on Mars we may be seeing “incipient plate tectonics . . . where one plate is beginning to break away . . . like the Earth, about two hundred million years ago.”598 The towering Olympus Mons (formerly “Nix Olympica”1323), at 26 kilometers high the largest mountain in the solar system, bears mute testimony to the presence of extensive and fairly recent volcanism on Mars.2072

Earth has well-developed tectonic activity, plenty of active volcanoes, and a crust only about 30 kilometers thick.367 Radar probes of Venus, our sister world, have found low mountain chains suggestive of at least a moderately active lithospheric environment.1214,2041

Presumably, the core of a still larger terrestrial planet would be more massive and hotter, pushing the mantle closer to the surface. The thinner crustal sheet would buckle, slip and shake far more readily than does Earth’s rocky skin. Quakes would probably be more violent and more numerous, and breakthroughs in the crust by hot magma (volcanoes) should be widespread and commonplace.

Factors and forces of mountain building

What kinds of mountains are alien worlds likely to possess? The building of mountains is an extremely complex process, depending on planetary mass, gravity, composition, heat flow rate through minerals, air pressure and wind velocity, and a host of other factors. For instance, on larger worlds rivers may flow downhill faster because of the higher gravity, which may cut deeper valleys and canyons.

Perhaps one of the most significant astrogeological advances in this century has been the development and elaboration of the theory of continental drift. Continents are now known to be small plateaus of granite embedded in much larger “tectonic plates.” The entire Earth’s crust is believed to be fragmented into a mosaic of perhaps eight of these plates, rigid shifting masses of solidified lithosphere which have been described as great tabular “icebergs” of rock floating on the surface of a “sea” of denser subjacent mantle material.2140,2141

Plate tectonics

Plates are believed to be about 100 kilometers thick,2140 and may move literally thousands of kilometers across the surface of the planet in only 100 million years or so.2142 Convection currents in the deep mantle have been proposed as the prime mover of the plates, circulating the viscous magma in localized “cells” much like the currents of water in a flat pan which is heated from below.2141

Because the continents are always on the move (though they change shape very little as they travel piggyback around the world2142), each has a trailing edge and a leading edge. The trailing edge is tectonically stable, so mountain-building is minimal. But the leading edge is forced downward with the descending mantle currents; the lighter, more siliceous materials that comprise the continents pile up at the site of subduction.2141 Great mountains are born. (One of the clearest examples of this process occurred during the Cenozoic Period, when the Indian Plate smashed into and dove under the Eurasian Plate, throwing up the mammoth Himalayan ranges.2140)

From the arguments presented earlier, it is at least plausible to advance the hypothesis that more massive planets will have more internal energy available to drive the thermal convection currents in the mantle, and should therefore produce greater tectonic thrusting and more extensive mountain chains.

Square-Cube Law

Figure 5.10 Maximum Size for a Planet's Mountains1279

figure 5 10 350px

The graph at right gives the “maximum statically loaded topography” supportable by a range of different materials.

  • The curves are based on the assumption that if the interior pressure created by building the mountain exceeds the compressive strength of the materials, then the mountain will “fall down.”
  • Planetary radius R is the horizontal axis, and h, the maximum height of mountains (or depth of depressions), is the vertical axis, both in kilometers.
  • For weaker materials — such as water-ice — the topographic relief must be far less than if rock is used.
  • No materials are expected to have much greater strength than taenite, so all planets should be found below this line. (Note the extreme position of Jinx, a hypothetical egg-shaped planet devised by science fiction writer Larry Niven.451)
  • Note the relative weakness of the ices — if Titan has only ammonia-ice mountains, they cannot be larger than two or three kilometers.

Maximum mountain heights in our solar system are roughly as follows:

  • Mercury — 3 km,1563
  • Venus — from 1-2 km,2041
  • Earth — from 8-11 km
  • Luna — highest peak is 6.8 km high (Theophilus)
  • Mars — highest peak is 26 km high (the volcano Olympus Mons)2072

Like all material bodies, mountains are subject to the Square-Cube Law. This principle is, quite simply, that volume increases faster than area as size increases. For a mountain to remain standing and not collapse, it must be strong enough to support its own weight. This weight is distributed over an area. The weight that must be supported, however, increases with the volume. (For example, mountains with eight times more mass have only about four times more base area to support that mass.) Consequently, a mountain should be less capable of sustaining its own bulk as it increases in size.

The maximum height of rocky ranges is therefore proportional to their weight, the product of the mass and the force of gravity (Figure 5.10). Higher gravity planets will have smaller, squatter mountains, because the limits of compressive strength of rock are reached much sooner. At least down to about 0.1 Mearth or so, smaller worlds should tend to have taller formations.

As has been discovered with craters on the bodies in our solar system,1277the height of mountains should statistically vary inversely as the force of surface gravity.*

* Astrogeologists will recognize that I have made a gross oversimplification here. The mountains of large differentiated planets are actually supported by isostatic forces. Only small bodies can accurately be considered to have statically loaded topography.1279

Densities and Compression Strengths

Table 5.12 Densities and Compression Strengths

table 05 1

Mountain size will also be related to the compressive and shear strength of the building materials used.1233,1279 The maximum height of ranges will vary approximately linearly with the compression strength (Table 5.12). For Earth mountains, rock is the usual orogen* with a maximum sustainable load of about 107 kilograms/meter2. However, were we to find mountains of carbon dioxide on another planet, the greatest height would be far lower. This is because the compressive strength of “dry ice” is less than 10-30% that of rock.1569

Volcanism could be a peculiar affair on other worlds. On a planet as cold as Titan, for instance, water could be an orogen instead of a thalassogen. If sufficient crustal radioactivity exists, and if the planet is roughly terrestrial-sized, we might observe cold volcanoes spewing forth molten water instead of lava.1947 Dr. Donald M. Hunten, a physicist at the Kitt Peak National Observatory, believes that Titan may possess just such a subsurface magma of liquid water.2046 The magma would lie atop a rocky mantle and would contain large amounts of dissolved ammonia. The relatively thin crust should then be a mixture of methane and water-ice, frozen solid.

A curious phenomenon is the flowing of glaciers (mountains of water-ice). There is some evidence that this may be virtually a unique property of H2O “mountains,” One of the more unusual characteristics of water is its ability to drop its melting point when subjected to pressure. Underneath a glacier pressures rise to hundreds of atmospheres. A lubricating layer of melted ice can form at the base, and the object proceeds to slide downhill on this thin, slippery film of water.

While ice exhibits the freezing point depression effect up to pressures of more than 2500 atm, solid carbon dioxide and other ices cannot duplicate this behavior. Only water-ice will flow rapidly down valleys like rivers. One Alpine formation, the Quarayaq Glacier, is known to flow between 20 and 24 meters per day.1850 (Of course, CO2 glaciers are still subject to slow creep,1569 but this is far less dramatic.)

If mountains are subject to the Square-Cube Law, are not worlds as well? Small, mountain-sized hunks of matter may be very irregular in shape, because the internal stresses are relatively low. But as mass increases, pressures build: Inside any terrestrial planet rock begins to flow and seek a spherical shape — energetically the most stable configuration.

* Derived from the Greek roots, meaning, literally, “something that produces mountains.” I use the word to signify “any substance capable of forming planetary mountains.”

Maximum Size of Oblong

Table 5.13 Maximum Size of Oblong (e = ½) Bodies,

for Various Orogens1279

table 05 13 maximum size of oblong 400

Stephen Dole has estimated that the largest mass of a body that can maintain a highly irregular shape is on the order of 10-5 to 10-4 Mearth.214 To get some idea of the degree to which an object may deviate from sphericity, Table 5.13 gives the largest size of a body whose mountains are as tall as the planetary radius itself (the long axis is twice the short). These worlds must be very small to retain their egg-shape.

Finally, returning once again to peculiar surface effects, the astrogeologists may have some real surprises in store for us on other worlds. For example, we know that Venus’ air is deficient in oxygen, and one explanation is that the surface rocks have all been well-oxidized. But at temperatures beyond 620 K and pressures above 50 atm, superheated steam dissolves alumino-silicate rocks. If the oxygen depletion theory is correct, Venus might once have been molten to considerable depths and served as a factory for huge, exquisite gemstones.1293 The surface of the Morning Star may well be studded with garnets, sapphires, rubies and topaz!

5.5 Planetary Habitability
Planetary Mass and Pelagic Worlds

Figure 5.11 Planetary Mass and Pelagic Worlds367,2044,2046

figure 5 11 500px

We have barely scratched the surface of the total field of general planetology in this brief survey, and most if not all of the discussions have been simplifications of vastly more complicated processes. The concept of habitable zones, for instance, is a very old and respected idea but one which should not be engraved in stone and rendered sacred. Countless ways can be imagined to “beat the heat.” Some of the more obvious of these are surface effects on the planet itself and have nothing to do with the stellar class of the primary.

For example, the greenhouse effect adds about 30 K to Earth’s temperature, and about 500 K to that of Venus. In Titan’s air, methane and hydrogen might trap solar energy and heat the planet significantly. Calculations indicate that if the surface pressure is on the order of 0.1-0.4 atm, the greenhouse effect could easily add 60-110 K. This would raise the temperature at the surface of Titan to 150-200 K.1280,1281 Were Titan at the distance of Jupiter instead of Saturn, another 30 K or so increase could probably be arranged — putting it very close to Mars, temperature-wise. There are indications that even chilly Neptune may have a greenhouse amounting to some 80-90 K.2046

A second warming factor is the presence of small-particle smog suspended in the air of Titan. These darkened organic dust motes can absorb sunlight and transfer still more heat to the surrounding atmosphere.2046 So we see that perfectly valid arguments may be made to extend the outer reach of the habitable zone of Sol as far Jupiter and possibly even Saturn!

What are the limits of mass for habitable planets? Again, the answers don’t come easily. In selecting worlds that might be habitable for human life, Dole set forth the following values: Mass should be greater than 0.4 Mearth, to ensure that a heavy enough atmosphere can evolve and remain trapped, and should be less than 2.35 Mearth, to keep the force of gravity below 1.5 Earth-gees.214 Planetary mass will also affect the likelihood of finding planetwide oceans (Figure 5.11).

While these are useful estimates, they are clearly rather conservative when applied to all ET lifeforms instead of just to humans. Rasool expects that in a few eons, Mars’ atmosphere will thicken sufficiently for it to begin evolving towards a more Earth-like clime.2065 The mass of Mars, however, is only 0.11 Mearth. And while human life may be uncomfortable at more than 1.5 gees, there is absolutely no rationale for using this as the cutoff for all carbon-based intelligent life. Accretion models suggest that terrestrial worlds may form with masses as high as 5-10 Mearth,1258 with surface gravity reaching at least 2.2 gees.

Tides Raised on an Earthlike Planet by Satellites of Various Masses and Distances

Figure 5.12 Tides Raised on an Earthlike Planet

by Satellites of Various Masses and Distances

figure 5 12 329px

Assuming a very homogenous pair of fluid bodies, the tidal height H may be expressed mathematically as:

 H = constant x


Msat RP4

— — — — — —

, or
 H = constant x

— — — — — —


■ where Mp and Msat are planetary & satellite mass,

■ Rp and Rsat are planetary and satellite radius,

■ rp and rsat are the respective densities,

■ r is the average distance between the two bodies.1980

(Those equations are based on a highly oversimplified model — for fuller treatmentsee Alfvén and Arrhenius1980 or Goldreich and Soter.1243)

Another factor we have not really considered is the tides caused by satellites (or by the primary). Tides may occur in the lithosphere and atmosphere, but are most effective when they arise in the hydrosphere — the ocean. A moon which is very massive, or quite close, will tug at its primary much more insistently and raise higher tides (Figure 5.12).

The tides are important because they will alter the erosion of continents, wave motions in the sea, the weather, and so forth. Larger tides will slow the rotation of the planet, depending on the distribution of land masses, and may have enormous implications in the emergence of life from the sea.

There are additional complicating factors. Peculiar tidal resonances are known to occur. For instance, we now know that Mercury is not a one-face planet as was once thought. Instead, it turns on its axis exactly three times for every two trips around the sun. (A case of “spin-orbit coupling.”2048) Venus also appears to be “tidally locked” — but to Earth.2041 The sun must similarly be taken into account. Sol is responsible for only about one-third of Earth’s oceanic tides, but a planet in the habitable zone of a K2 star would experience far greater tides even if it had no moon.

Tilt of the planet’s axis

The tilt of the planet’s axis is likewise significant with respect to habitability.* All of the ecospheres computed in this and the previous chapter were based on the assumption of a relatively low inclination to the orbital plane. (Earth is about 23°, which is fairly typical.) A planet with high inclination will have more extreme seasonal temperature variations across its surface. Large tracts of land may become totally uninhabitable, although marginal livability apparently can be retained for tilts as high as 81°.214

The tilt of a world is responsible for its seasons. Planets with 0° inclination should have relatively humdrum, monotonous climates all year long (although an especially eccentric orbit might produce season like effects). With no seasons, there would be no regularly changing weather patterns, no cycles of autumnal death and vernal rebirth in the plant kingdom, no migrations of fish and fowl. The entire rhythm of existence would be lacking, and the influence on culture, religion, philosophy, and the agricultural sciences must necessarily be enormous.

Many rare and exotic environments for life may exist in our Galaxy.214 A “superjovian orbiter” might derive life-giving heat from the gas giant it circled. Inhabitants of this terrestrial world on the side that permanently faced away from the superjovian would scoff at tales of a giant Thing in the sky and reports of strange native religions brought back by intrepid explorers who had visited the other side. (The auroras there should be fantastic, if Io turns out to have beautiful yellow displays as many believe.2047,2090)

Earth-Moon system, a double planet

The Earth-Moon system is for all practical purposes a double planet, and it is not unreasonable to suppose that in many stellar systems across the Galaxy two Earths orbit one another. A world with two habitable belts, which might be found nearer the inside edge of the stellar ecosphere, is also a distinct possibility. Only the polar regions could be livable — the tropics would be unbearably hot.

There may be starless worlds, as the late astronomer Harlow Shapley suggested, bodies which lie alone out in the cold of interstellar space.816 Life is possible only if these planets are self-heating.18,2061 (Hal Clement used this idea in his science fiction story entitled “The Logical Life.”)

Perhaps we will find pelagic worlds, or terrestrials with Saturn-like (or Uranus-like) rings, or planets with large liquid bodies at the surface maintained near the triple point of the thalassogen. The ocean would boil furiously while gleaming icebergs floated and tossed on the frothy sea. The possibilities are as limitless as the imagination.

* Orbital eccentricity is also important — e must be less than 0.2 if at least 10% of the surface is to remain human-habitable.214+

Part II ♦ Xenobiology

Chapter 6 ♦ A Definition of Life
6.0 A Definition of Life

The ubiquity of life

manfred clynes 283In earlier chapters we considered the astronomical environment which extraterrestrial lifeforms must cope with. Other galaxies, stars, and countless planets appear amenable, if not perfectly hospitable, to life.

Since no ETs have been detected outside the Earth to date, it might be argued that any statements regarding the ubiquity of life in the universe must necessarily be pure speculation. But this is not so. We have the incredibly good fortune to be alive at the first moment in history when this tantalizing question can be approached with rigor and in some detail.20 Not only can we draw certain tentative conclusions regarding the existence of extrasolar planetary systems, but we may also seriously discuss whether or not other worlds will possess environs which permit, encourage, or demand the emergence of life.

It is probably true that a good many planets are merely dead bodies of rock washed by sterile seas.939 Much depends on whether life originates quickly and regularly given suitable conditions, or if it requires an event so improbable that evolution in any reasonable time is scarcely possible on any world.


The study of the origin of life, called "abiogenesis" by many researchers in the field, is highly relevant to xenology and xenologists. By determining the conditions that existed on the primitive Earth, and by duplicating them in the laboratory, scientists can attempt to recreate events that must have occurred on this planet billions of years ago. Should these experiments indicate that the fundamental chemical building blocks of life are easy to generate — perhaps even inevitable under the proper circumstances — then we might well be justified in concluding that biology is a fairly widespread phenomenon among the many worlds of the Milky Way.

Studies in abiogenesis give some clues as to the universality of those processes which lead to the emergence of life. Of course, any rigorous discussion must include a good working definition of the subject of discourse. When we say we are searching for "life," what do we really mean? The traditional wisdom that "if it wiggles, it’s alive" is insufficient to deal with exotic lifeforms which may have little in common with organisms on Earth.50

We must also remain sensitive to yet another aspect of the problem of the origin of life. We 180-centimeter-high lifeforms with mere 70-year lifespans all too easily lose sight of the broader perspective we need to appreciate the vastness of space and time. This "chauvinism of scale" is simple to identify but almost impossible to overcome.

… in terms of mere planetary
spatial frames, biology is only
an impurity, a trace constituent
of the cosmos.

In one sense, life is both abundant and ubiquitous on Earth. The live weight of microscopic organisms in an acre of soil to the plow depth of 18 cm has been estimated as more than two tons.* But viewed from a slightly different perspective, life fades into obscurity. The entire Earth weighs 6 × 1024 kg, the whole atmosphere only 5 × 1018 kg. The total mass of the biosphere is no more than 1016 kg, about 0.2% as much as air or 0.0000002% of the entire planet. The mighty works of man and nature are a kind of biological rust, clinging doggedly to the surface of a small world.20

So even in terms of mere planetary spatial frames, biology is only an impurity, a trace constituent of the cosmos.

Perhaps an even more relevant problem of scale is what might be called "temporal chauvinism." Man tends to think in terms of timescales commensurate with his own puny lifespan. But if we are to comprehend the meaning and the magnitude of evolutionary processes that lead to the origin and development of life, it becomes necessary to overcome temporal chauvinism. Centuries are of little concern in this arena — it is only the millions and billions of years that count.

Events which seem unfathomable in the usual time frame become more sensible on geological timescales. Indeed, it appears that the key to evolution is time. As one scientist puts it,

… in two billion years the impossible becomes the inevitable.702

A proper sense of the passage of time enables us to firmly grasp, not only the origin of life and the evolution of intelligence in the universe, but also such seemingly diverse topics as comparative culturology, technology gaps and alien thought processes, suboptic communications lag times, and the mechanics of galactic colonization.

* This includes 900 kg of molds, 450 kg of bacteria, 450 kg of branching unicellular organisms (Actinomycetes), 100 kg of protozoa, 50 kg of algae, and 50 kg of yeasts. Viruses are present in great numbers, but their mass is insignificant.38

6.1 Chronology
Cosmic evolution

Figure 6.1 Timescale of Cosmic Evolution

(from Barney Oliver, in Duckworth2296)

figure 06 1
In 15 billion years the universe has evolved from the blazing inferno of the primordial fireball into galaxies of stars surrounded by planets, many of which may support intelligent life.
  • The earth has existed for approximately one-third this time or 4.5 billion years.
  • While man has been around for only 1.5 million years or one ten thousandths the estimated age of the universe.

In 1648 James Ussher, the Archbishop of Armagh, announced that the creation of Earth occurred promptly at 10 A.M., October 23, 4004 B.C. This span of roughly six thousand years was calculated in accordance with the descriptions and geneologies found in the Bible, and enjoyed wide currency until about two centuries ago.

Today we know that the material universe is far older. The primieval fireball is believed to have exploded perhaps sixteen billion years ago, the Milky Way coalescing a few eons later. Such vastness is scarcely conceivable in any meaningful terms.

How does one conveniently comprehend a span of time equal to millions of human lifetimes? Imagine that we draw a line from top to bottom of this page, a linear scale to portray the entire history of the universe.

On this map, the sum total of human civilization would be represented by an invisible sliver a few hundred atoms long. On the same scale, the time man has known electricity is measured by the span of two or three atoms. Even the segment illustrating the entire Age of Mammals would hardly exceed a millimeter in length (Figure 6.1).

A million years is just a day

Figure 6.2 Radical changes on the Earth due to 50 million years of continental

drift are predicted by three University of Chicago paleoclimatologists477

One rather well-known visualization was set forth by the famous British astronomer Sir James Jeans many decades ago. Imagine a penny carefully balanced atop the Washington Monument. Affixed to the cent is a postage stamp. Proportionately, the Monument represents the age of the Earth, the coin the entire age of the species of man, and the stamp the length of time since humans first learned to use tools.2109

Our minds are easily boggled. The whole history of the United States spans a mere two hundred years, a series of only eight generations of humankind. The differences between the late 18th century and the modern world seem immense. To contemplate our world as it may exist two hundred years hence sorely taxes our imagination (Figure 6.2).

But hundreds or even thousands of years are nothing to the xenologist.143 As biochemist and Nobelist George Wald aptly observes, "in geological time, even one million years is just a day.867 It is inconceivable that all other lifeforms throughout the Galaxy began evolving at exactly the same time as we, and at the same rate. If ETs do exist, many of them undoubtedly possess civilizations millions of years our senior — if not hundreds of millions or even billions of years more advanced.


Figure 6.3 Timescales of Responses to Change(from Wilson565)

figure 06 3 timescales of responses to change 500

Before such numbing timescales, humanity pales into relative insignificance in view of the mission of intelligence in the cosmos. Even if mankind were to be virtually annihilated in some terrible natural catastrophe, over a span of millions of years other mammals might evolve to take up the niche vacated by ourselves. Considering the broad sweep of the evolution of sentience, there seems no reason to doubt that higher intelligence would reassert itself on this planet.

Barring such catastrophes, humanity and its progeny may have literally eons of life and development ahead of it.* The Age of Dinosaurs lasted only a hundred million years, roughly 0.2% the age of the Earth. Says Arthur C. Clarke: "If we last a tenth as long as the great reptiles which we sometimes speak of disparagingly as one of nature’s failures, we will have time enough to make our mark on countless worlds and suns."81

Part of our problem in understanding time is due to the differing order of change in nature (Figure 6.3). Humans are accustomed to dealing with events that can best be classified as "organismic responses" — instincts and reflexes, learning, cycles of reproduction and so forth.565 We are only now, in the 20th century, becoming dimly aware of the concept of ecological time, the scale upon which demographic (population) and ecospheric changes take place. And the next highest levels — of evolutionary and geological times — still remain beyond our ken.

* Ultimately, we are limited only by the lifetime of our sun. Another 8-10 billion years remain before it flickers and dies, although Earth will probably become uninhabitably hot in 4-5 eons.20, 2056 Perhaps by then, humanity will have discovered a new homeland.

A brief history of Earth

One interesting example of a long-term trend is the change in the length of day. Every million years, because of tidal friction caused by the Moon, Earth’s day becomes about 3.3 minutes longer.2206 A couple hundred million years ago, during the Age of Dinosaurs, our planet revolved about one hour faster. In the steamy Carboniferous Period, when giant insects cruised forests of giant ferns, the day was only 22 hours long. One eon ago the components of Earth’s air were stabilizing near their present values and marine organisms were reeling with the discovery of sex. But they had to accomplish in only 18 hours what we take 24 to do.

Projecting into the future, a day in 1,000,000,000 A.D. will last about 30 hours. The Earth is gently slowing, a giant top marking time in eons.

"I perceived that I was on a little round grain of rock and metal," wrote Olaf Stapledon in his 1937 science fiction classic Star Maker,

filmed with water and with air, whirling in sunlight and darkness. And on the skin of that little grain all the swarms of men, generation by generation, had lived in labor and blindness, with intermittent joy and intermittent lucidity of spirit. And all their history, with its folk-wanderings, its empires, its philosophies, its proud sciences, its social revolutions, its increasing hunger for community, was but a flicker in one day of the lives of stars.1946

All of these considerations are of great significance to the origin of life on this planet. Until recently, scientists were of the opinion that the creation event itself might have taken place one or two eons after the formation of the Earth. But how much time was really required? As late as the middle of this century, no one really knew the answer to this question. The skeletal fossil record extends back only to the beginning of the Cambrian Period, about 600 million years ago. The Precambrian, comprising the first 87% of our world’s history, remained enshrouded in mystery and ambiguity.

In the last decade or two, improved techniques and several major finds have lifted the veil of ignorance. Scientists now hunt for molecular fossils, traces of the biochemical signatures left behind by the remains of microscopic organisms long dead.1420 The evidence now seems fairly clear that single-celled life existed some 3.4-3.6 billion years ago. (But note Schopf.2369) It is plausible that extremely primitive replicative lifeforms existed for several hundred million years prior to these earliest finds.41

Temporal chauvinism

We cannot even prove
that 10,000 years is
too short a period.

The implication is that life had half a billion years, perhaps even less, in which to assemble itself from nonliving chemical precursors. As two pioneers in abiogenesis research have noted:

There is no way at present to estimate when, during this (first) billion or so years, life arose. Periods of a hundred million years are so removed from our experience that we can have no feeling or judgement as to what is likely or unlikely, probable or improbable, within them. If the formation of the first living organism took only one million years, we would not be very surprised. We cannot even prove that 10,000 years is too short a period.521

The process of biological evolution must have begun as soon as the first living system emerged from the primieval "soup" four eons ago. Early forms of anaerobic photosynthesis probably arose three eons ago, in response to what one scientist has called "the world’s first energy crisis." Energy-laden molecules floating in the seas had become depleted. Photosynthesis allowed organisms to directly tap the power of the sun, which partially solved the crisis.

Unicellular life began to diversify about 2.3 billion years after the formation of the Earth, with the appearance of the first metazoans (multicellular animals).939 Aerobic photosynthesis was invented a short while later, and the concentration of oxygen — a harmful, poisonous waste product detrimental to most lifeforms in existence at the time — rose dramatically. In response to this "smog crisis," nature invented organisms able to consume the harmful oxidant and return carbon dioxide, thus detoxifying the air. These efforts were not entirely successful, however: Burning oxygen proved more efficient and made possible the conquest of land.

Man - footnote to history

Perhaps the vastness of time and our place in it can best be illustrated by the chronology in Table 6.1. Earth’s biography is plotted as a series of slow, painstaking steps from the formation of our planet 4600 million years ago up through the present. Truly, man is a mere footnote to history.

Our descendants …
will see our present age
as the misty morning
of human history.

Sir James Jeans gracefully surmounts the barriers of temporal chauvinism:

We are living at the very beginning of time. We have come into being in the fresh glory of the dawn, and a day of almost unthinkable length stretches before us with unimaginable opportunities for accomplishment. Our descendants of far-off ages, looking down this long vista of time from the other end, will see our present age as the misty morning of human history. Our contemporaries of today will appear as dim, heroic figures who fought their way through jungles of ignorance, error and superstition to discover truth.2109

Table 6.1 ♦ A Brief History of Earth
Geological Era
Period (Duration in Mys -- millions of years)
Length of Day
and Major Events
Archean or 
(~1000 Mys)
  H2 CH4, NH3, H2O
(N2 CO, HCl, H2S
trace constituents)
Formation and 
of planet Earth 
-- none --
(~2100 Mys)





very highly reducing
solidification & 
stabilization of 
the crust
strongly reducing
Oceans filled to 
10% their present volume
      Great volcanic activity,
granite intrusions, some
sedimentary deposition,
and extensive erosion
    Great increase in 
N2 production
Oldest dated rocks
(~1100 Mys)
  H2 CH4, NH3, N2 
(CO2, H2O, HCN, 
H2S as trace 

Graphites of possible
organic origin
Oldest crustal rocks

"Age of 
Unicellular Life"
less reducing
      Oceans essentially filled   Onverwacht and Fig-Tree:
Organisms resembling
blue-green algae (chemical  molecular fossils)

Early protozoans
Nonoxygenic photosynthesis
    CO2 rises to 
1% of total 
      First limestone deposits   Microfossils and bacteria  
slightly reducing
    Bulawayan Group,
South Rhodesian limestones 
          Soudan Shale microfossils  
Early Proterozoic
(~300 Mys)
  N2, CO2 
(NH3, CH3, H2O, O2
as trace constituents)
    Wealth of evidence of
biological activity
Strikingly advanced flora,
Macro-fossils remain rare 
"Age of 
Primitive Marine Invertebrates"
(300 Mys)
Middle Proterozoic 
(~120 Mys)
(~600 Mys)
    Witwatersrand Supergroup,
South Africa (microbiota)
      Great sedimentation;
sedimentary rocks
extremely thick;
repeated glaciations;
extensive erosion
  Anaerobic and 
oxygenic life 
flourish, the former 
slowly giving way 
to the latter 
Gunflint Iron Formation
blue-green algae,
flagellates, & fungi
Invention and deployment of
    O2 begins a dramatic
rise CO2 drops to
present level
CH4, NH3 vanish
Invention of oxygenic respiration
    Some volcanic activity  
    Atmosphere oxidizing        
          Primitive aquatic plants, 
marine protozoa and 
aerobic metazoa




    Atmosphere strongly oxidizing        


Late Proterozoic 
(~400 Mys)
  N2  78% 
O2  21% 
Ar   ~1% 
CO  ~0.1% 
H2O (trace)

Modern atmosphere established

    Bitter Springs: 
blue-green algae, 
red algae, fungi, 
Mollusks, worms, & other
marine invertebrates
(sponges, brachiopods)
Invention of Sex
        Algonkian Ice Age      
          All animal & plant 
phyla established 
"Age of 
(~375 Mys)
Cambrian (100)   Climate warm;
formation of major
Paleozoic geosynclines
  Spores, tracheids 
Trilobites and brachiopods dominant 
Spread of land plants Freshwater fish, coral, 
Marine arachnids
  Ordovician (75)   Low continents; warm
Arctic; extensive land
submergence & flooding

Silurian (25)
Devonjan (50)
Carboniferous (75)


Eocambrian Ice Age
Land rises; mountain
building arid lowlands;

21 .7h
Wingless insects; 
amphibians, lungfishes, 
First reptiles
"Age of Fishes and Land Plants"
  Permian (50)   Climate warm & humid
 first, cooler later
Giant ferns, cool swamps 
Large insects, thernlonts
"Age of 
Insects and Amphibians"
(~150 Mys)
Triassic (50)   Dry and cool; continental 
uplifting, Pangea breakup
Modern insects 
First dinosaurs 
First mammals 
"Age of Reptiles"

Jurassic (50)
Cretaceous (50)

  Climate warm, last
shallow inland seas;
Alps, Andes, Himalayas,
Rockies rose
Giant dinosaurs, 
toothed birds 
Dinosaurs decline; 
first flowering plants
(~75 Mys)
Tertiary (75)   Climate cooler 
Quaternary Ice Age
(repeated global glaciation)
  Rise of birds, 
higher mammals, 
and arthropoids (including the genus Homo)
"Age of Mammals"
    Quaternary (1)    
6.2 What Is Life?
6.2.0 What is Life?
Differentiate the living from the nonliving

samuel butlerAnthropologists jokingly tell of two cannibals watching an airplane fly overhead. Eyeing the craft wistfully, one says to the other, "It’s very much like lobster. It’s hard to get into, but very good once you get inside."

Kenneth Boulding, Director of the Institute of Behavioral Science at the University of Colorado, insists that the cars, planes and factories which surround us bear an analogous relation to the life inhabiting them as the lobster’s shell does to the lobster. "If a being from outer space were observing this planet," Boulding suggests, "he might well report that the process of evolution had produced a species of large four-wheeled bugs with soft, detachable brains."30

How can we accurately differentiate the living from the nonliving? For years, science fiction writers have been teasing our imaginations, giving us stories about plants that act like animals,564, 2115 animals that act like plants,607, 2168 and other organisms that almost defy classification.1561, 2163, 2210, 2221 Countless stories have been written around the theme of "machine life,"983, 1755, 1836, 1912 and a well-known Stanford radioastronomer has speculated that there may exist aliens which are simply spherical balls. Instead of handling objects as we do, Dr. Ronald Bracewell suggests that "they might have to ingurgitate them and manipulate them as we can manipulate things with our tongues. Perhaps their tongues would be luminescent and there would be an eye in the roof of their mouth, or a microscope."1040

Science fictioneers have devoted a great deal of time to an attempt to identify some of the problems we may encounter simply in recognizing that an object on another world is alive. False calls in either direction are possible. We may, for instance, mistakenly ascribe life to what is in reality a purely physical process. Conversely, there is the more frightening possibility that we might fail to identify a fascinating but unusual lifeform, which could cause irreparable harm before the error was discovered.

Hypothetical organisms

Hypothetical organisms

  • ■ Polymorph
  • ■ Lithomorphs
  • ■ Macromorphs
  • ■ Amorphs
  • ■ Electromorphs
  • ■ Plantimals

Such hypothetical organisms generally fall into five broad categories (although there are numerous exceptions). First we have the polymorph, a creature having a plural or changeable form. In Olaf Stapledon’s First and Last Men, Earth is invaded by a host of microscopic organisms from Mars. On occasion, these microbes form themselves into a rational entity by solidifying as a kind of "intelligent cloud."81 Such is not without precedent even on Earth: It has long been debated whether the sponge (Porifera) is a true organism or a colony of unicellular organisms.443


Ralph Milne Farley wrote "Liquid Life" back in 1936, in which a virus in a pond achieves group-collective consciousness.581 This is similar to the "scum-intelligence" proposed by Bracewell80 or the "mold-intelligence" proposed by Academician A. Kolmogorov, a Soviet writer.1330 Perhaps easier to view as living but equally difficult to understand are Arthur Clarke’s Palladorians, each of which is described as possessing "no identity of its own, being merely a mobile but still dependent cell in the consciousness of its race."2207 Another class of exotic fictional lifeforms are the lithomorphs, organisms having the form of rock. Such creatures have actually been discussed in sober scientific circles.1238 The two extremes of the problem of false calls are nicely illustrated by a pair of science fiction tales involving lithomorphs.

The Star Trek episode entitled "Devil in the Dark" deals with the discovery of a silicon-based organism that lives in the rocky mantle of a small planetoid. The human miners had been collecting and destroying apparently useless spherical silicon nodules — which turned out to be the Horta’s eggs. In Clarke’s novel Imperial Earth, exactly the opposite difficulty is encountered. Early settlers on Titan, the largest moon of Saturn, discover the "waxworms," entities snaking around on the surface at speeds up to fifty kilometers per hour and often pausing to climb over hills. "To the bitter disappointment of the exobiologists," Clarke writes, "they had turned out to be a purely natural phenomenon…"1947

Many variants


Macromorphs are beings having a large or elongated distribution. Typical of this class is the huge single-cell lifeform encountered in the Star Trek adventure "Immunity Syndrome," or the Gaia concept of the living planet sponsored by scientists Margulis and Lovecock.1293

Perhaps the most fascinating suggestion along these lines was made by the Swedish writer Gosta Ehrensvärd, who pointed out that organisms in the sense we understand may not even be a prerequisite for life.257 As an example, he envisions a coordinated network of lakes and streams covering a planet, participating in a complementary carbon cycle together with a sun-activated circum-planetary flow of water. Such a system, Ehrensvärd claims, "would undeniably constitute life, but it would hardly correspond to our idea of organism life. We could hardly recognize at first that we were dealing with something living, for we would not see any mass, body, or anything moving, but only a global activity in chemical serenity."


The fourth class of unusual creatures are the amorphs, those entities which exist without form or shape. Perhaps the best-known amorph is from the 1958 Paramount Studios movie "The Blob," the story line of which will not be gone into here. Suffice it to say that such organisms are not wholly without precedent on Earth. Slime molds are acellular plants which, because their construction is not unlike a sheet of water, find it possible to slowly creep about on the ground.


Blobs could also arise by natural evolution from Euglena ancestors (a photosynthetic microbial animal), or by artificial evolution as a direct consequence of genetic experimentation with "plantimal" cells. Plantimals are created by fusing animal cells with plant cells to form viable interkingdom protoplasts. To date, human tissues have been mated with carrot and with tobacco cells, and rooster cells have been joined with tobacco as well. According to Dr. James X. Hartmann of Florida Atlantic University at Boca Raton, a living, meatlike amorph might eventually be grown as livestock which could build animal protein by converting the sun’s energy directly into chemical energy — just as plants do.1617

Even this close it looked like a shadow. It also looked like
a very flat, monstrously large amoeba, or like a pool of oil
running across the ice. Uphill it ran, flowing slowly and
painfully up the side of a nitrogen mountain, trying
desperately to escape the searing light of my lamp. …
Helium II, the superfluid that flows uphill.548

There have been many variants on this theme in science fiction,1389 including petroleum-blobs such as in Brenda Pierce’s "Crazy Oil" on Venus.2071 In a familiar plot line, the human miners discover too late that the sticky black goo they’ve been extracting is part of a living organism. Still more fascinating is the possibility of superfluid amorphs, such as those described by Larry Niven in his "The Coldest Place":

Alien presence … might be undectable


Finally, we have the electromorphs — beings having the form of electronic energy, fields or plasmas. These ethereal creatures, first described by the Russian space pioneer K. E. Tsiolkovsky445 and later given a more public airing in the Kubrick-Clarke masterpiece 2001: A Space Odyssey,1912 are among the most beloved of science fiction writers. Hal Clement notes that "one must admit that very complex electric and magnetic field structures other than those supplied ready-formed by atoms and molecules are conceivable."878 One of the first science fiction novels the author ever read, decades ago, was about a form of intelligent ball lightning inhabiting the planet Mercury.*

Arthur Clarke has warned that we might not even be able to detect the presence of an alien species on a planet, save by the use of sophisticated electronic gadgetry. The lifeform could be gaseous, electronic, or could operate on timescales far faster or slower than our own.81 Hal Clement has fictionally created creatures constructed of densely packed electrons possessing quasi-solid properties and which live inside suns,2139 and still others that inhabit neutron stars, existing in a kind of superoptic quantum space and feeding directly on patterns and structures of information.2183

The classic electromorph of all time remains, however, astronomer Fred Hoyle’s Black Cloud — a kind of intelligent comet.62 (Being a lifeform of the dimensions of a solar system, it is also a macromorph.) In the novel, a great patch of ionized gas, which enters our solar system and engulfs Sol, is found to be alive when efforts to predict its movements using the simple laws of mechanics fail. Says the astronomer-protagonist in The Black Cloud: "All our mistakes have a certain hallmark about them. They’re just the sort of mistake that it’d be natural to make if instead of the Cloud being inanimate, it were alive."

It turns out that the biochemistry of this amazing organism is plasma physics instead of molecular chemistry. Memory and intelligence are stored on a conductive substrate of various solid materials, and are controlled, operated and manipulated purely by means of electromagnetic forces. Ionized gases carry substances to wherever they are needed, like a bloodstream. The Cloud must therefore be recognized as alive, at least in the sense of possessing intricate structures, a capacity for regeneration and energy utilization, and a complex behavior.

* I have since forgotten the title, which annoys me greatly. (Note added, 2 Jan 2011: Erik van Lhin (aka. Lester del Rey), Battle on Mercury, The John C. Winston Company, Philadelphia PA, 1953.)

6.2.1 The Traditional Answer
Finding a single definition

hw longfellow 305The possibility of discovering an exotic lifeform such as the above has spurred biologists to carefully reconsider their assessment of the nature of life. Scores of situations can easily be conjured up in which our tried-and true common sense rules break down horribly. The need for a more rigorous definition is clear.

If so many different kinds of life are possible, though, can we hope to reach them all with a single definition? Perhaps. For instance, one comprehensive characterization of life, at once exact and unsatisfying, might be: "Life is a highly improbable state of matter."1171 The difficulty arises when we try to be a bit more specific than this.

Some of the most generalized functional definitions have been rather ingenious. One author presents an ecological specification: A rock has small influence on another rock, but an organism profoundly affects all other living things around it. Living creatures alone can form ecological systems.64 Dr. Daniel Mazia has suggested that survival is the key to understanding what life is. As he correctly points out,

the living world thwarts time by survival, all the rest combats time by endurance.313

Another writer, Dr. V.A. Firsoff, has proposed that "mind" underlies all life, but is a quality denied to the nonliving.352 Others would claim that "the exclusive property of life is consciousness"171 or "self-direction."444 Still another definition hinges on the similar concept of "free will." As the late John Campbell, former editor of Analog, once put it: "Inorganic matter displays the characteristic that what it can do, it must. Any nonliving system always does everything it can do. Living systems don’t display that characteristic; if a living organism can do something, it — may."200

Life's unique properties

Life's unique properties

  • Growth
  • Feeding
  • Metabolism
  • Motility
  • Stimuli response
  • Reproduction w/ adaptation

The traditional biologist points out that all living things possess certain unique properties. One way to define life rigorously is in terms of specific, enumerated traits: Growth, feeding and metabolism, motility (physical movement), responsiveness to environmental stimuli, and reproduction with adaptation.

Let’s look at each of these in turn.

During the process of growth, a living organism takes in raw materials and integrates them into itself. Molecules of various substances are added, redistributed, or removed as the body changes shape and develops new structures. Growth also allows for replacement of old worn-out parts.

Unfortunately, many non-living systems also display growth. Crystals of table salt, for instance, or hailstones can be said to grow.

Reproduction is probably
the most frequently cited
"essential" defining characteristic
of living systems.

"Chemical gardens," made from heavy metal salts immersed in a bath of sodium silicate solution, also exhibit growth. It is true that most of these counterexamples involve only simple accretion from the outside, and the structure remains basically unchanged. But the flame of a candle appears to grow, and in a fire there is an actual throughput of new atoms. Hence, the candle flame is a valid exception to growth as a defining characteristic of life.

How about the criteria of feeding and metabolism? We know that living organisms eat primarily for two reasons. First, food is ingested and metabolized to provide an energy exchange with the surrounding medium. This gives a lifeform the ability to carry out any other functions it may wish to perform — reproduction, movement, thinking, more eating, etc.

Second, food must be accumulated to secure the raw materials necessary to effect repairs and to maintain growth. As has been pointed out, the kind of food consumed is really irrelevant. While humans and worms may prefer apples, some bacteria thrive on the most putrefactious sewage (and abhor oxygen), and plants "eat" carbon dioxide and sunlight.

But here again we note that the candle flame has a kind of metabolism. Fires may be said to digest their fuel and to leave wastes behind as chemical energy is converted to heat. Crystals too may eat, if we are willing to consider the saturated chemical solution in which they grow to be their food. Even machines metabolize their fuel, whether to manufacture spare parts or to build near-duplicates of themselves.

Replication plus adaptation

Motility is another oft-touted characteristic of life: Animals, and plants to a lesser extent, are capable of bodily movement. Yet there are many analogues in the world of the nonliving. Rocks and snow move in avalanches, cars travel highways, rivers flow, and under the proper thermal conditions metals will expand and contract. Granted, most of these are the result of the imposition of strictly external forces.444 Nevertheless, the fact that forest fires may spread under their own power constitutes an exception to the motility rule.

What about irritability? It has been said that if organisms are to profit from their association with the environment, they must be responsive to it at all times. Sensors and effectors thus become more and more highly developed as we climb the evolutionary ladder.

However, some non-motile bacteria show little evidence of reaction to stimuli,64 and plants are notorious laggard in their responses. Also, irritability is a property demonstrated by many nonliving systems. Crystals react quite sharply to changes in the solute concentration or temperature of their environment. The candle flame recoils when an open door admits a draft. A flask of nitroglycerine is highly responsive to certain environmental stimuli, particularly heat and shock.881

Reproduction is probably the most frequently cited "essential" defining characteristic of living systems.20, 521 Although a few scientists would demand the presence of DNA or RNA molecules, proteins, lipids, polysaccharides and the like as requirements for life, most stick to the basics: Replication plus adaptation.

According to these so-called "genetic" definitions of life, living things are entities capable of reproducing themselves, mutating, and subsequently re-reproducing the new mutated form. Organisms are required to multiply geometrically as well. Simple arithmetic reproduction, as in a printing press, is insufficient. The copies themselves must also be able to make more copies. When mutations arise, they are faithfully duplicated — variation is preserved in subsequent replications.

The central idea behind this attempt to define life by reproduction is that living organisms must be the subjects of natural selection, capable of adaptation and evolution. Any system that can replicate, mutate, and replicate mutations will be susceptible to normal evolutionary processes. Favored organisms with the highest potential for survival go on to multiply; others who fare more poorly in the struggle for existence eventually become extinct. Fundamental to the genetic definition of life, then, is the built-in and perhaps unwarranted assumption that a certain level of complexity cannot be achieved save by natural selection operating via adaptive replication.2358

Exceptions and objections to reproduction

It is entirely possible that some lifeforms may have no need to reproduce themselves. Such nonreproducers, if they exist, must be either immortal or very recent arrivals. One class of such beings would be self-creating but nonreplicating organisms, such as robots capable of making continual repairs and of upgrading their own mechanisms periodically, or such as astronomer Hoyle’s Black Cloud mentioned earlier.

There could even exist beings who evolve by means of acquired characteristics.2216 Such lifeforms could neither die nor reproduce, but would instead modify their parts in response to the changing environment. As Dr. P.H.A. Sneath of Leicester University puts it: "Evolution and selection would then operate internally on their constitution, rather than on a succession of descendent organisms."64 Dr. Sneath suggests that the closest analogy to this might be soils, which don’t reproduce in the usual sense but are complexly organized systems nevertheless. Soils respond to environmental changes, arise wherever there is rock and wind to erode it, and are virtually immortal. Organisms such as these would be unable to "compete" with their neighbors without blending together with a total loss of individuality.

Most bees, ants, wasps and
termites don’t reproduce…

There are other objections to the use of adaptive reproduction as the fundamental criterion for life. Mules, the offspring of a mare and a male donkey, are sterile and so technically are not "alive"- under the genetic definition. Most bees, ants, wasps and termites don’t reproduce either. Selection acts on the whole nest, rather than on individual units, so evolution proceeds through the queen and drones alone. Many varieties of hybrid flowering plants are similarly sterile.

The inorganic world too is rife with exceptions. Flames, driven by wind or with sparks, can reproduce and "mutate." Crystals placed in solutions doped with foreign ions are perfectly capable of reproduction, mutation, and of propagating the mutation (i.e., lattice imperfections).

We see that traditional concepts of life are unduly restrictive for our purposes. As Dr. Mazia laments: "The problem is not that our conception of a living thing is vague; on the contrary, our concern is that it is too definite because it is too provincial."313 We must seek more generalized means to identify and to define life.

6.2.2 Organization

Complex interrelatedness

minas ensanian 243Life is a process by which relatively unorganized environmental components are made more organized. That is to say, life is a building up-process, although to organize also means to cut down the possibilities. But certainly a basic characteristic of all lifeforms is that they are highly organized.2214

What do we mean by "organization"? The concept may be viewed in terms of what Sneath has called "complex interrelatedness.64

Interrelatedness means simply that all parts of the pattern are related to and somehow affect all other parts. Each component reacts to changes in its surroundings so as to preserve internal integrity and minimize the effects of any disturbances. This damping action is the principle of homeostasis, common among biological systems. Of course, biochemical homeostasis can be preserved only within certain critical tolerance limits. Death will rapidly overtake any system which is subject to stresses it cannot tolerate.

Complexity is the other facet of organization.64, 1643 "Complex" is used here in its normal sense, as opposed to "simple." Candle flames have a great deal of interrelatedness, yet they lack complexity and hence "organization" as well. Conversely, a lump of granite is highly complex, but because it lacks interrelatedness it cannot be considered "organized" in the sense of having life.

The key to life
may well be
information itself.

Dr. Sneath cites a most useful example of the role of complexity. If complexity is defined as the amount of information needed to completely characterize a system, the perplexing case of the growing crystal is greatly simplified. We might describe a small cube of rock salt as follows: "A simple-cubic Bravais crystal lattice structure with spacing of 2.82 × 10-8 cm, consisting of alternating sodium and chlorine atoms, containing a total of 1020 atoms of each kind." This requires only a few lines of print, and is complete.

On the other hand, living things are typically characterized by enormously more complicated descriptions. Life systems possess order on a scale far smaller than the macroscopic. Unlike the monotonous repetitiveness of the salt crystal, even the simplest bacterium needs some 103-104 different enzymes, each with a unique sequence of perhaps a hundred or so amino acids.64, 630 This is real complexity. On the microscopic level, life might best be characterized as a highly "aperiodic crystal."2213, 2364

The key to life may well be information itself. The living world is built from the stuff of the nonliving world, different only in its complexity and organization. Organisms find it possible to actually store and replicate the information that specifies their organization.

Yet it is purely capricious to set some arbitrary level of complexity as the threshold of life.1717 A frozen amoeba, for example, has an amazingly detailed and intricate structure without being alive — it has only the potential for life. Organization, as we shall see presently, is a most useful parameter for assessing the intensity or efficiency of life. However, it is more reasonable to base our definition on the fundamental processes and functions displayed uniquely by living systems.

6.2.3 Towards a Definition of Life
What is entropy?


  • Thermodynamic measure of energy
  • Statistical measure of disorder

Thermodynamic and statistical principles are among the most fruitful tools of scientific inquiry. They are equally applicable to simple and to complex systems, living or nonliving, terrestrial or extraterrestrial. As Dr. James P. Wesley, Associate Professor of Physics at the University of Missouri in Rolla, tells us:

The relationship of life to the environment is, above all, a thermodynamic relationship. Wherever man may go and whatever alien lifeforms he may encounter, the thermodynamic behavior of life will always be basically predictable.1717

The idea of entropy is often involved in modern discussions of the definition of life.

What is entropy? There are really two relatively straightforward aspects of this concept. The first ties in to the thermodynamic aspects of matter, having to do with heat and energy; the second pertains to statistics and order in any system.

Entropy in the thermodynamic sense is a distinct, physically measurable quantity, much like length, temperature, or weight. At a temperature of absolute zero, to take one example, the entropy in a lump of matter is exactly zero. If the temperature is slowly increased in tiny, reversible little steps, the increase in entropy is mathematically equal to the amount of energy (in joules) divided by the temperature at which it was supplied. This holds even if a change of state occurs, as from solid to liquid.

Suppose that we melt a cube of solid ice at 0 °C. If the mass is 1 kg, the increase in entropy can be calculated as exactly 1223 joules/degree. Entropy in the thermodynamic sense is thus a very real, physical quantity.

In the statistical sense, entropy is a measure of the disorderliness of a system. It seems rather clear that when we melt our 1 kg block of ice, the neat orderly arrangement of water molecules in the cube is destroyed. The rigid crystal lattice is converted into a less ordered system — the continually changing, sloshing, randomized distribution of molecules in a liquid.

When the orderliness of a system decreases, the entropy correspondingly increases. The situation is analogous to the state of the public library when the shelvers are out on strike. Books are removed from their proper places but are not returned. Disorder and randomness — entropy — increase.

The greater the structural complexity of a system, the more information is required to describe it. The more organization a system has, the more information and the less entropy it possesses. But information and orderliness, on the one hand, and entropy, on the other hand, are irreconcilable.

It is the business of the
universe to destroy complexity
and to become progressively
more randomized.

The Second Law of Thermodynamics states that entropy and disorder shall always increase and that information will naturally tend to be degraded and lost in any isolated physical system. (Such isolated systems drift from less probable states to more probable ones.) It is the business of the universe to destroy complexity and to become progressively more randomized.

How does this relate to life? Organisms appear to present a rather curious thermodynamic anomaly. Living systems "violate" the Second Law, by developing well-ordered systems (themselves) out of relatively chaotic systems (their food).85 At first blush, lifeforms seemingly oppose the "universal drive to disorder" mandated by thermodynamic principles. They organize their surroundings and produce order where there was little or none before. Entropy is actually reduced.

This apparent conflict has only been resolved in the last decade or so. Classical treatments dealt with idealized, isolated systems which transfer no energy or matter between themselves and the external environment.2213 In sharp contrast, most systems in nature are nonisolated "open" systems, exchanging matter and energy with the surroundings.

Energy flow

Energy emerges from the source,
flows to the sink,
and is there absorbed.

Energy by itself is not enough — there must be a useful flow of it. This means that to support life, an environment must possess both a "source" and a "sink." Energy emerges from the source, flows to the sink, and is there absorbed.

Living systems customarily establish themselves as intermediate systems, interposed between some source and some sink in the environment. Then, they utilize the energy flow from source to sink to power their own internal functions.

The total entropy of the entire system, which we shall label E, is the sum of the entropies generated in two separate places. First, there is the entropy caused by the source-to-sink energy flow which we shall call S. Then there is the entropy generated by the intermediate system (the living organism) due to exchanges of matter and energy with the surroundings. If we call this L, then we know that the change in total entropy DE = DS + DL.

The second Law of Thermodynamics demands that the total entropy E of any isolated system always increase. Hence, the amount of change must always be positive, so DE > 0. The flow of energy from source to sink (S) consists of irreversible processes, so it too must always cause entropy to increase: DS > 0. Consequently, -DL < DS is our only constraint.


Local pockets of negative entropy change.

What does this mean in plain English? The last equation above simply says that while the entropy of a living system is always permitted to increase by the Second Law (e.g., upon death), a short range of decrease is allowed as well. That is, it is thermodynamically permissible to have local pockets of negative entropy change — "negentropy."

… the essence of life is
that it feeds on negentropy.

Life involves a continuing struggle
against increasing entropy.

Dr. Erwin Schroedinger was really the first to point out that the essence of life is that it feeds on negentropy.1678 An organism able to transfer disorder from itself to its environment can reach a plateau for which the steady-state entropy within the living system is less than the formal entropy entering it.85 Life involves a continuing struggle against increasing entropy.

Living systems thus increase local order at the expense of a larger decrease in order within the environment.

Does life really violate the Second Law of Thermodynamics? We’ve seen that organisms can effect a local decrease in entropy by maintaining an energy flow.* This leads to an ordering of the intermediate (living) system. So the Second Law does not hold for nonisolated systems (L), but only for isolated ones (E). It is invalid for lifeforms alone, but does hold if that same living system is considered in conjunction with the medium in which it is immersed.

Life by itself is a nonisolated system capable of achieving negentropic conditions locally. Life plus environment is an isolated system, for which the total amount of entropy must always increase.

As one writer puts it:

Living systems convert order in their surroundings into disorder, and thereby increase their own internal order.
To say that living systems feed on negentropy is equivalent to saying that their existence depends upon increasing the entropy of the rest of the Universe.2213

* A probable corollary is the necessity for "phase separation." In some sense, the sources and sinks should be physically separated with the living system inserted between them. So we expect barriers to exist between an organism’s sources and its sinks. This prevents dissipation of the system, protects it from adverse changes in the environment, and insures the lifeform’s ability to exert and maintain control over its interior.2213 The exact nature of these barriers — whether gravitational, electromagnetic, or utilizing some hitherto unsuspected principle — has not been widely discussed.64

Negentropic processes

Figure 6.4 Life, Entropy, and Organizational

Structure(after Morrison1279)

Figure 6.4A

The first graph shows the population of structures — molecules, for instance — against some measure of size or complexity for a case where a free-energy flow has spread the composition to include species of high free energy.

Figure 6.4B

The second graph shows what happens when some autocatalytic process begins to increase the population beyond some level of complexity by further degrading the less elaborate, material present.

Figure 6.4C

figure 06 4 C 50pc

The last graph suggests how artifacts might be looked at in this process, a second and rarer, but more costly, bump.

At the most fundamental level, negentropic ordering processes are achieved by living organisms. Life drives its environment to physical or chemical disequilibrium, establishing an entropy gradient between itself and its surroundings.1144 All living systems possess this feature, and it is contended that any system engaging in such negentropic operations must be considered "living" to a certain extent (Figure 6.4).

It is the business of life to accumulate
information and complexity.

The question is, of course, to what extent?

Rather than viewing the question of life in absolutist terms, it seems more fruitful to establish the intensity of negentropic processes as a measure of the extent of the life-quality. One of the more fundamental distinctions between "life" and "nonlife" is the degree of organization and internal structure possessed by living systems. Order and structure are virtually synonymous with information content.1012

That is, living systems do more than merely establish a thermodynamic entropy gradient — they establish an organizational / informational gradient as well. As organisms feed on negentropy, they in effect remove information from the surrounding medium and store it within themselves. It is the business of life to accumulate information and complexity.

Aperiodic crystal

Aperiodic crystal
a biological lattice with highly
irregular small-scale nonuniformities

In a physical sense, these data bits which permeate all lifeforms may be thought of as being stored in an "aperiodic crystal" — a biological lattice with highly irregular small-scale nonuniformities.2213 The more effective the negentropic processes, the greater the organization which will arise and hence the more aperiodic the physical structure will become. Organization is maintained by the extraction of order from the environment.

If we consider every "autonegentropic" system to be alive, then its character or richness of expression may be defined along a spectrum from lesser to greater levels of organization. At one extreme are the viruses, which are not negentropic systems by themselves and thus cannot be considered alive in the absence of living hosts. At the other extreme are mules and bees, earlier rejected by the genetic definition of life because of their individual inability to reproduce. These animals are quite clearly auto negentropic systems possessing a vast degree of organization both in the macroscopic and microscopic realms. Thus they are not only "alive" (because they feed on negentropy to build internal complexity) but also "very alive" (because they are so internally complex).*

The refrigerator in my house technically should be considered a "live" system in the very broadest sense, as it is a well-defined intermediate system which uses an energy flow to decrease entropy within (the icebox gets colder, and well-ordered ice crystals collect on the freezer walls) at the expense of increasing the entropy in the external environment (the kitchen air gets warmer). Yet its organizational structure is minimal. Little information is stored, and there is only trivial interrelatedness even on the macroscopic scale. There is scant evidence of aperiodic crystal, no complexity at all on the microscopic stage. So the intensity of life in my refrigerator is negligibly small.

Note that "machine life" or "solid state life" per se is not ruled out. As machines become more and more sophisticated, complexity follows. Large-scale integrated circuits available today pack millions of components onto a tiny silicon wafer the size of a postage stamp. Under the microscope, significant aperiodicities have begun to appear in the latest generation of electronic devices. It is entirely possible that, in time, machines will evolve beyond the point of negligible life-quality. This is true, despite the fact that modern digital computers (which merely process data without adding any of it to their internal structure) are not yet alive at all.

* While evolution and the capacity to reproduce are of immense biological importance, a system need not be capable of reproduction for it to be classified as living.2213, 62


In conclusion, xenologists suspect that there are two fundamental properties any system must possess before it can be considered alive:

For those who prefer succinct and pithy definitions, the author would
like to offer the following as a starting point for further discussion:

Life is negentropic and self-organizing aperiodic crystal.

    • First, it must be thermodynamically negentropic, establishing an entropy gradient between itself and the environment.
    • Second, it must utilize the entropy gradient to create or to maintain structural order internally — that is, it must be autonegentropic or self-organizing.

Then there is the quality of organization, known as complex interrelatedness or aperiodic crystal, which reflects the intensity of the life process displayed by a given entity.

Chapter 7 ♦ The Origin of Life
7.0 The Origin of Life

rig veda 345
Two central themes of xenobiology

Scientists today will still admit that they really don’t know how life began on our planet. Laboratory work is tricky, and nobody was present to witness events at first hand on the primitive Earth. Researchers in abiogenesis can only invent some reasonable story about how life arose, and then maximize its plausibility by theoretical and experimental investigations.20

There are two central themes that run as undercurrents throughout the whole of xenobiology. First, what is the probability that life of our kind will evolve on other worlds? By illuminating the abiogenic processes of this planet in ancient times, scientists hope to get a handle on the exact combination of conditions and events necessary for the origin of carbon-based Earthlike life anywhere in the Galaxy.

The second central theme of xenobiology, to which we shall return in later chapters, is the likelihood that life, once having emerged in a planetary environment, will constitute a form of biota more or less similar to that found on Earth. The laws of biochemistry demand that molecules combine only in certain specific ways, and usually only in a very few most probable ways. In other words, what are the physical and biochemical limits of the possible?

7.1 Historical Views on the Origin of Life
Abundant throughout recorded history

van helmontSpeculations on the source of life have been abundant throughout recorded history. The Rig Veda mentions that biology began from the primary elements, and the Atharva Veda suggests that the oceans were the cradle of life. The Bible, with its contradictory accounts of the Creation in Genesis (did man arrive before or after the beasts?), is strictly adhered to by many fundamentalists. Philip Henry Gosse, an eminent 19th century zoologist and Christian, found it a simple task to reconcile the growing mass of paleontological evidence with the Scriptures. God, he declared, created the Earth entirely in accordance with scientific findings. The Lord fabricated geological strata, embedded fossils and the like for the sole purpose of fooling geologists. The apparently extreme age of Earth is only an illusion.


Peculiar ideas abound. Hylozoism, for instance, is the belief that matter and life are one and inseparable. From this viewpoint, life either has no origin and has always existed, or else the question may be deferred to the origin of all matter.

The theory of pyrozoa, to cite another example, was advanced by William Preyer in the last century. Preyer believed that life has existed at all times, even when our planet was still in the molten state. These first fiery living things, the pyrozoa, slowly modified and adapted themselves as the environment cooled and changed, eventually assuming the form in which life presents itself to us today.2218

Four theories on The origin of life

Most theories on the origin of life have fallen into one of four distinct categories:

  1. Life has no origin — both life and matter have existed forever;
  2. Life is the consequence of a supernatural event, intractable and in explicable by the methods of science;
  3. Life originated via ordinary chemical evolution in a deterministic fashion — under similar circumstances, the same general evolutionary patterns would repeat themselves on any world; and
  4. Life originated elsewhere by means unknown, and was subsequently transported to Earth (panspermia).

The first two are self-explanatory, and the third closely approximates the leading modern theories. The last deserves a word of explanation.

Lithopanspermia / radiopanspermia / cosmospermia

The Greek philosopher Anaxagoras of Clazomenae (ca. 500-428 B.C.) was possibly the first to suggest that the seeds of life permeated the universe. With the downfall of spontaneous generation millennia later, panspermia enjoyed a brief revival. The theory was sponsored by many 19th-century notables, including Richter, Kelvin, Helmholtz, Arrhenius, and the great Italian chemist Avogadro.


The doctrine of lithopanspermia held that meteorites were the means by which life wandered from planet to planet throughout the cosmos. Lord Kelvin, a central proponent of this view, considered it probable that countless life-bearing "stones" existed in space, perhaps as the result of collisions between inhabited worlds. Hermann von Helmholtz, a German philosopher and a pioneer in physics, believed that the interior of a meteorite would be a safe retreat for interplanetary microbes during the long incandescent journey through thick planetary atmospheres. The presence of hydrocarbons in the carbonaceous chondrites was cited as evidence of the biological activities of the tiny organisms from space.

Modern analyses suggest that microscopic lifeforms embedded in interstellar comets are possible, but unlikely. The accumulated radiation dose from cosmic rays and natural internal radioactivity is "embarrassingly high" over the large transit times involved between worlds.22 Furthermore, it is now known that meteorites are of roughly the same age as the rest of the solar system, and that the organic molecules found in chondrites are reproducible by strictly chemical means.208,2030,2219


The famous Swedish physical chemist and Nobelist Svante Arrhenius was the loudest advocate for the theory of radiopanspermia.2304,2305,2306 He suggested that minute spores might be carried upward through planetary atmospheres by convection, where electrical forces could provide sufficient energy to expel them from the body. The pressure of sunlight would then be enough to propel these cosmozoa to other solar systems. Tramping through space, or riding piggyback on small grains of dust, these legions of microscopic interstellar emissaries thus brought the good news of life to the rest of the Galaxy.

Carl Sagan has done a careful analysis of the problem,20 the details of which will not be repeated here. His conclusion is that radiopanspermia is not a viable theory of the origin of life on Earth. Those microbes ejected from a stellar system by radiation pressure accumulate a dose of x-rays and UV three or four orders of magnitude higher than the maximum lethal irradiation sustain able by even the hardiest terrestrial organisms. Shielding won’t help: Life-forms large enough not to be killed aren’t ejected by radiation pressure because they are too heavy.22


The theory that life arose in the ancient swirling gas and dust clouds of interstellar space and then traversed the cosmos, seeding the Galaxy with life, may be called cosmospermia. Dr. J. Mayo Greenberg at New York State University set up a laboratory experiment a few years ago, using tiny grains of matter the size of space dust and appropriate gases. He found that many compounds of relatively high molecular weight could be formed under the influence of ultraviolet radiation. Greenberg evidently believes that a similar mechanism could lead to the production of grains of a size and composition similar to that of viruses.

Dr. Sagan has disputed such theories, noting that any hypothetical extraterrestrial organism of 10-5 cm — the size of a rabies virus or the PPLO (the smallest lifeform known) — would have a replication time on the order of two hundred million years. There could only have been fifty or so generations since the Galaxy first formed, insufficient time for natural selection and evolution to operate.141 It is hard to imagine a smaller yet viable organism; the replication time for a larger microbe would be even longer, permitting still fewer generations.

Gold Garbage theory and directed panspermia

Panspermia does not address
the phenomenon of abiogenesiss
but merely displacess
the problem in space and time.

Accidental panspermia is a class of theory typified by the "Gold Garbage Theory," popularized by Dr. Thomas Gold, a leading astrophysicist at the Center for Radiophysics and Space Research at Cornell University. The Garbage Theory was first announced in a paper read before a Los Angeles meeting of space scientists in late 1958,139 and proposes that Earth may have been visited by an expedition of advanced ETs who carelessly allowed some of their native microbiota (picnic basket litter?) to escape. "While this garbage theory of the origin of life understandably lacks appeal," one xenologist notes wryly, "we should not exclude it altogether."20

A similar idea is the concept of directed panspermia, which suggests that organisms were deliberately transmitted to Earth by intelligent beings on another planet.1283 Advanced civilizations might intentionally seed sterile worlds, either as a prelude to colonization or perhaps simply to perpetuate the heritage of life on the home planet as insurance against catastrophe.

Panspermia does not address the phenomenon of abiogenesis but merely displaces the problem in space and time.* Consequently, panspermia hypotheses aren’t strictly relevant to the ultimate origin of life in the universe but simply explain how any particular world might have come to be inhabited.

* One science fiction story suggests that life on Earth may have arisen from biota left behind by a careless time traveler from our planet’s future.636 If any theory begs the question it is this one!

7.2 Cosmochemical Evolution
charles darwin

The ubiquity of Life's essential compounds

The building blocks for life are lying around everywhere.

Great clouds of organic molecules have been discovered drifting between the stars, presumably formed by various natural processes.1002,2219,2220,2221 Radioastronomers have seen relatively complex compounds hiding deep in inter stellar space, including methyl alcohol, ethyl alcohol, cyanogen, formaldehyde, formic acid and ether,1002,2217 and the search is on for amino acids.

Compounds of carbon and hydrogen, particularly cyanogen, methane and hydrocarbon radicals, are detected on the surfaces of stars.1973,2297 To find the limits of such processes, Dr. John Oró performed an experiment which simulated a hot stellar plasma. Using a graphite resistance apparatus and a plasma torch device temperatures from 1500-4000°C were obtained. Methane, ammonia and water were introduced continuously. The products were condensed at room temperature and allowed to interact for a few hours before analysis. Three amino acids appeared — alanine, glycine, and aspartic acid — along with hydrogen cyanide and a host of other organics.1072

There is no doubt that the carbon compounds essential for the development of Earthly life are ubiquitous. Organics have been detected on the Moon,2443 other planets,2037, 2046 asteroids,2037 and in comets.1973,2222

The basic constituents
necessary for the emergence
of life are universal.

The carbon chemistry of meteorites is also well-documented.702,2219

The Murcheson rock which fell in Australia on September 28, 1969 contains 2 × 10-7 moles of amino acids per gram of meteorite, which is more than many desert sands on Earth.521 These amino acids correspond rather closely to those produced in prebiotic synthesis experiments performed in the laboratory.225

The Orgueil meteorite contains approximately 7% organic matter, including hydrocarbons, fatty acids, aromatics, porphyrins, nucleic acid bases, optically active lipids, and a variety of polymeric material.1075 On the basis of the amounts of carbon compounds detected in various meteorites, researchers have concluded that these interplanetary wanderers could have brought as much as 5 × 1010  kg of formaldehyde and 3 × 1011 kg of amino acids to Earth during the first eon of its existence.134

Taken together, these studies of meteorites, comets, planets and interstellar matter strongly suggest that chemical evolution is a continuing and commonplace occurrence in all parts of the cosmos. The basic constituents necessary for the emergence of life are universal. This implies that life should be widely distributed throughout the Galaxy, wherever conditions are clement, since the required ingredients of abiogenic processes are abundantly available everywhere.

7.3 Early Chemical Evolution on Earth
The alphabet of life

Chemical evolution refers to the period in Earth’s history during which the chemical components on the surface changed from simple forms into complex substances from which the first living organisms — protobionts — could develop. The primary investigative tool in abiogenesis research has been the prebiotic synthesis experiment. Plausible primitive Earth conditions are arranged in a closed laboratory apparatus, and the changes that take place are carefully monitored.

The argument has long been made that since no geological record of the origin of life exists, the course of events leading up to the creative event is fundamentally unknowable. While most biochemists today would dispute this supposition, how close to reality are the simulated prebiotic experiments?

The alphabet of life is extremely
simple; the wide variety of life
observed today may be traced
to a mere handful of chemicals.

It is unnecessary for scientists to heat together water, methane, ammonia and hydrogen (components of the primitive atmosphere), irradiate the mess with various forms of energy, and then sit back to wait for a recognizable lifeform to reach its slimy paw over the edge of the beaker and crawl out onto the lab desktop. We won’t ever achieve this kind of completeness, because that takes evolution and the secret to evolution is time.225 (But it has been seriously suggested that a complete artificial seashore be set up to test some of the proposed mechanisms in the origin of life.1630)

From chemical equilibria we know the kinds of substances that had to be floating around in the primitive atmosphere and seas. Protein molecules ultimately consist of different combinations of only twenty different amino acids. Nucleic acids are composed of one of five bases, one of two sugars, and a single type of phosphate group. As Cyril Ponnamperuma of the NASA/Ames Exobiology Division once remarked: "The alphabet of life is extremely simple; the wide variety of life observed today may be traced to a mere handful of chemicals."85

Abiogenesis: an unverifiable historical process

Abiogenesis research differs markedly from most other scientific work, in that an unverifiable historical process is being reconstructed. It probably is not practical to run through an entire origin of life "from scratch," so different criteria must be used to evaluate hypotheses. For instance, postulates must at least be consistent with known astronomical, geophysical, and biochemical principles insofar as this is possible. And stepwise experiments, in which only one step of abiogenesis at a time is simulated, are reasonable if plausible and appropriate prebiotic conditions are maintained.

It is believed that the origin of life may have happened very fast, certainly less than a billion years521 and possibly less than a hundred million years.225,305,2160 Most estimates today place the creative event in the primitive seas, roughly 4.2 to 3.6 eons ago.

7.3.1 Prebiotic Synthesis
Miller Apparatus

Figure 7.1 Miller Apparatus for Prebiotic Synthesis

Figure 7.1 Miller Apparatus
for Prebiotic Synthesis

In this schematic of the apparatus used in Stanley Miller's s historical experiment, a variety of organic compounds are synthesized as the atmosphere of methane (CH4), ammonia (NH3), hydrogen (H2) and water vapor (H2O) is subjected to an electric spark discharge.

Circulation is maintained in the system by the boiling water on one end and the condensing jacket on the ether.

After one week of continuous operation, the water was removed and tested by paper chromatography.

A great abundance of amino acids and other organics was detected.

For many years it was known that mixtures of carbon dioxide, ammonia and water vapor would produce small amounts of simple organic chemicals if energy was supplied. But the results of these experiments were generally very discouraging and the yields miniscule under these oxidizing conditions. To originate life in such a poor, thin broth would be well-nigh impossible.

In 1953 a graduate student named Stanley Miller, working under Nobelist Harold C. Urey at the University of Chicago, constructed an apparatus to imitate the conditions of the primitive Earth (Figure 7.1). Previous investigators had always assumed the atmosphere to be oxidizing or neutral. Miller and Urey, following the suggestions of A. I. Oparin in the Soviet Union and J. B. S. Haldane in Britain during the 1920’s, took the unprecedented step of devising a reducing environment instead.2258

Miller mixed together methane, hydrogen, ammonia and water, and carefully eliminated all oxygen from the system. This gaseous concoction was then circulated past an electric spark discharge, followed by a water bath to simulate the primitive sea. After about one week of continuous operation, the "ocean" had turned a deep reddish-brown.

The experiment was halted and the contaminated water removed for analysis. Miller discovered to his amazement and delight that many amino acids had been produced in surprisingly high yields. Two percent of the total amount of carbon in the system was converted into glycine alone. Sugars, urea, and long tarlike polymers too complex to identify were also present in unusually high concentrations.

Prebiotic Chemical Evolution

Figure 7.2 Prebiotic Chemical Evolution on the Primitive Earth

figure 7 2 621px

Of course, electrical energy was only one of the many sources of energy available on the primitive Earth (Figure 7.2)

In fact, ultraviolet radiation was probably the principle source: UV would have been able to penetrate to the surface be cause the protective ozone layer in the upper atmosphere did not yet exist.

A Miller-type experiment using ultraviolet rays and a reducing atmosphere was performed in 1957 by the German biochemists W. Groth and H. von Weyssenhoff at the University of Bonn.2307 Their results closely paralleled those obtained at the University of Chicago half a decade earlier.

Other Prebiotic Synthesis Experiments

Table 7.1 Summary of Prebiotic Synthesis Experiments through 1975

Amino Acids, Fatty Acids, and Simple Organics

table 07 1 A 500px

top: Simple Sugars and Carbohydrates

bottom: Polypeptides, Amino Acid Polymers, Proteinoids

table 07 1 B 500px

top: Nucleic Acid Bases: Purines and Pyrimidines

bottom: Nucleotides and Polynucleotides

table 07 1 C 500px

Countless prebiotic simulations have since been achieved which confirm Miller’s original conclusions. One bibliography, current through 1974, lists more than three thousand papers on the subject.1679 An exhaustive treatment of all of them is clearly beyond the scope of this book, but the interested reader in encouraged to dive into the literature (Table 7.1).

Ultraviolet radiation

Table 7.2 lists the sources of energy believed to be present during the first eon or so of Earth’s history. Ultraviolet radiation leads the pack. Carl Sagan and others have completed experiments with UV which seem to indicate rather high yields for prebiotic amino acids, the building blocks of proteins. Over the first billion years of chemical evolution on this world something like a hundred kilograms of amino acids per square centimeter may have been produced, resulting in a "soup" of about 1% concentration. This is the approximate consistency of chicken bouillon.

But ultraviolet radiation is a two-edged sword. While it may be the most abundant form of energy for molecule building, it is also the most destructive. Early researchers were concerned that organics would be destroyed as fast as they were created. Fortunately, the primitive oceans probably turned opaque like the brownish glop in Miller’s apparatus rather quickly. Vital chemicals newly synthesized and carried a short distance beneath the surface of the soup by convection undoubtedly escaped decomposition.

Electrical discharge

Of the remaining energy sources, electrical discharge was the most potent. As much as 5-15% of the carbon in a mixture of methane, ammonia and water may be converted to amino acids and other organics by the energy of the discharge. Various forms of ionizing radiation give high yields as well. a particles, b particles, and g rays were common on the surface of the primitive Earth because of the presence of intense natural radioactive sources in the crust — such as potassium-40, thorium-232, and isotopes of uranium.

Table 7.2 Energy Available for Synthesis

of Organic Compounds on the Primitive Earth

Volcanic heat and other power sources

Volcanic heat was another prebiotic power supply.2368,2380 It has been shown that lava-heated seawater and underwater volcanoes may be effective in producing biologically important compounds. Heat and sonic energy would have been released by infalling meteorites — certainly a significant factor in the environment of the primitive solar system.1417,2375 In fact, experiments performed recently by Bar-Nun and others have conclusively demonstrated that as much as 30% of the nitrogen in an ammonia atmosphere can be converted into amino acids in this manner.315,1664,2375 Torrential rains have even been suggested as a possible source of energy for prebiotic synthesis, and experiments have shown that a flask of formaldehyde, allowed to stand for a few days at room temperature, will produce some simple sugars.

The great lesson appears to be that the exact nature of the power supply is relatively unimportant. Amino acids, sugars, and other chemical precursors to life probably arise on any planet possessing an initially reducing atmosphere and quantities of hydrogen, carbon, nitrogen and oxygen in gaseous reduced form — regardless of the particular source, or sources, of energy available.*

* Other factors may also be important. For instance, early-type stars (F) are more likely to emit ultraviolet radiation in copious quantities than are late-type stars (K, M). The speed of chemical evolution in primitive planetary environments may actually slow as we move from class F through classes G to K stars among habitable solar systems.

7.4 Proteins and Cells
Cellular construction

The cell is the fundamental biological unit and a common denominator among all terrestrial lifeforms. Living things on this planet are made up of cells which vary in size from less than one micron to several centimeters in diameter. While the simplest organisms are unicellular, the typical human is an ambulatory assemblage of from fifty to a hundred trillion (1014) individual cells.

… no biologist today would claim that proteinoid
microspheres are alive in the sense of representing
the first protocell. And yet, to the extent that they
self-organize, accumulate information from their
surroundings, and exhibit both structure and
behavior, they are certainly near the borrline of life.

The cellular construction of Earth life is remarkably uniform: Similar water content, similar kinds of proteins, similar lipids and so forth. All have at least one membrane, perhaps no more than 100 Angstroms thick, which protects the inner workings from the harsh vagaries of the external environment. The first protobionts undoubtedly had no such complex organizational qualities.
But how can structure arise in the first place?

It has been shown by Ilya Prigogine that thermodynamic chemical systems may develop certain states wherein some of the chemical constituents have periodic, oscillating values.2230 A biologist, J. Pringle, has demonstrated that initially homogeneous systems can undergo a progressive change, leading to the appearance of "spatial heterogeneity."2231,2232 That is, structure can arise spontaneously. These two treatments of the problem of organization suggest that mechanisms may exist for collecting material into small, localized concentrations, perhaps leading to ordered structures we would recognize as cells.2368

Protein construction

But to build cells, we must have protein. Protein is the most fundamental construction material, used in building cell walls, enzymes, and so forth. To make proteins, there are two requirements.

  • First, we need an abundance of amino acids. From our discussion above, we see that this is virtually inevitable on any normal world possessing at least small aqueous oceans and a primitive hydrogenous atmosphere.
  • Second, there must be some way to hook up a long string of amino acids into a polymer of protein. Polymerization (linking together) of amino acids leads to the production of protein, which can then be used for cell-building.* It is true that even dilute primordial soups can coagulate into gelatinous masses. But such conditions are far from ideal. In all likelihood, most prebiotic syntheses probably took place in local regions of increased concentration. The efficient construction of amino acid polymers undoubtedly occurred elsewhere than in the open seas.

Numerous concentration mechanisms have been proposed which might conceivably lead to the creation of small pockets of more potent broth. The simplest method is evaporation. Primordial soup, caught in a narrow, shallow lagoon, would slowly thicken as the water that held the components in solution evaporated away.85 As suggested by Miller and Orgel, similar effects result from slowly freezing the solution in the lagoon: The solvent freezes out in the pure form first, leaving the solute concentrated in ever-smaller quantities of solvent.521

A combination of air-water and water-solid interfaces provides mechanical consolidation of suspended matter, as evidenced by the accumulation of scums and oil slicks near coastlines.1667 Another possibility is that organic compounds may have been trapped on solid surfaces such as aluminum silicate clays, quartz, and other minerals which allow polymerization reactions to proceed.1430,2381

Proteinoid properties

To date, however, there is really only one proven method which yields polymers of amino acids under plausible prebiotic conditions. Dr. Sidney W. Fox at the University of Miami has obtained long-chain molecules with the following essential properties:

  • They contain all amino acids common in contemporary terrestrial organisms.
  • They have high molecular weights (the chains are relatively long).
  • They are "active" because they interact in the catalytic or rate-enhancing sense. (This anticipates metabolic activities mediated by enzymes — which are also proteins.
  • They are as heterogeneous as contemporary proteins.
  • They yield "organized units" upon contact with water which have many properties in common with modern cells.1625

* We will not discuss here the significance of molecular optical activity. The curious reader is referred to Sagan,20 Jackson and Moore,47 Glasstone,72 Gabel and Ponnamperuma,315 Miller and Orgel,521 Ulbricht,1445 Hochstim,1446 Bonner et al,1447 Wald,1665 and Walker.2382

Thermal Proteinoid Microspheres

Figure 7.3 Possible Model Protobionts:

Thermal Proteinoid Microspheres1625

figure 07 3 a 400

These proteinoid microspheres were produced by slowly cooling
a hot, clear solution of thermally polymerized amino acids.

figure 07 3 b 400

Structured thermal proteinoid microspheres.

figure 07 3 d 400

Various stages of binary fission of proteinoid microsphere

figure 07 3 c 400

Parent microspheres spout buds.

figure 07 3 e 400

Second-generation laid on second generation microsphere.
After the bud grows to maturity, it sprouts its own new buds.
Is this a form of incipient reproductive capability?

Dr. Fox calls his substances "proteinoids," because they greatly resemble living protein polymers. His method for producing them is quite simple. A mixture of amino acids is cooked at 120-170 °C for a few hours, and substantial yields (10-40%) of polymeric material are obtained.

Fox decided to test his method under more realistic field conditions. He secured a large piece of lava from the site of an Hawaiian volcano.1702 The temperature of the rock was raised to 170 °C, and the appropriate amino acids seated in a small depression at the top. Heating continued for several hours, after which the lava was washed off with a small spray of sterilized boiling salty water — as might have occurred naturally near a volcanic shoreline in ancient times.

Proteinoid polymers were formed, but there was more! To Dr. Fox’s surprise, billions of tiny "microspheres" appeared in the wash water: spherical, microscopic particles of uniform diameter bearing a striking resemblance to living cells (Figure 7.3).

Scenario for microsphere protocell origin

The Miami scientist presented a scenario for the origin of microsphere protocells in prebiotic times:

  • 1. Hot lava meets soup;
  • 2. water boils away, leaving sticky brown goop on lava;
  • 3. contact with water (rain, sea spray, etc.) causes proteinoids to assemble themselves; and
  • 4. microspheres are washed back into the soup.

These initial experiments were completed nearly two decades ago,1702 and since that time Fox and his colleagues have refined their methods and perfected their theories on the origin of cells and life. Protein-like materials are now produced with molecular weights ranging from 3000 to 10,000 under plausible primitive Earth conditions.2371 And it has been shown that a primitive “cell” with most of the attributes of life can arise spontaneously in a very brief period of time.

Microspheres studies

Detailed studies of microspheres have confirmed the researchers’ initial optimism. What makes these spherules so unique is their “active” nature. Dr. Fox has observed and recorded the following characteristic behavior of his proteinoid microspheres under various chemical and physical conditions:

  • Spherical shape — 0.5 to 7.0 microns, uniformly.
  • Single-walled membranes (like plants) and double-walled membranes (like animals).
  • Simulation of osmosis — microspheres swell and shrink in response to changes in the chemical environment.
  • Selectivity of diffusion — microspheres possess semipermeable membranes analogous to those of living cells. For instance, in one case Fox discovered that polysaccharides were selectively retained under conditions in which monosaccharides diffused freely through the microsphere walls. (Polynucleotides and other organics are also absorbed from aqueous solution.1624)
  • Cleavage — a kind of binary fission of a single “cell” has been observed in acidic proteinoid microspheres.
  • Motility — the microspheres, when viewed under a microscope, move non-randomly in preferred directions under certain special conditions. The addition of ATP appears to enhance the movement.
  • Budding — buds appear spontaneously on proteinoid microspheres allowed to stand undisturbed in their mother liquor.
  • Growth by accretion — buds which have been liberated by mild heating or electric shock will swell by diffusion to the same approximate size as the “parent” cell.
  • Proliferation through budding — second generation budding has frequently been observed on buds that grew to the size of normal microspheres. The buds are apparently engaging in a kind of “reproduction.”
  • Formation of junctions — microspheres are capable of approaching one another and physically attaching together in a more or less permanent fashion.
  • Transmission of information — when two spheres have joined, small proteinoid microparticles within the larger sphere are observed to pass through the junctions. The whole process is highly suggestive of microbial conjugation.
  • Stability — the activity of the proteinoids does not diminish with storage over a period of 5-10 years.2370

The best-known of all physical cell models prior to the discovery of proteinoids was the coacervates, thoroughly researched by the Soviet biochemist A. I. Oparin, the Dutch biochemist H. G. B. de Jong, and others. Coacervates are produced by combining solutions of oppositely charged colloids such as gelatin or histone with gum arabic. When solutions of the two substances are commingled, they interact to yield clusters of microscopic structures having the appearance of tiny liquid droplets. Coacervates have many interesting properties from the point of view of the origin of life.

For instance, after these uniform spherules have aggregated, they are able to absorb various simple organic molecules from the external medium (sugars, dyes, etc.). However, Oparin has admitted that this process quickly leads to static equilibrium, and the coacervate "protocell" then becomes a passive system, unstable and prone to break-up upon standing. Another property of coacervates is their ability to convert certain chemical monomers to polymers after diffusion through the "cell" wall, although it is generally recognized that the dynamic behavior of these droplets is fairly limited.

There is another reason why coacervates,1432 sulphobes,1625 "biphasic vesicles,"1630 and many other prospective pseudocells1211,1415 do not compare favorably with Dr. Sidney Fox’s microspheres as model protocells. Coacervate droplets are formed from polymers which themselves were synthesized by living organisms. The gum arabic used to manufacture Oparin’s droplets was not produced abiogenetically, nor is it at all clear how this might be done. The great advantage of the microspheres is that they are the direct product of single, simple amino acids — amino acids that must have been common on the shores and seas of the primitive Earth eons ago.

Of course, no biologist today would claim that proteinoid microspheres are alive in the sense of representing the first protocell. And yet, to the extent that they self-organize, accumulate information from their surroundings, and exhibit both structure and behavior, they are certainly near the borderline of life. 

7.5 Nucleic Acids and DNA
Replication and the genetic code

Figure 7.4 The Role of Nucleic Acids in Terrestrial Biochemistry

Figure 7.4A The Role of Nucleic Acids in Terrestrial Biochemistry

figure 7.4A 500px

Figure 7.4B Chemical Structure of Nucleic Acid

figure 7.4B 500px

Figure 7.4C DNA Content

figure 7.4C 500px

… the development of molecular
self-replication was probably
the most critical single event
in the origin of life on Earth.

DNA – the primary information-
carrying molecule used by
all lifeforms on this planet.

In the previous section it was mentioned that there are two requirements for the production of proteins. First, there must be amino acids, and second, there must be a way to hook them together to form polymers.

There is, however, a third requirement for the origin of living systems on Earth. It will be recalled from the discussion of the definition of life that "it is the business of life to accumulate information and complexity." Let us consider this mandate in view of the problem of building proteins.

To abiogenetically produce a living system, that system must be capable of accumulating information and order from its environment. The proteins constructed by a cell must have the proper architecture for whatever job needs to be done. So our third requirement may be stated: There must be a way to hook the amino acids together in the correct sequence. Any old proteins will not do — they must be the right ones.


There exist simple chemical techniques to achieve this kind of ordering. One common example is called "autocatalysis" by chemists. Autocatalysis is a way for a process to catalyze its own production. Once a tiny bit of it has been produced, that bit catalyses the rate of reaction to yield still more, and faster.

Aside from this simple selective feedback effect, the development of molecular self-replication was probably the most critical single event in the origin of life on Earth. The origin of replication and the genetic code, as opposed to the origin of proteins and cells, allowed natural selection to begin to operate on stored information. And once evolution begins, selective advantages of superior membranes and of multicellular colonies can be expressed in the form of increased organismal complexity.

DNA — the primary information-carrying molecule used by all lifeforms on this planet — is a polymeric nucleic acid (Figure 7.4). We’ve already seen how easy it is to get amino acids and their polymers. But what about nucleic acids? Can they be demonstrated in prebiotic synthesis experiments, along with their polymers?

Nucleotide synthesis

Prebiotic assembly of purines and
pyrimidines into full-fledged nucleotides
has proven more difficult, … The main
obstacle to success seems to be the
formidable complexity of the nucleotide
molecules themselves.

In 1963, Dr. Cyril Ponnamperuma managed to synthesize adenine (one of the two most important nucleic acid purine bases) under simulated primitive Earth conditions. The NASA scientist and his three colleagues used a Miller-type apparatus, and began their synthesis with nothing more than methane, ammonia and water in the system. The mixture was bombarded with energetic electrons, and about 0.01% of the carbon in the methane was converted into adenine.304 This is highly significant because adenine is useful, not only for making DNA, but also RNA, ATP, ADP, FAD, and a host of other critical life-molecules.

In a related experiment two years later, Dr. John Oró of the University of Houston and A. P. Kimball produced adenine is a closed reaction system which included ammonia, water, and hydrogen cyanide. Heat was supplied as the energy source, and this time the production of the purine base rose to 0.5% of the available carbon.303 This value was observed over a wide range of chemical conditions, indicating the relative ease with which this complex molecule must arise in a plausible prebiotic environment. The synthesis of the other important purine, guanine, has also been convincingly demonstrated.

There have been various attempts to fabricate the three major varieties of pyrimidine bases which are also necessary in the production of nucleic acids. However, the appearance of these substances under conditions similar to the primitive Earth has not been investigated as thoroughly as the purines.

One experiment that yields a hefty 20% of cytosine requires a three-step process involving methane and nitrogen initially to create a cyanoacetylene intermediate, which then goes on to produce the pyrimidine when combined with cyanate ion. Uracil, another pyrimidine, is obtained in very good yield by the direct hydrolysis of cytosine — a prebiotically reasonable reaction. All the pyrimidines have been synthesized in environments at least arguably analogous to that of the early Earth.

Prebiotic assembly of purines and pyrimidines into full-fledged nucleotides has proven more difficult, and intensive investigations are now underway to determine and eliminate the problem. The main obstacle to success seems to be the formidable complexity of the nucleotide molecules themselves. While bases and sugars are relatively easy to produce, combining them together is a much harder task.

Nevertheless, demonstrations of nucleotide synthesis under geologically plausible constraints have been made. One such technique involves the use of a mediating mineral called apatite, which contains phosphates and oxalate ion, in an "evaporating pond" scenario.

Polymerization of nucleotides into DNA

Purines and pyrimidines are comparatively
simple to manufacture abiogenetically.
The assembly of nucleotides has also met
with some imited success, but to date
it has proven difficult to synthesize more
than six-unit polymeric chains in
a prebiotically plausible way.

We are not quite home yet. Just as amino acids needed polymerization to become protein, so must nucleotides by polymerized into DNA. What progress has been made in the prebiotic synthesis of polynucleotides?

The experimental record is admittedly spotty. When adenine nucleotides were heated in the presence of polyphosphate for 18 hours at 55 °C, adenine polynucleotide polymers were obtained ranging from 20-30 nucleotides per chain. However, in the words of the experimenter, "the concentration of the reactants had to be as high as possible when the formation of high polymeric material was desired."1625

That is, unless quite artificial conditions were contrived, the adenine nucleotides could not be forged into very long chains. In another experiment, solutions of adenine nucleotide were irradiated with UV light. Long chains were again obtained, but only when extraordinarily high concentrations of polyphosphate were maintained.1628 Under similarly unrealistic conditions, uracil polynucleotides with chain lengths ranging from 10-50 units have been found.1626,1627

One good experiment has been performed by John Oró and E. Stephen-Sherwood, using a plausible "evaporating lakebed" scenario and temperatures from 60-80 °C. Uracil two-unit chains were formed with a yield of 23%, and three-unit segments with a 12% yield. Cytosine polynucleotide chains were obtained by these experimenters with up to six nucleotides in straight-line linkages. Thymine polynucleotides 2-12 units long were produced when an unreasonable chemical environment was used; with more closely matched prebiotic conditions, five-unit chains were obtained in yields of 1% or less.1429

The polymerization of some nucleotides has proven unexpectedly difficult, partly because of the inevitable formation of unnatural side chains and partly because the reaction just doesn’t seem to want to go. Various solutions to these problems have been suggested. For instance, there are enzymes — ordinary proteins — that are capable of catalyzing these polymerization reactions with ease. These enzymes, or enzymes like them, could have arisen by nonbiological means. If this is the case, claims one researcher, "such catalysts may have been responsible for the first polymerization of nucleotides on the primitive Earth."72

So at present, here is where we stand. Purines and pyrimidines are comparatively simple to manufacture abiogenetically. The assembly of nucleotides has also met with some limited success, but to date it has proven difficult to synthesize more than six-unit polymeric chains in a prebiotically plausible way.2370

Can these short strands alone make a stab at primitive replication? Dr. Leslie Orgel at the Salk Institute in San Diego, California, mixed up a solution of nucleic acids that might be considered prebiotically reasonable. He then placed some of the six-nucleotide polymers in his specially-enriched "soup." The short-chain DNA polymers correctly replicated themselves once out of every ten tries.

7.6 Early Biological Systems
Coevolution of proteins and DNA

Q. How did Life arise?

A. No one knows.

Thus far we have concentrated on the parallel development of polymeric amino acids (proteins) and polymeric nucleotides (DNA). We’ve seen that Dr. Sidney Fox’s proteinoid microspheres exhibit many properties which are strikingly similar to those displayed by contemporary living cells. We’ve also seen that Dr. Leslie Orgel has succeeded in demonstrating accurate, if erratic, replication in primitive polynucleotides. And yet, despite these remarkable achievements, the great final question remains untackled: How and when did the first living organism arise?*(see note on Tab# 5)

It has been fairly clearly demonstrated
that life as we know it could not have
arisen if either one or the other
[proteins or nucleic acids] was wholly absent.

The answer is as unsatisfying as it is precise: No one knows. The arguments on this score smack of the "chicken-or-the-egg" controversy. It is unknown at present if proteins and protocells came first, to be followed later by replicative nucleic acids, or whether the nucleic acids were first, and from them the cells later spawned.

It has been fairly clearly demonstrated that life as we know it could not have arisen if either one or the other was wholly absent.521 Organisms lacking nucleic acids would have no means of achieving genetic continuity and evolutionary progression, while organisms without proteins would find themselves severely limited in their ability to utilize the chemicals in their environment. Some manner of coevolution seems to be indicated.

The Naked Gene theory
the signs

One theory holds that nucleic acids evolved some kind of boundary layer, a proteinous skin to protect themselves from their surroundings — the so-called "naked gene" theory. When this invention inhibited or prevented reproduction, the parent nucleic acid molecule became extinct. When the new boundary layer served to protect the DNA without interfering with replication, these were the "protobionts" which survived.

There is some experimental evidence to support the view that polynucleotides might be able to influence protein synthesis directly.1431,2255 To do this, they must cause a selective linear organization of amino acids, and must facilitate amino acid polymerization.1444 Unfortunately, other studies have shown that the interaction between polynucleotides with individual (monomeric) amino acids is relatively weak.1248

Self-assembly in molecular structures

Most valid evolutionary sequence:

From the simple to the more complex

More convincing, perhaps, is the idea that cells were first. Self-assembly in molecular structures has been known for many decades, and experimental evidence to date favors the easy synthesis of proteins in comparison to polynucleotides.1634 Sidney Fox has remarked that the sequence:

  • protoprotein → protocell → nucleic-acid-coded contemporary cell

is the most valid evolutionary sequence because it proceeds from the simple to the more complex.1625

The primitive protocell, as modeled by the proteinoid microspheres, could have exhibited many of the properties customarily regarded as belonging only to "living" things. Under Fox’s theory, the cell would have developed nucleic acids to serve its ends, rather than the other way around.

One final piece of evidence seems to argue for the primacy of cells. In 1974, Dr. Fox and his colleagues published some experimental findings on micro-spheres which seem to imply that the proteinoid protocell can do everything optimistically predicted for it. The abstract of the paper reads, in part:

roteinoid microspheres of appropriate sorts promote the conversion of ATP to adenine dinucleotide and adenine trinucleotide. When viewed in a context with the origin and properties of proteinoid microspheres, these results model the origin from a protocell of a more contemporary type of cell able to synthesize its own polyamino acids and polynucleotides.1435 (emphasis added)

From the stuff of stars to the stuff of life

[Life is] an obligatory result
of the general growth
of the universe.

We’ve seen that scientists have discovered a relatively smooth chain of synthesis from the stuff of stars to the stuff of life. On the basis of pre biotic experiments performed to date, it is probable that most of the organic molecules of life with a molecular weight less than 1000 spontaneously appeared in significant quantities during the early years of our world. While a number of problems remain, most indications are that the origin and development of life on Earth had a certain inevitability about it.

From the simplest compounds present when our planet first congealed about 4.6 eons ago, to the first viable protobiont some half a billion years later, the patterns of development and the upward march of complexity seem unavoidable. Only the most general conditions must be needed for carbon-based life to arise: A body of water, a primitive reducing atmosphere, some source of energy, and lots of time. Life, in Soviet Academician Oparin’s own words, is "an obligatory result of the general growth of the universe."2297

Even now we humans just begin to suspect the truth: The universe is not ours alone to keep.

Countless side issues crying out to be discussed

There are countless side issues that cry out to be discussed at this point, but which unfortunately can be given only a passing nod. First of all, there is the absolutely fascinating question of the genetic code. As is well-known, genetic information is written on the DNA strand in short, three-nucleotide "words" called codons. By properly reading these encoded blueprints, a cell can construct exactly the right protein molecules.

The series of adenine nucleotides in a codon

For instance, a series of three adenine nucleotides in a codon tells the cellular machinery to use one molecule of an amino acid called lysine at that location. Three guanines in a row means that a molecule of the amino acid glycine should be used. One by one, the codons tell which amino acid to use and in what order, and proteins are built up in precisely the right way.

What is the origin of this marvelous code?1064,1444,2383 Is a three-nucleotide codon somehow optimal,1777 or would four have been more evolutionarily efficient?1064,1066 Why not the simplicity of only two? And what determined the rules of the coding itself? Three guanines mean glycine to a virus, a dandelion, or a human. Is the code somehow efficiency-maximized or error-minimized?1065 (It appears to be!1066,2378)

Origin of chromosomes / purpose of genes / methods for storage of genetic information

What is the origin of chromosomes,2301 and the true purpose of genes?2322 These are important questions for xenobiologists to be asking, because the universality of our genetic mechanisms will determine the limits of variation that can be expected in alien biochemistries.

For xenologists, of course, there are far more fundamental issues that must be raised. For example, why must genetic information be stored digitally in a linear sequence of monomer units? Could not some form of analog system serve? What of the possibility of genetic systems whose information was stored, replicated and transcribed in a planar fashion rather than linearly?1777

… on Earth, DNA is used for "replication" and
proteins are used solely for "expression" or "action."

Dr. Francis Crick has pointed out that on Earth, DNA is used for "replication" and proteins are used solely for "expression" or "action." Is it possible, he asks, "to devise a system in which one molecule does both jobs, or are there perhaps strong arguments, from systems analysis, which might suggest that to divide the job into two gives a great advantage?"22 Others have echoed this idea.521

Necessity for genotype distinct from a phenotype?

Similarly, Michael Arbib of the University of Massachusetts at Amherst questions "whether it is necessary for any lifeform to have a genotype distinct from a phenotype; in other words, whether we have to have a program to direct growth and change, or whether in fact the organism might be able to reproduce itself as a whole."85 Crick seems to agree,, suggesting that it might be possible to "design a system which was based on the inheritance of acquired characteristics."22 (At least one science fiction story has been written along these lines.2216) Arbib also wonders: "One might imagine some planet whose beings reproduce by xerography with no gene required!"85 The possibility of inheritance without genes has been suggested before,1178 although in a different context. (A general review of replication was published in 2004 by Freitas amd Merkle.)

Nucleic acid chauvinism

And we must take care not to be guilty of "nucleic acid chauvinism." We are familiar with only one molecular replicating system, but there is no reason why others should not be possible. Gordon Allen writes: "Life on other planets need not be based on nucleic acids or proteins if their catalytic functions can be otherwise provided."1591

Dr. Alexander Rich at MIT also suspects that the functions of Earthly nucleic acid are not unique. Rich believes that "other molecules could be used to form other polymers which could be used as information carriers for living systems." Later, he elaborates:

I think it would be amusing to make a chemical system of complementary polymers based on monomers that are not nucleic acid derivatives, simply to demonstrate that it can be done. In about ten years’ time, I think we will have a well-developed field of synthetic polymeric information carriers that will give us a great deal of insight into our own terrestrial system. That another system is possible might have relevance, if not to biology on this planet then perhaps to another.1587,1632

Clearly, a search should be made for non-nucleic acid self-replicating molecules. Exotic systems based on silicon, boron, or nitrogen-phosphorus chemistries are possible: Specialists in these fields expect an abundance of compounds comparable to that of carbon chemistry.1777 But we must not anticipate the subject matter of the next chapter.

Chapter 8 ♦ Exotic Biochemistries
8.0 Exotic Biochemistries

In the previous chapter, we asked the question: What is the likelihood that life may evolve somewhere else in the universe? We answered by showing that, given a primitive environment similar to that of ancient Earth, some form of proteinous life is not unreasonable.

But how deterministic are the processes that occurred on this planet four eons ago? What are the chances that life must follow the identical biochemical pathways taken by organisms on Earth? It is the principle aim of xenobiology to ascertain where life exists in the universe, and what form it takes.

At first glance, the Hypothesis of Mediocrity might seem to rule out the possibility of alternative life biochemistries. There are no silicon beasts or chlorine-breathers present on this world, ergo natural selection does not favor them and they cannot exist.

This is, however, an incredibly chauvinistic argument. The only rigorous conclusion that can be drawn from the lack of exotic biochemistries on Earth is that contemporary conditions do not favor those other systems. Since a rich diversity of habitats is possible in the Galaxy, peculiar life chemistries cannot be categorically ruled out.

Life adapts itself to its environment

Life adapts itself to its environment. Change the environment, and the nature of life itself will change. It may be that no negentropic life-system can arise spontaneously under non-Earthlike conditions, but it is poor science to tie one’s hands with this assumption from the outset. Owing to the unique adaptivity of living things, the Hypothesis of Mediocrity must be applied cautiously when we venture out into new environs.

Experimental investigations have brought to light new facts which appear to indicate that significant variations on terran biochemistry are possible — even probable — on other planets.

[Note: See also the author's article "Xenobiology", published in 1981.]

8.1 The Argument for Diversity
Chauvinisms are common in xenology
larry niven

The word chauvinism is derived from the cognomen of a highly jingoistic French soldier by the name of Nicolas Chauvin, born at Rochefort in the late 18th century. In 1815 Chauvin gained great notoriety by his obstinate, bellicose attachment to the lost cause of Napoleon’s crumbling imperium. The term has since become identified with the absurd, unreasoning, single-minded devotion to one’s own race, nationality, sex, or, most recently, to one’s own point of view.

Solar chauvinism

Chauvinisms are predictably common in xenology. For example, we have argued against what might be called "G-star chauvinism," the idea that a home sun exactly like Sol is a prerequisite for life.15 Although our sun is class G, F stars and K stars undoubtedly are also hospitable to life. But stars may not be necessary at all. The possibility exists that interstellar space may contain a large number of starless planets, objects having jovian or superjovian mass. Neglecting such an alternative could be condemned as "solar chauvinism."1470

Planetary chauvinis

There have been discussions of "planetary chauvinism," the belief that life can only exist on the surface of planets. Fred Hoyle exposes this parochialism in his science fiction novel The Black Cloud. After humans manage to open a communications link with the gaseous lifeform, the interstellar electromorph is quite astonished. "Your first transmission," says the Cloud, "came as a surprise, for it is most unusual to find animals with technical skills inhabiting planets — which are in the nature of extreme outposts of life."62


absurd, unreasoning, single-minded devotion

to one’s own race, nationality, sex,

or, most recently, to one’s own point of view.

Atmospheric chauvinism

And atmospheric chauvinism? Carl Sagan has imagined organisms trapped on a planet whose air is slowly leaking away to space. Over time, such creatures might evolve mechanisms to cope with what is essentially an interstellar environment.15 Another possibility might be an advanced spacefaring civilization that had set up outposts in deep space or on airless worlds.


There are many biochauvinisms in xenology, various preconceptions relating to indigenous alien lifeforms. For instance, one biochauvinism this author finds exceedingly difficult to overcome is "phase separation chauvinism."2371,2393 The requirement that all organisms must retain some sort of boundary layer between themselves and their surroundings seems to follow directly from the basic thermodynamic nature of life processes.

The extremes of life on Earth are well documented;
microorganisms are especially hardy.

Other things seem not so fundamental. The extremes of life on Earth are well documented; microorganisms are especially hardy.

Acontium velatum and Thiobacillus thiooxidans flourish in some of the strongest acids known (pH = 0.0), while a blue-green algae known as Plectonema nostocorum thrives in the strongest bases (pH =13.0). (Normal water is neutral, with pH = 7.0).

Microbes can tolerate poisons of many kinds in their environment, such as corrosive sublimate (mercuric chloride), sulfuric acid, and arsenic. The tardigrades can stand prolonged periods of virtually complete desiccation,* and organisms have survived pressures ranging from the vacuum of space to more than 8000 atm (the barotolerant deep sea bacteria).

Growth and reproduction have been demonstrated from -24 °C (psychrophilic bacteria) up to 104 °C, and a few organisms (tardigrades, spores) have been frozen to near absolute zero, or heated to more than 120 °C, and survived the ordeal.

Radiation resistance is low in mammals and other higher lifeforms — whole body lethal dose for man is a few hundred roentgens. But Deinococcus radiodurans and certain algae have endured as much as ten million roentgens of neutron bombardment, owing in part to special protective chemicals contained within their cells. Ultraviolet chauvinists claim that life is impossible on Mars because of the intense, unshielded solar radiation (UV) there, but many protective adaptations readily can be imagined.15,26,1238

Oxygen chauvinism

Perhaps one of the most persistent biochauvinisms is "oxygen chauvinism." A few decades ago, before the matter was given the serious thought it deserves, it was alleged that any planet lacking this "vital" gas was ipso facto uninhabitable. However, O2 is not a requirement for survival for many organisms alive on Earth today (e.g., yeasts, tetanus bacillus, etc.) and was not present in appreciable quantities on the primitive Earth when the origin of life occurred.

Oxygen needs life, rather than
the other way around

It has been shown that the present level of O2 is not optimal for plant growth. Greenery evidently grows more luxuriantly in an atmosphere containing only about half the normal amount of oxygen.53 Human scuba divers are poisoned by the pure gas at more than a couple atmospheres of pressure. The presence of O2 in the nuclear regions of contemporary living cells is usually fatal.

Oxygen is basically a reactive, toxic gas which chemically combines with and degrades virtually all useful biomaterials. The disastrous and widespread contamination of Earth’s atmosphere with O2 a few eons ago (the first real "smog crisis") might have spelled the end of life on this world had nature not been able to quickly readjust to the new situation. It is as Arthur Clarke says: "Oxygen needs life," rather than the other way around.609

The mere absence of oxygen on a planet cannot, by itself, argue against the presence of life there.

* The kangaroo rat, a common resident of American deserts, never needs to drink water. Its metabolism breaks down chemical compounds in sufficient quantities to enable it to live on the water manufactured from the food it eats. Other animals, such as the flour beetle, are known to have similar abilities, and camels can sustain themselves for weeks in this fashion. In the plant world, the Spanish moss can grow without contact with any groundwater — when humidity is high, it can extract the needed moisture directly from the air.

Liquid phase vs. the solid, gaseous or plasmic phases

The liquid phase is probably
the preferred mode of existence
for extraterrestrial lifeforms.

Perhaps a more general biochemical question is whether or not the chemistry of life must occur in the liquid state. Most biologists would probably insist on a liquid solvent.

But life in the gaseous state cannot categorically be ruled out. One can imagine a "soap-bubble beast," laced with innumerable compartments and sub-compartments throughout. Probably a creature of the air, its metabolism might consist of chemical redox reactions taking place within its many "cells" in a controlled manner with the reaction products slowly diffusing outward. Because of the lower concentration of chemicals in such a gaseous medium, the organism’s structure, complexity, size and behavior would be sharply limited.

Solid life, too, is not out of the question. Although it has been alleged that reaction rates would be too slow for such lifeforms to exist, we know that timescales are relative and highly subjective. Trees often take hundreds of years to grow to full maturity, and many are thousands of years old. There is nothing a priori absurd in positing a form of life which has extremely slow negentropic processes.

Of course, it must be admitted that the liquid phase seems rather more convenient than the solid, gaseous or plasmic phases. Ions form easily, transport is greatly simplified, the breakup and recombination of chemical bonds is facilitated, and crude environmental stability is assured. The liquid phase is probably the preferred mode of existence for extraterrestrial lifeforms.

8.1.1 Temperature Chauvinism

Table 8.1 Chemical Bond Energies for

Some Combinations of Xenobiological Interest

table 08 1

Any life chemistry will inevitably be subject to a narrow, or at least specific, temperature range. This is because a successful biochemistry is based on large assemblages of complex, delicately balanced molecules. These molecules must walk the thin line between overstability and overreactivity. Too cold, and the system grows sluggish and grinds to a halt; too hot, and reactions become uncontrollably rapid and the metabolism destabilizes.

The dedicated temperature chauvinist wants to restrict the viable range of all lifeforms to less than 100 °C, hardly enough to cover the gamut of terrestrial organisms alone. More sophisticated arguments suggest that even unfamiliar carbon-based systems probably could not exist much above 500 °C, because large carbon macromolecules shake themselves to pieces long before things get even that hot.

At the cold end of the scale, carbon-based biochemistries may be much less successful below about -100 °C. Reaction rates become extremely low, and there are fewer and fewer solvents in which the life-chemistry may proceed.

But are these valid limits for all conceivable living systems?

Perhaps not.* Table 8.1 gives the energy of various chemical bonds that might possibly occur in biologically significant molecules. If a structure is given more than this energy, the bonds may snap and the molecule falls apart. The higher the bond energy, the more stable the molecular structure. And stability is essential for any chemistry that aspires to live.

There seem to exist chemical bonds of
appropriate structural stability for life, and it
would appear premature to exclude the possibility
of life on any planet on grounds of temperature.

Carl Sagan suggests that for life to exist, the fraction of bonds disrupted due to random thermal motions must be no larger than 0.0001%. If this is true, then lifeforms whose biochemistry is based solely on van der Waals forces (a weak attraction between atomic electrons and the nucleus of an adjacent atom) alone could survive at temperatures as high as 40 K. Biochemistries relying on hydrogen bonds alone could exist up to 400 K. Bonds of strength 2.0 eV or higher would suffer less than 0.0001% random breakage up to 2000 K, and for 5 eV bonds the molecules survive up to 5000 K.2358

This spans the range of temperatures from the coldest worlds to the surfaces of stars. Concludes Dr. Sagan: "There seem to exist chemical bonds of appropriate structural stability for life, and it would appear premature to exclude the possibility of life on any planet on grounds of temperature."

* Hal Clement’s two science fiction novels, Mission of Gravity (low temperature life)2069 and Iceworld (high temperature life),292 are highly entertaining.

8.2 Alternative Biochemistries
8.2.0 Alternative Biochemistries
isaac asimiv

Professor G.C. Pimentel, chairman of a NASA Study Group on Exobiology in 1966, remarked that perhaps the most interesting and important discovery that could ever be made in the entire field of xenobiology would be the detection of extraterrestrial lifeforms based on a chemistry radically different from our own. Space probe experiments designed solely to search for Earthlike organisms cannot firmly rule out the possible presence of life solely on the basis of a negative result.  Cautions Pimentel, we must beware of the hazards of "down-to-earth thinking."2353

Yet all living creatures with whom we are acquainted are comprised of complex carbon compounds immersed in liquid water. Two classes of molecules always seem to be present: Nucleic acids, the blueprints of inherited instructions, and proteins, the materials and tools with which the architecture of life is constructed.

Must life always be based on carbon chemistry in aqueous solution? If we can agree that a biochemistry is the proper format for living systems, and that a liquid phase is probably essential, does it follow that carbon and water are our only choices in the matter?

8.2.1 The Limits of Carbon Aqueous
from exobiology

Water: ideal for our kind of life

Carbon chemistry in terrestrial organisms proceeds by chemical reactions in the medium of water — an amazing substance with a whole set of properties which make it ideal for our kind of life. Some have even contended that "water is the only possible candidate material."

In 1913, Harvard University biochemist Lawrence J. Henderson published a little book entitled The Fitness of the Environment in which he assembled for the first time the many points in favor of water as a life-fluid.879 Henderson’s analysis extends to the other molecules of life as well, and his main contribution is to show that the very chemical properties of the elements gives each of them a certain unique status and irreplaceability.

Excellent solvent

Among the many advantages of water, Henderson notes that it is an excellent solvent for countless substances, making it quite useful as a mediator of chemical activity in the liquid phase. Water, too, is an ionizing solvent, which means that an acid-base chemistry is permitted and an ever wider range of reactions can take place. (Acid-base chemistry is fundamental to Earth life but is not necessarily a requirement for all life.1074)

Water — an amazing substance with a whole set
of properties which make it ideal for our kind of life.
Some have even contended that
"water is the only possible candidate material."
High heat capacity

Hydrogen bonding between water molecules gives the liquid a high heat capacity — the ability to store lots of heat without changing temperature very much. Organisms which use water are thus at a distinct advantage in an environment in which sudden swings between hot and cold are common. This same bonding force also holds biomolecules together so that reaction rates are enhanced64 (although it has been pointed out that H-bonding may not be absolutely essential for life2353).

Wide liquidity range

Furthermore, water has a comfortably wide liquidity range — a full 100 K under normal terrestrial conditions. However, the extent to which this temperature span may be broadened is not generally appreciated. Saturated salt water may freeze as low as 250 K; under 100-200 atm of pressure, the boiling point may be elevated to as much as 640 K.

In the proper environment, water could remain a liquid over a range of 400 degrees. It is not unreasonable to conclude that H2O may well be the solvent of choice from 250-500 K, particularly in view of its extremely high cosmic abundance.

Serious laboratory work aimed at defining and measuring the limits of carbon-based, aqueous biochemistries has just gotten under way in earnest in the 1970s. Consequently, direct evidence is only beginning to emerge from the scanty data.peter moulton

Alternatives within a carbon-water system

In spite of this handicap, there are early signs that many alternatives are possible even within the confines of a carbon-water system.

Dr. Peter M. Molton at the University of Maryland has suggested that simple changes in the early prebiotic environment may drastically affect the chemical species which later turn up as the dominant actors on the biochemical stage of evolution.1094 His example is drawn from Miller-type experiments involving the prebiotic synthesis of amino acids, the building blocks of proteins.

Two forms taken by amino acids: alpha and beta

In the lab, chemists have learned that there are two common structural forms taken by amino acids. They are called alpha and beta.

The basic layout of an amino acid molecule is a chain of carbon atoms with a small -NH2 ("amino group") stuck on somewhere. In the alpha form, the amino group appears near the tail end of the molecule. In the beta form, the amino group is displaced more towards the front of the chain.

All amino acids used in terrestrial biochemistry,
with one minor exception, are of the alpha variety.
The beta forms are absent. Why?

All amino acids used in terrestrial biochemistry, with one minor exception, are of the alpha variety. The beta forms are absent. Why?

Molton shows that this peculiarity may be due to nothing more complicated than the order in which water is introduced during the early stages of chemical evolution. If H2O enters into the prebiotic reactions when the first simple compounds are being synthesized, then life will evolve with proteins consisting exclusively of alpha amino acids.

This was probably the situation on the primitive Earth, eons ago.

But what if the initial products of chemical evolution never come into contact with water at all in the early stages? According to Molton, when water is thus absent the beta amino acids will predominate. The proteins comprising the resulting extraterrestrial lifeforms would then be of the beta, rather than the alpha, variety.*

The next step, says Molton, is to try to synthesize plausible alternative nucleotides in the laboratory, simply by altering the prebiotic conditions under which they arise. Scientists are just beginning to see the myriad possibilities that may be open to carbon-water biochemistry on other worlds.

* Proteins made from the beta forms would probably not be edible by humans. Indeed, they might even be poisonous — a fact of considerable importance for future interstellar astronauts and colonists.

8.2.2 Alternatives to Water
Properties of a good solvent for life

Figure 8.1 "Ammonia! Ammonia!"(from Bracewell80)

figure 08 1

Can living processes be based on a liquid other than water (Figure 8.1)? To answer this question we must address a more fundamental problem: What are the properties of a good solvent for life?

  • First of all is availability. If the substance is exceedingly rare, there will not be enough of it around to sustain an ecology.
  • Next, it should be a good solvent for both inorganic and organic compounds, and in this regard an acid-base chemistry is highly desirable.
  • Further, the fluid ought to have a reasonably large liquidity range, so that organisms will enjoy a wide span of temperatures in which they remain biochemically operational.
  • A high dielectric constant is preferable — the liquid medium should provide adequate electrical insulation from the surroundings.
  • Also, a large specific heat would be nice, because this would give the organism thermal stability in the face of sudden or extreme temperature variations in the environment.
  • Finally, the solvent ought to have a low viscosity — it should not be too thick and resistant to flow (not an essential characteristic but certainly convenient).
Life based on liquid ammonia

J.B.S. Haldane, speaking at the Symposium on the Origin of Life in 1954, speculated on the possible nature of life based on a solvent of liquid ammonia.2328 The British astronomer V. Axel Firsoff picked up on this a few years later, and extended the analysis considerably.352,1217 Today, ammonia is considered one of the leading alternatives to water. Let’s see why.

Ammonia is known to exist in the atmospheres of all the gas giant planets in our solar system, and was plentiful on Earth during the first eon of its existence. Ammonia may be a reasonable thalassogen, so it should be available in sufficient quantities for use as a life-fluid on other worlds.

Chemically, liquid ammonia is an unusually close analogue of water. There is a whole system of organic and inorganic chemistry that takes place in ammono, instead of aqueous, solution.1579,1584

Ammonia has the further advantage of dissolving most organics as well as or better than water,2345 and it has the unprecedented ability to dissolve many elemental metallic substances directly into solution — such as sodium, magnesium, aluminum, and several others. Iodine, sulfur, selenium and phosphorus are also somewhat soluble with minimal reaction. Each of these elements is important to life chemistry and the pathways of prebiotic synthesis.

The objection is often heard that the liquidity range of liquid NH3 — 44° C at 1 atm pressure — is a trifle low for comfortable existence. But as with water, raising the planetary surface pressure broadens the liquidity range. At only 60 atm, far less than Jupiter or Venus in our solar system, ammonia boils at 98 °C instead of -33 °C. ("Ammonia life" is not necessarily "low temperature life.") So at 60 atm the liquidity range has climbed to 175 °C, which should be ample for life.

Acid-Base reactions for ammonia-based life

Table 8.2 Acid-Base Reactions for Ammonia-based Life

table 08 2
As a solvent for life,
ammonia cannot be
considered inferior to water.

Ammonia has a dielectric constant about ¼ that of water, so it is a much poorer insulator than H2O. But ammonia’s heat of fusion is higher, so it is relatively harder to freeze at the melting point.* The specific heat of NH3 is slightly greater than that of water, and it is far less viscous (it is freer-flowing too).

The acid-base chemistry of liquid ammonia has been studied extensively throughout this century, and it has proven to be almost as rich in detail as that of the water system (Table 8.2).

The differences between the two are more of degree than of kind. As a solvent for life, ammonia cannot be considered inferior to water.


Has the virtually unique property

of expanding upon freezing

* The point is sometimes made that water has the virtually unique property of expanding upon freezing, which means that ice will float atop a cooling mass of water and protect the lifeforms beneath. However, water freezing within the cells of living tissue exposes the organism to a new hazard — mechanical damage by expansion. Since ammonia shrinks when it freezes, the very property responsible for massive oceanic freeze-ups should also allow ammono lifeforms to be much more successful hibernators in a frozen clime.

Dissociation of the vital solvent

Table 8.3 Dissociation of the Vital Solvent1217

table 08 3
In the ammonia system, water, which rests with liquid NH3 to yield NH4+ ion, would seem as a strong acid, quite hostile to life. Ammono-life astronomers, eyeing our planet from their chilly observatories, would doubtless view the beautiful, rolling blue oceans of Earth as little more than "vats of hot acid."

Compelling analogues to the macromolecules of Earthly life may be designed in the ammonia system.

But Firsoff has urged restraint: An ammonia-based biochemistry might well develop along wholly different lines. There are probably as many different possibilities in carbon-ammonia as in carbon-water systems.1172

The vital solvent of a living organism should be capable of dissociating into anions (negative ions) and cations (positive ions), which permits acid-base reactions to occur (Table 8.3).

In the NH3 solvent system, acids and bases are different than in the water system-acidity and basicity, of course, are defined relative to the medium in which they are dissolved.

Living in liquid ammonia

Figure 8.2 Living in Liquid Ammonia

figure 08 2 500px

After all, water and ammonia are not chemically identical. They are simply analogous. There will necessarily be many differences in the biochemical particulars.

Molton has suggested, for example, that ammonia-based lifeforms may use cesium and rubidium chlorides to regulate the electrical potential of cell membranes. These salts are more soluble in liquid NH3 than the potassium or sodium salts used by Earth life.1132

Dr. Molton concludes: Life based on ammonia instead of water is certainly possible (Figure 8.2), theoretically, at the superficial level.

If we delve further into the complex biochemistry of the cell, we could find some insuperable barrier to ammonia-based life — but it is hard to conceive of any obstacle so insuperable that it would rule it out altogether.

Other xenobiochemical solvents

Table 8.4 Physical Constants for Xenobiochemical Solvents352,879,1578,2082

table 08 4 physical constants for xenobiochemical solvents 600 top
table 08 4 physical constants for xenobiochemical solvents 600 bottom

There are many other life-solvents (Table 8.4) which have been studied to varying degrees, though none so extensively as ammonia. Hydrogen fluoride (HF), for instance, has often been proposed. HF is an excellent solvent in theory both for inorganics and organics vital to carbon-based life.

Hydrogen fluoride has a larger liquidity range than water and has hydrogen bonding as well as an acid-base chemistry (in which nitric and sulfuric acids act as bases!).1583 It also has a large dielectric constant and a sizable specific heat. The major difficulty with HF is its extreme cosmic scarcity. However, this need not be a fatal objection in view of the widespread use of the equally rare element phosphorus in terrestrial biochemistry.

Liquid hydrogen cyanide (HCN) is another possibility. Unlike HF, hydrogen cyanide has a reasonably high cosmic abundance — although it still may be too low to be of xenobiochemical significance. HCN is a good inorganic and organic solvent, has an adequate liquidity range, has hydrogen bonding, a large dielectric constant and specific heat, and a viscosity five times lower than that of water. Its chemistry, however, may be complicated by its tendency to polymerize.

Other analogues of water

Table 8.5 The Periodic Table of the Elements

table 08 5
Sulfur analogues

Hydrogen sulfide (H2S) is the sulfur analogue of water, in which S atoms replace those of oxygen. (The two elements are of the same family in the Periodic Table (Table 8.5), and have similar chemical properties.) We might expect that H2S would have similar solvating abilities to water, but such is not the case. Hydrogen sulfide has only weak hydrogen bonding, a low dielectric constant, and is a very poor inorganic solvent.1578 Its narrow liquidity range (25 °C) means that it should be suitable, if at all, only for planets with heavy atmospheres and small daily temperature variations.

Sulfur dioxide, another possible thalassogen, is an ionizing substance which is a good organic and a fair inorganic solvent. It has an adequate liquidity range, but a very low dielectric constant.

Carbon disulfide, a wide liquidity range fluid, solvates sulfur and a number of organic compounds. But it is relatively unstable with heat and is expected to be rare on most planetary surfaces.

Fluorine oxide: direct analogue of water

Little is known about chemistry in liquid chlorine (Cl2). While it has a good liquidity range, it is five times more viscous than water. One peculiar halogen hybrid, fluorine oxide (F2O), is a direct analogue of water. This intensely yellow fluid is a good ionizing solvent, unstable at high temperatures but ideal for biochemistry below 100 K. At such temperatures, F2O might serve as solvent for the coordination chemistry of the noble gases.1172

There are many, many other less likely solvents that have been discussed in the literature.*

* Dr. Allen M. Schoffstall at the University of Colorado at Colorado Springs has performed some preliminary experiments with possible prebiotic syntheses in exotic solvents, such as formic acid, acetic acid, liquid formamide and other nonaqueous solvents. His experiments have demonstrated the feasibility of prebiotically converting nucleosides to nucleotides or nucleoside diphosphates in anhydrous liquid formamide — an alternative solvent to water.2384 Similar research is just now getting started at several other laboratories.4086

8.2.3 Alternatives to Carbon
Carbon: uniquely qualified

Figure 8.3 Depiction of a silicon-based lifeform

in science fiction

figure 08 3
Depiction of a silicon-based lifeform in science fiction
The Horta, a silicon-based lifeform depicted in an episode of Star Trek,
crouches in fear of the approaching humans. The small mineral nodules
littering the subterranean lair are the creature’s eggs.

Why do lifeforms prefer carbon?

Few elements can compete with its ability to combine with many different kinds of other atoms. As for its ability to form long, polymeric chains, carbon knows no equal. There are many who believe that the element is "uniquely qualified" for the job of life. They may well be correct.

The idea of living systems founded on a radically different chemical basis from ours has been around for a long time. It was already old hat in 1908 when Dr. J.E. Reynolds, a British biochemist, delivered a paper on the subject at a meeting of the Royal Institution in London. The reviewer for Chemical Abstracts wearily reported:

… It contains no new matter. The author advances a speculative theory as to the probability of a "high temperature protoplasm" containing silicon in place of carbon and phosphorus in place of nitrogen, and points out that silicon found in certain animal and plant cells may actually be a constituent of the protoplasm of such cells.1608

Among xenologists, the possibility of silicon (Si) -based extraterrestrial lifeforms was raised by the British astronomer Sir Harold Spencer Jones as early as 1940.44 In more recent times, silicon-based structures have become perhaps the best-known and most commonly advanced proposal as an exotic biochemistry for aliens (Figure 8.3).

Silicon-based life

Table 8.6 Cosmic Abundance of the Elements1413

Atomic Number 1-20
table 08 6
Atomic Number 21-40
table 08 6
Atomic Number 41-60
table 08 6
Atomic Number 62-92
table 08 6

This is because Si lies directly below C in the Carbon Family of the Periodic Table of the Elements (Table 8.5). Members of the same family are expected, more or less, to have similar chemical properties and to form analogous compounds.

There have been numerous objections to silicon life from all quarters of the scientific community.

A common protest, for example, is based on the relative cosmic scarcity of Si as compared to C. From Table 8.6, we note that carbon is roughly an order of magnitude more abundant than silicon in the universe.

Comparative elemental abundancess

Table 8.7 Comparative Elemental Abundancess6,96,1413,1470

table 08 7

But the real business of biochemical evolution takes place on planetary surfaces.  The Earth, Moon, and Mars are remarkably similar in their silicon content — roughly 25-30% of the total topsoil.

But on this planet, Si atoms outnumber those of C by more than two orders of magnitude (Table 8.7). Organics are present in lunar soil only to the extent of a few parts per million, and on Mars there is no trace of carbon in the crust even at the parts-per-billion level.

Carbon is actually rare!*

A few have suggested that since carbon-based Earth life exhales carbon dioxide, a gas, silicon-based lifeforms must surely "breathe out silicon dioxide, SiO2, which is quartz: a painful process …"49 It is difficult to find any merit to this biochauvinistic objection. Silicon organisms probably are able to survive only in a reducing, oxygen-free environment — so SiO2 should not be produced at all. Even if it is, it’s not clear why an extraterrestrial lithomorph should find the excretion of sand at all painful.

A seemingly more valid challenge is the contention that any available prebiotic silicon atoms will be irreversibly locked into large, heavy SiO2 polymers, making it impossible for them to participate in any life chemistry. But silicon dioxide is far from absolutely stable. In fact, it is the original material in the synthesis of many silicon-organic molecules under the action of various chemical reagents.26

Another common complaint is that the number of carbon compounds catalogued — perhaps two million or so — greatly exceeds the total number of silicon-based substances known to chemists today — about 20,000, two orders of magnitude less.

Much more attention has been paid to carbon organic chemistry
than to silicon organic chemistry, largely because most biochemists
we know are of the carbon, rather than the silicon, variety.

But the only reason a class of compounds is found may be because someone went looking for them. As few as twenty-seven organosilicon molecules were known at the turn of the century, and real interest in silicon chemistry began to accelerate just a few decades ago. Furthermore, the pitiful number of scientists currently engaged in silicon research is dwarfed by the armada of pharmaceutical houses and petrochemists flying the flag of carbon.

As Carl Sagan notes with some amusement: "Much more attention has been paid to carbon organic chemistry than to silicon organic chemistry, largely because most biochemists we know are of the carbon, rather than the silicon, variety."15

The inability of lone Si atoms to readily hook together to form very long chain polymers is often cited as the fatal flaw in all silicon biochemistry schemes. But exactly how crucial is this ability to concatenate?

* While more than thirty carbonaceous molecules have been detected in the interstellar void by radioastronomers, only two silicon compounds — the monoxide (SiO) and the sulfide (SiS) — had been found as of 1976.1002 This may, however, reflect more the zeal and interests of the searchers than the true ubiquity of molecular species containing silicon.

Carbon-Family analogues for life

Figure 8.4 Carbon-Family Analogues for Life: Polymers of

Silicon (Si),1603,1649,2348Germanium (Ge),1572Tin (Sn),1596and Lead (Pb)1696

figure 08 4 500px

In Earthly proteins, carbohydrates, and nucleic acids — the three most important and common polymer types — the C-C linkages rarely include more than a few consecutive atoms. Organic side chains may contain up to eight, and fats and various vitamin complexes use even more successive carbons, but the basic molecular backbone of life is served by only a few.

For instance, most proteins consist of a repeating -C-C-N- unit, a mere two carbons in a row.

Biochemistries need stable polymers, not long chains of similar backbone atoms (Figure 8.4).

Silicon, in combination with nitrogen and oxygen, forms a variety of ring-shaped and chain-polymer macromolecules stable in high ultraviolet radiation fluxes (such as might be found near a class F star or on the surface of an unshielded planet like Mars) and at low temperatures as well.1597

Silane (SiH4), the silicon analogue of methane with a repulsive odor, remains a liquid between 88.l K and 161.4 K. It might serve as a solvent for a cold silicon biochemistry under anhydrous reducing conditions. The Si halides might also work, though at somewhat higher temperatures.

Unfortunately, Si-Si bonds tend to break up in the presence of ammonia, oxygen, or water, all of which are more likely to appear on a colder world.

This difficulty disappears in a hot environment in which the role of oxygen has been usurped by its chemical cousin, sulfur. The problem then becomes one of preventing the low-energy Si-Si bonds from tearing themselves to pieces in the blistering heat.1172

Possible prebiotic biochemicals usable by Si or Si—C life

Figure 8.5 Possible Prebiotic Biochemicals

Usable by Si or Si—C Life352,1132,1597,1649

figure 08 5 500px

At present, the biggest obstacle is in devising plausible pathways of prebiotic evolution (Figure 8.5). Carbon seems more competitive under most conditions we can readily imagine.* Yet as Dr. Molton says, "this may be due to our own ignorance of silicon chemistry as much as to any inherent theoretical difficulty."1132

Siloxanes: "silicones" in popular parlance

In the last few decades a broad, new class of silicon polymers has been discovered which might serve as a basis for life. These substances, known as siloxanes to the chemist and as "silicones" in popular parlance, are extremely stable in the presence of oxygen and water. In fact, many silicones are formed by the action of water on the Si-Si bond.

This novel class of compounds is now under intensive investigation, as they have been found to exhibit a wide range of fascinating properties. There are rubbery silicones, analogous to soft living tissue, which remain flexible and "elastomeric" across a span of temperature that few organic polymers can match. There are hard silicone resins with impact and tensile strengths comparable to those of bone, and which retain their stoutness in hot environments.1607,1610

Silicone liquids are useful as hydraulic fluids, and some of them have very handy peculiarities. For example, polydimethylsiloxane is an oil with variable mechanical properties strikingly similar to those of mammalian synovial fluid (a kind of bone joint lubricant).230

Some silicone rubbers are selectively permeable to specific gases. One rubber which passes oxygen has been tested in artificial gill devices designed to extract the dissolved gas from seawater for the benefit of human divers.2348 These compounds are generally less active chemically, stronger, more heat-resistant and more durable than their carbon counterparts.

The molecular architecture of the silicones is relatively simple. Silicones have a backbone, not of Si atoms alone but rather of alternating silicon and oxygen atoms. The side chains can be organic, and are as complicated as any in terrestrial organic chemistry. Silicones appear to possess an information-carrying capacity and a complexity of structure as required for a successful biochemistry.

There remain two problems with such silicon-oxygen lifeforms, which must be dealt with before the plausibility of their existence can be acknowledged.

First, many silicones tend to disassemble into ring molecules at temperatures of roughly 300-350 °C. (Similar behavior is observed in most complex carbon compounds, but at somewhat lower temperatures.) It would be difficult for silicones to remain stable in much hotter climes, and it is unclear whether this slight thermal advantage is enough to enable Si to out-compete C in a high temperature regime.

There do exist a few silicon polymers that can really get out of carbon’s league. Certain Si-C combinations are good to at least 500 °C, and various aluminum-silicon structures can reach 600 °C without destruction.

The second problem that must be faced is a familiar one: How do we arrange for a plausible prebiotic evolutionary sequence? Natural planetary conditions, by and large, are not conducive to the prebiotic synthesis of silicones.

Worse, recall that most of the complexity of the silicones is derived from the carbon side chains they possess. In spite of their greater thermal stability, these Si polymers may find themselves in an indirect competition with carbon-based macromolecules.

On any world in which the carbon chemistry had evolved sufficiently far to allow C side chains (as on the Si backbone) of the requisite complexity, it is far more likely that these carbon chains would form polymers among themselves rather than splicing onto an "alien" silicone backbone molecule.

Of course, silicon is not the only game in town. Other members of the Carbon Family might stand in for C, although this is much less likely.

Germanium as an analogue

Germanium has been suggested as an analogue to carbon in some biochemical systems. N.W. Pirie has cited some rather dubious evidence for germanium-based protobionts in Earth’s past: The excessive concentration of Ge in the Hartley coal seam in Northumberland, England.2347

But we are not restricted to the Carbon Family in our quest for analogues to C. One alternative not widely known outside specialist circles involves a tricky arrangement with the element boron (B).1172,2089,2446

Looking at the Periodic Table, we see that boron lies just to the left, and nitrogen just to the right, of carbon. One might well suspect that a kind of averaging effect could take place if the two elements were combined, resulting in some sort of "pseudocarbon" system.

* It should be noted that partial substitution of Si for C occurs even in terrestrial skeletal components (e.g., diatoms, some grasses, etc.) and in protoplasm.1551,1649 Dr. Alan G. MacDiarmid, Professor of Chemistry at the University of Pennsylvania, has succeeded in forcing bacteria to take up silicon analogues of various carbon compounds in their nutrients. He has conducted similar experiments using analogues based on germanium (Ge),1172 the element directly below silicon in the Periodic Table and whose compounds have long been known to possess certain medical properties.1576

Boron-Nitrogen analogues

Figure 8.6 Boron-Nitrogen Analogues

figure 08 6 500px

Indeed, this does occur. There are compounds made of alternating boron and nitrogen atoms which closely parallel their organic counterparts in many ways. They have the same types of bonds, similar molecular weights, similar physical and chemical properties, and so forth. A few possibilities are illustrated on the following page by comparing a series of common carbon compounds with their boron-nitrogen analogues (Figure 8.6).

While some B-N polymers are known to be stable to high temperatures, many such substances turn out to be less stable with heat. Borazine, the boron-nitrogen analogue to benzene, is more susceptible to chemical attack because of its greater reactivity. The presence of water tends to degrade most B-N polymeric compounds.

Part of these difficulties can be eliminated by switching to other combinations which also give a "pseudocarbon" effect. There are the boron-phosphorus (borophane) and the boron-arsenic (boroarsane) systems, which are known to be extraordinarily stable and inert to thermal decomposition. These substances might serve on high temperature worlds if the abundance problem could be licked.

Halogen life

A completely different kind of exotic biochemistry is the possibility of halogen life. Members of the Halogen Family, of which fluorine and chlorine are the most abundant, could conceivably replace hydrogen atoms in whole or in part. This would apply to biological macromolecules constructed on the basis of carbon, silicon, or any other viable backbone system.

An oxygen-poor star might give rise to planets with abnormally high concentrations of free halogen. This is not as unreasonable as it might sound at first. The element phosphorus, a common atom in Earthly biochemistry, has a cosmic abundance approximately equal to that of fluorine and chlorine. Thus, the availability and use of halogens by alien lifeforms cannot be categorically ruled out.

There might exist water oceans and an atmosphere rich in chlorine or fluorine. Peter Molton has proposed a respiration-photosynthesis cycle for such a world, involving carbon tetrachloride as the halogen analogue of methane.1132

Going still further out on a limb, Isaac Asimov has set forth the possibility of fluorocarbon (Teflon) or chlorocarbon polymers floating in seas of molten sulfur. "No one," the Doctor gently chides, "has yet dealt with the problem of fluoroproteins or has even thought of dealing with it."2344 No one, that is, except science fiction writers.1359

Other polymers of possible xenobiochemical interest

Figure 8.7 Other Polymers of Possible

Xenobiochemical Interest

figure 08 7 500px

Actually, polymers of any kind should be of interest to xenobiologists (Figure 8.7). Since the basis of all life appears to be the polymeric organization of small molecules into larger ones, polymer chemistry seems a reasonable avenue to explore for alternative biochemistries.

In view of various deficiencies in normal carbonaceous organic chains, many other classes have been examined in recent times.2348 According to H. R. Allcock, a chemist at Pennsylvania State University, "a new revolution based on organic polymers is about to begin."

Used in "silly putty"

Silicon-nitrogen rubbers and oils have been known for many years. These compounds, called silazanes, are unstable in the presence of water or in an oxygen atmosphere.1598 Inorganic polymers with alternating silicon and boron atoms have turned up recently, and a boron-oxygen-silicon linkage is used in the well-known "silly putty." Various carbon-boron ("carborane") polymers which are quite stable have been discussed in the literature,1575 along with short-chain nitrogen, sulfur, and silicon-sulfur arrangements.

Phosphorus, nitrogen, and chlorine combine to form a kind of rubber in a water-free environment. These "polyphosphazines," as the chemists love to call them, are normally highly unstable in the presence of H2. However, it has recently been learned that short segments can be polymerized and made water-stable.

Soon after this discovery, the elated researchers wrote: "… it now seems likely that almost any set of required properties can be designed into the polymer by a judicious choice of side groups." The proposal that polyphosphazine polymers be used in biomedical applications to transport fixed metal ions2351 suggests a wide range of xenobiochemical applications, perhaps analogous to the metal-containing complexes in chlorophyll and hemoglobin.

8.3 Exotic Lifeforms
norman horowitz

The bottom line in xenobiochemistry

What is the bottom line in xenobiochemistry?

It must be admitted that the mere ability of atoms to assemble themselves into polymers, while significant, is yet a far cry from the complex biochemistry needed to sustain a living system. There remains a vast gulf between the simple silicone and polyphosphazine polymers and the orchestrated symphony of life.

While chemists have been vaguely aware of the possibility of exotic life schemes for more than a century, no coherent, well-integrated alternative system has been proposed and none is on the horizon. Without actual specimens of alien organisms to examine, the task suddenly takes on staggering dimensions. Imagine trying to speculatively reconstruct our entire terrestrial biochemical basis, having no prior knowledge of its nature or even of its existence!

It is quite premature to conclude
that ours is the only, or even the
best of all possible, biochemistries.

The evidence admittedly is against the existence of silicon-based life-forms: The evolutionary mechanisms and planetary conditions appear much too unwieldy. Ammonia life seems far more feasible, if for no other reason than it is so closely analogous to terran biochemistry.

It would also appear that carbon is the backbone element of choice in Earthlike environments, although this should impose no real restriction on diversity. One must agree with Shklovskii and Sagan when they assert: "It is quite premature to conclude that ours is the only, or even the best of all possible, biochemistries."20

In spite of the difficulties, there probably exist many different kinds of life in our Galaxy, including some very exotic forms based on different physical interactions than ours. But we cannot be certain of this until we travel into space and seek them out.

Chapter 10 ♦ Alien Bioenergetics
10.0 Alien Bioenergetics
christian huggens

The bioenergetic challenge

Why are xenologists so concerned about bioenergetics? Bioenergetics means, simply, the study of biological energy. The engine of life, as any machine, needs a supply and a flow of energy — chemical, electrical, thermal, or whatever — to keep it running. If power is suddenly cut off, both mechanical and biological machines soon grind to a halt.

Fundamental requirement of life:

a flow of energy from a source to a sink

The thermodynamic definition of life discussed in an earlier chapter — that living systems "feed on negentropy" and thereby manage to maintain themselves against the universal drive to disorder mandated by the Second Law of Thermodynamics — demands a flow of energy from a source to a sink. This requirement is so fundamental to the basic character of life itself that we may confidently predict that bioenergetics will be a favorite discipline among alien zoologists and physiologists. But can we be as certain about the specifics?

The Viking Lander biology package assumed so. The Pyrolytic Release device tested for photosynthetic activity on Mars, and the other two experiments sought evidence of respiration and simple metabolism. But were these assumptions reasonable? Must lifeforms evolving under alien suns on distant worlds conform to Earthly patterns and cycles?

ET photosynthetic animals

ETs will have met the bioenergetic challenge in many diverse and unexpectedly clever ways. Each new race independently must have evolved intriguing and totally unique methods for absorbing, storing, distributing and regulating energy. While we don’t know for certain if extraterrestrial photosynthetic animals are possible elsewhere in our Galaxy, or if alien bloodstreams will run red, green or blue, or whether in some distant corner of the universe there exist "biological refrigerators" which can stabilize body temperatures on sweltering worlds as hot as blast furnaces, xenologists cannot resist the temptation to pose these and other fascinating questions.

10.1 Finding the Energy to Live
Energy transduction mechanisms

All life on Earth ultimately depends upon one of two sources:
Photons from Sol (photosynthetic organisms) and
Chemical energy (all nonphotosynthetic terran lifeforms).

Active life requires a flow of energy between a source (a region of high energy) and a sink (a region of lower energy). To use heat energy, for example, a difference in temperature between two points in space must be maintained. A steam engine works not because it is hot, but rather because the boiler is hotter than the condenser.

Plant life

How does plant life fit into this scheme? It has been said that the only reason photosynthesis works at all is that the surface of the sun is at 6000 K, whereas the surface of the Earth (and its plant life) is only at 300 K. Photons emitted at the higher solar temperature travel through space to this planet, enter the chlorophyll molecule and power the plant’s metabolism. Later, photons of waste heat, a form of degraded energy, are radiated off at the far lower planetary surface temperature.

Animal sources and sinks

Animals too need sources and sinks. The food they eat is burned by the oxygen they breathe, and this constitutes a useful source of energy. The external environment, by accepting bodily waste heat, serves as a sink.

All life on Earth ultimately depends upon one of two sources: Photons from Sol (photosynthetic organisms) and chemical energy (all nonphotosynthetic terran lifeforms). However, various other possibilities have been suggested for hypothetical alien beings on other worlds.

Super-jovian self-heating

One widely discussed alternative involves the evolution of life on so-called "starless planets."128,816 Such worlds, if they exist, lie in the dark plumbless abyss of interstellar space far from the coddling embrace of any friendly star. Were the object large enough, say, a massive jovian or super-jovian, it might be warmly self-heating with a tepid surface crust.

Of course, we know that heat alone will not power a living organism. And it is difficult to imagine how to establish a flow of energy in an environment heated to some relatively fixed, unvarying temperature. Most writers have ruled out life on starless planets on this basis.18,22,714

Dr. Thomas Gold at Cornell University disagrees. If we consider the surface of a starless planet as a source at 300 K, he points out, then all we need to do is find a sink somewhere at a lower temperature to establish a life-giving flow.

Space is very cold, only about 2.7 K. If this or something very close to it could be used for the energy sink, then biological thermodynamic efficiencies approaching those of terrestrial photosynthesis might in principle be possible.22

A thermal gradient in time

Many other imaginative and exotic energy systems have been
postulated by various writers, including geothermal heat and
volcanism, piezoelectricity, solar wind ions, planetary magnetic fields,
atmospheric electricity (lightning), and radioactive decay (fission).

Extending this idea just a bit further, Gold suggests that some alien lifeforms may base their processes on a thermal gradient in time rather than in space. Imagine a uniformly heated environment in which there was a slow but regular diurnal temperature variation. Usable bioenergy could be extracted through the use of a chemical system which coupled only to the equilibrium state established at each extreme.

At the hottest extreme, certain reactions might take place which stored energy in chemical form. This energy would then be released only when the temperature swung down to the coolest extreme. In this scheme, the source and sink are no longer coextensive in time. As temperature fluctuates, the surroundings would be first the source, later the sink, and so on.

Many other imaginative and exotic energy systems have been postulated by various writers, including geothermal heat and volcanism, piezoelectricity, solar wind ions, planetary magnetic fields, atmospheric electricity (e.g. lightning), and radioactive decay (fission power).

Chemical osmosis

J.W. Ycas has come up with a novel form of energy transduction, to which he has given the formidable appellation "palirrhotrophy."2379 His organisms, should they exist, are powered by chemical osmosis. A flow of bioenergy — an "osmotic current" — is established "by exploiting the rhythmic variations in salinity which occur in the estuarine environment." As the palirrhotrophic lifeform is periodically flushed, first with salty seawater and later with upriver freshwater, energy is pumped into its system osmotically.

Such creatures might exist on a predominantly watery world, one with a large moon or moons and a fast rotation to make the tides frequent but brief. A tropical climate would ensure plenty of rainfall and a bountiful source of freshwater, and high gravity would cause mountain water runoff to cut deep channels and fjords to the sea — a viable niche for palirrhotrophic ETs.

Mechanical energy, Thermoelectric organism, Thermonuclear lifeforms

Mechanical energy

Another distant possibility is the use of mechanical energy. The waves, winds or tides might be harnessed to power a shore-dwelling alien creature. A slowly rotating planet with a massive moon in a fast orbit would have plenty of mechanical wave energy available at the surface. Yet organisms would find themselves without sunshine for such long night-time stretches that they might find it useful to evolve a biomechanical energy system as an auxiliary power supply.

Thermoelectric organism

A similar proposal is that extraterrestrial lifeforms might be able to use the internal heat flowing up though the surface of a terrestrial world. Unfortunately, even on a world as far removed from the stellar campfire as cold, distant Pluto, the sun out radiates internal planetary sources by nearly two orders of magnitude. A more viable proposition, perhaps, is the concept of the thermoelectric organism.607 On a planet with thin air, located close to its star, the temperature differential between direct sunlight and shade might be sufficient to adequately power an alien biochemistry.

Thermonuclear lifeforms

A few hardy souls have even suggested thermonuclear lifeforms. At a meeting of the British Interplanetary Society back in 1948, Olaf Stapledon proposed that the fusion power of the sun might conceivably be harnessed as an alternative to biochemical processes.556 Although Isaac Asimov has used this idea in the context of a small, planetbound animal,94 such a power supply might be more apropos for electromorphs akin to Hoyle’s Black Cloud.*

* It is interesting to note that Sol, the only nearby entity we know of that uses fusion power, has an overall energy output of only about 0.0002 watts/kg. The human body, on the other hand, operates at a whopping 2.0 watts/kg, about four orders of magnitude higher than the sun!

10.2 Photosynthesis
Chemodynamic machines
konstantin tsiolkovskii

Despite that many energy schemes noted above, not all processes theoretically permissible under the Second Law of Thermodynamics are commonly or easily available to living organisms. The methods for the performance of useful work used, say, in modern industry are generally not utilized by Earthly lifeforms. For instance, changes in temperature such as might result from combustion or nuclear reactions are not found in biology. Instead, the creatures on this world uniformly may be characterized as "chemodynamic machines," operating by chemical rather than by thermal energy.

This is not a serious restriction. As Dr. E. Broda at the Institute of Physical Chemistry in Vienna points out, nutritionists have observed that "surprisingly high yields of useful energy can be obtained from food." Comparing chemical and thermal systems, Broda continues: "With a yield of 25% as observed, the {equivalent} temperature difference works out as 105 °C. Hence, if the body was a heat engine, local temperatures of at least 310 K (body temperature) + 105 K (food-conversion temperature) = 415 K would be needed."1013

We conclude, … that nonexotic chemical energy
systems will normally be the method of choice
for the majority of extraterrestrial lifeforms.

While we recall that arguments on the basis of temperature alone cannot rule out the possibility of life, this example serves to illustrate the superior competitiveness of chemodynamic as opposed to strictly thermodynamic energy systems. We conclude, perhaps somewhat chauvinistically, that nonexotic chemical energy systems will normally be the method of choice for the majority of extraterrestrial lifeforms. Naturally, the most convenient and abundant source of usable energy for most ETs will be their sun.

Designing a metabolic system

It is difficult to imagine an easier or more elegant
solution to the fundamental bioenergetic problem.

The process of energy utilization by a living creature is its metabolism.

Given the problem of designing a metabolic system, starting from the sole assumptions that:

  • a chemical framework
  • powered by sunlight

must be used, we quickly arrive at two logical conclusions.



Harvest energy:

  • Straight from the original source


Pirate energy-riches:

  • From the complacent autotrophs
  • Steal energy from each other

First, the simplest organisms in a planetary ecology will be those capable of tapping the given energy source directly. These lifeforms accumulate energy from photons received from the sun, absorb any needed inorganic matter that happens to be lying around, and put the two to work in an integrated biochemical system. Because they are able to harvest energy straight from the original source all by themselves, such creatures are called "autotrophs."


Second, we might imagine another kind of lifeform which cannot tap the energy source directly. This class of organisms is either too lazy or too incompetent to manufacture its own food. So what powers them? Instead of patiently accumulating solar energy, these larcenous "heterotrophs" pirate energy-riches from the complacent autotrophs. Since there is no honor among thieves, we would also expect to find heterotrophs stealing energy from each other as well. An entire chain of robbery would develop, with the strong taking from the weak, the stronger taking from the strong, and so forth.

With a few minor variations, this is the basic scheme of life on Earth. The autotrophs are our plant life, which take up carbon dioxide and convert it to carbohydrates and other energy-rich goodies. The heterotrophs are the animals.

Clearly, the organization of an ecology into two major groupings (producers and consumers) is not at all arbitrary but follows logically from the twin assumptions stated earlier. It is difficult to imagine an easier or more elegant solution to the fundamental bioenergetic problem. Although other ecological systems may exist, the dual autotroph/heterotroph arrangement is probably the preferred technique for chemically-based, solar-energized metabolizers.1428

Each year about 150 billion tons of carbon are taken in by the autotrophic plants on this planet and are combined photosynthetically with some 25 billion tons of hydrogen (split from the oxygen in water) to make carbohydrates. In the process, 400 billion tons of oxygen are set free. On the average, a typical molecule of carbon dioxide wends its way through the system once every 200 years; each O2 cycles less frequently, perhaps once every 2000 years.997

Universality of the photosynthetic process

Of course, it is not absolutely necessary for alien autotrophs and heterotrophs to participate in a carbon cycle biochemistry powered by the breakdown of water to oxygen. While the photosynthetic process itself is so simple as to suggest a certain measure of universality, there is nothing sacred about which chemicals are recycled. In fact, there are a number of other systems in use today right here on Earth.

Sulfur cycle as alternative

One alternative to the photosynthetic H2/O2 cycle of which humanity is a part is the H2/H2SO4 process of the sulfur bacteria — an entirely different oxidation-reduction system than the one we use. Purple sulfur bacteria take in hydrogen sulfide (H2S) and oxidize it to sulfuric acid (H2SO4). Desulfovibrio, another class of sulfur bacteria, completes the cycle by reducing the acid back to the original hydrogen sulfide gas.

Many other systems are in use on Earth besides this "sulfur cycle." There is an H2/H2O cycle, a CH4/CO2 cycle, an NH3/N2 cycle, and so forth. Microorganisms on this planet are capable of metabolizing such peculiar and diverse substances as selenium, iron sulfide, arsenic, thiosulfate ion, cyanides, and methanol. But the main hangup with using any of these exotic non-oxygenic systems to power large extraterrestrial organisms is their relative inefficiency.

Most are at least an order of magnitude less energetic than the water/oxygen cycle which dominates the biochemistry of Earth.

Because they are so woefully inefficient, non-oxygen-cycle lifeforms are significantly out-competed in most terrestrial environments and "have been driven to the fringes of life-as-we-know-it."1390,1651 Nevertheless, there have been many valiant attempts to design viable extraterrestrial ecologies around various alternatives, notably by Asimov,1358 Clement,292 Glasstone,72 Mitz,1424 Salisbury,1658 and Vishniac et al.313

Chlorophyll and possible analogues

Porphyrins are very simple ring-shaped molecules
which have been produced in many prebiotic synthesis
experiments,and which are believed capable of
autocatalyzing their own production. Once formed,
the porphyrin ring has enormous stability against
decomposition. This may help to explain why these
substances are so widely distributed on Earth today.

If photosynthetic activity is extremely useful if not essential on other worlds, what is the best way to do it? Although there are many other molecules at work, chlorophyll predominates on Earth. Chlorophyll, the green active pigment in plants, is a member of a general class of carbon compounds known to biochemists as porphyrins.

Porphyrins are very simple ring-shaped molecules which have been produced in many prebiotic synthesis experiments,1590 and which are believed capable of autocatalyzing their own production. Once formed, the porphyrin ring has enormous stability against decomposition. This may help to explain why these substances are so widely distributed on Earth today.

The porphyrin pigment chlorophyll has a single magnesium atom located in dead center. The exact function of this metal atom has yet to be clarified, but it is believed to play a crucial role in trapping and utilizing the energy of incoming photons used in photosynthesis.

If alien autotrophs use porphyrins too, will they be restricted to green, magnesium-based chlorophyll?

The case for magnesium porphyrins

A few have argued that we should consider only the most abundant metallic elements in Earth’s crust — say, the top 99% in abundance — as candidates for the central atom.2374 If we buy this assumption, then a fairly good case can be made for the exclusivity of magnesium porphyrins in any water-solvent oxygenic biochemistry.1423,2399

Of course, this is only a plausibility argument — one which utterly fails if alternative liquid media (other than water) are considered. And even in water, despite the many points in favor of Mg, some doubt remains. Other possibilities may be open to ETs.

While photochemists have so far been unable to produce a substitute porphyrin complex "which involves relatively large storage acts" per photon of energy absorbed,993 it is well-known that zinc (Zn) porphyrin complexes are capable of undergoing reversible photochemical oxidation-reduction reactions similar to those exhibited by Mg-porphyrins.1422,1423

One chlorophyll near-analogue, called zinc tetraphenylporphyrin, has shown weak photoactivity.993 Other zinc porphyrins, although admittedly rare on this planet, have been found in several organisms including Rhodopsuedomonas apheroides, the diphtheria bacillus, various mammalian organs, and in leaf tissue homogenates.994,1069 Copper porphyrins have also been found in the diphtheria bacillus, and other substitutions using nickel, cobalt, or manganese are remotely possible but seriously questioned.1422,1442

Chlorophyll stand-ins

But perhaps we are being overly restrictive. What, after all, is so magical about the porphyrins? True, they arise in prebiotic experiments, and true, they are relatively simple molecules and they get the job done. But maybe there exist other equally suitable substances that could stand in for chlorophyll in alien plant biochemistries.

It seems difficult for many to believe that porphyrins are the best-suited class of molecules for the photosynthetic function. In fact, according to Bernard Pullman of the Institut de Biologie Physico-Chimique, University of Paris in France, "it certainly is not."315 George Wald, Carl Sagan, and countless others have pointed out that chlorophyll actually absorbs light rather poorly in the green portion of the spectrum — paradoxically, the very region where solar radiation is most intense. It is likely that a variety of dyes other than chlorophyll could have been used by plants.

Terran photopigments

Several alternative pigments are known in terrestrial biology to participate in the process of light absorption. For example, the carotenoids — found in many species of bacteria, algae, and higher autotrophs — absorb primarily blue light (which has more energy per photon) and thus are red, orange, or yellow in color. Carotenoids have no metal atoms and contain no porphyrin-like substances. Another category of terran photopigments are the phycobilins, which give both the red and the blue-green algae their distinctive, vivid color.

In the purple "halobacteria" (salt-loving), chlorophyll is entirely replaced by another Earthly photosynthetic pigment called bacteriorhodopsin. This substance has a deep purple color and is chemically related to rhodopsin, the photosensitive pigment called "visual purple" found in the rods of all mammalian eyeballs.

Research has suggested that this pigment may be selectively more advantageous in certain specific environments. This is particularly true under conditions of intense sunlight, elevated temperature, high salinity, and low oxygen concentrations.2402 As a possible photosynthetic agent for extraterrestrial plant life, purple bacteriorhodopsin is less efficient (by one-third) but chemically simpler than chlorophyll.

Oxides possessing high photosensitizing activity

Going still farther afield, why must complicated organic compounds be used at all? It has been amply demonstrated that the oxides of titanium (white), tungsten (canary yellow), and zinc (white) all possess high photosensitizing activity in oxidation-reduction reactions comparable to the activity displayed by chlorophyll. And these particular pigments are known to store light-energy in stable terminal products.2374 This may be useful for biology.

Organic photocells

Finally, it is also well-known that many carbon-based organisms are capable of utilizing silicon and germanium to varying degrees. Why could not alien autotrophs, instead of sporting leaves impregnated with chlorophyll, sprout thin platelets of "organic photocells" analogous to the solar cells used by NASA to power spacecraft? Water could be split up by some electrolytic process, and the hydrogen thus liberated incorporated into useful energy-rich molecules.

Any of these substances could serve as photosynthetic pigments for alien plants. When human explorers reach out to other worlds, they may discover beautiful white, blue, red, yellow, orange, purple, glittering steel-gray — yes, even green! — landscapes of thriving vegetation.

10.3 Animal Metabolism and Respiration
Limits to chlorophyllic autotrophic animals

Given optimum shape and plausible
environmental conditions,
autotrophic turtles are quite possible.

We have seen that photosynthesis is a highly useful means for collecting and storing solar energy. But only plants have been discussed. Could "animals" use this technique as well? This idea has cropped up from time to time in science fiction, so it is worthwhile to deal with it briefly here.

The basic idea is that it might be possible to design an alien metabolism falling somewhere between pure autotrophism and pure heterotrophism. The microscopic flagellate Euglena could be a possible ancestor of such creatures. This tiny microbe feeds both by chlorophyllic photosynthesis (like a plant) and by direct absorption of organic food (like an animal).

Plant men

But when it comes to larger organisms, many writers have been unable to conceive of plausible autotrophic animals. Usually it is alleged that "plant men" are impossible because they would be incapable of collecting enough energy fast enough, and that the only remedy for this failing is to become a sessile, vegetable-like being, perhaps akin to a tall, green-skinned saguaro cactus with corrugated skin and large, leafy limbs. But is this really true?

Water breathers, to inhale the same amount of