|Chapter 5 ♦ General and Comparative Planetology||© 1979 Robert A. Freitas Jr. All Rights Reserved|
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.
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
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 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 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
The terrestrials form closest to the protosun, where pressures range from
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.
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.
Figure 5.4 Computer Synthesis of Multiple-Star Systems1258
Examples of binary and multiple star systems
generated by computer model
Examples of binary and multiple star systems generated by Stephen Dole's computer model are shown above.
þ Terrestrials are represented as solid circles.
Another set of sample solar systems
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.
Table 5.3 Cosmic Abundance of the Elements
(number of atoms)6
|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
Must have a prominent liquid phase under
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.
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*
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!
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
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.
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.
Table 5.6. Element Abundances on Earth as Compared
to the Primitive Solar Nebula315,521
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.
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
Table 5.7. Summary of Terrestrial Planetary
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.
Figure 5.6 Rasool's Model of Planetary Atmospheric Evolution
CO2/H2O Phase Diagram for Terrestrial Planets2065
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
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.
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
Figure 5.7 Biological Modulation of Planetary Atmospheres1293
Figure 5.7a Atmosphere of a Lifeless Earth
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
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).
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.
Table 5.8 Exotic Biological Modulation Schemes: Theoretical Atmosphere/
Thalassogen Biochemical Energy Systems, Neglecting Abundance Problems (after Asimov1358)
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).
So 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.
Table 5.9 Wind Speed and Planetary Surface
Conditions for Terrestrial Planets1566,2066,2087
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
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
Figure 5.8 Different Patterns of Cyclonic Meteorology
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
- Climate powered by large temperature differential between equator and poles (ΔT > 10-100°C).
- Vertical pressure gradient minimal.
- 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)
- Climate powered by vertical pressure gradient forces.
- Temperature differential between equator and pole minimal (ΔT < 5°C).
- 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:
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.)
Table 5.10 Densities of Some Thalassogens of Interest2062,2063,2069
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.
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
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)
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.
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.
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.
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
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.
Table 5.11 Rayleigh Molecular Scattering in Planetary
Atmospheres as a Function of Stellar Class
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
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
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
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.
Figure 5.10 Maximum Size for a Planet's Mountains1279
The graph at right gives the “maximum statically loaded topography” supportable by a range of different materials.
Maximum mountain heights in our solar system are roughly as follows:
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
Table 5.12 Densities and Compression Strengths
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.”
Table 5.13 Maximum Size of Oblong (e = ½) Bodies,
for Various Orogens1279
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!
Figure 5.11 Planetary Mass and Pelagic Worlds367,2044,2046
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.
Figure 5.12 Tides Raised on an Earthlike Planet
by Satellites of Various Masses and Distances
Assuming a very homogenous pair of fluid bodies, the tidal height H may be expressed mathematically as:
■ 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.
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+