© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

5.3  Planetary Atmospheres

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.

Atmospheric constituents whose molecular weight (Figure 5.5, Table 5.5) places them above a planet are retained, those below are not. The closer a planet lies to the molecular weight (MW) = 1.0 line (corresponding to molecular hydrogen), the more massive its atmosphere is likely to be. Planets lying below this line will probably be gas giants.

Figure 5.5 Retention of Planetary Atmospheres as a Function of Molecular Weight (after Dole²¹⁴)

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

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.

Part of the mystery may be cleared up by considering the information contained in Table 5.6 below. 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}

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.

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

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.

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 M_earth world at Mars’ orbit should eventually become quite Earthlike, though it will naturally take much more time.)

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 under goes 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

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?

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.

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 M_earth 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).

* 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.

** 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.

Last updated on 6 December 2008