© 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

5.2  Thalassogens

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.

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

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.

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), and 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.

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 below -- 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.)

^* At higher pressures these values become slightly higher. T_c, the critical temperature, is the highest temperature at which the compound stays liquefied (at any pressure). P_c, the critical pressure, is likewise the highest pressure for which the substance remains in the liquid state (at any temperature).

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!

Last updated on 6 December 2008