Life on other Planets

Thomas Gold
May 1997

Meteorites have been collected from the ice fields of Antarctica and several of them appear to have come from Mars. Trace element ratios such as the sequence of noble gases from neon to xenon, as well as the rather unusual nitrogen isotopic ratio of the Martian atmosphere, are so specific that it seems very improbable that any other body would match this so closely. Some of these meteorites contain unoxidized carbon, some of it in the form of hydrocarbons similar to molecules that are commonly found in petroleum on the Earth. One of the Martian carbon-bearing meteorites, denoted ALH84001, was analyzed and gave an indication that microbial activity had taken place in this material. Detailed examination made it seem very improbable that this evidence was due to contamination in Antarctica, but rather that the biological imprint had been present in the interior of the stone before it fell to the Earth.

For an object to be shot off from Mars into an orbit that could eventually end on the Earth, a very large meteorite impact on Mars would have to have been responsible. There are many large impact craters on Mars, so that this does not seem improbable. But in a large impact, most of the material excavated and possibly propelled to a high velocity, will have come from a considerable depth, and the contribution made by surface or near-surface materials is likely to be a very small one. In that case the past surface conditions on Mars are not significant factors for the evaluation of the evidence provided by this meteorite.

Mars, like the Earth, will have internal heat sources, and temperatures will be increasing with depth. Water is so common on planetary bodies that it seems almost certain it will be present in large quantity also on Mars, and there must then be a depth range in which it is liquid. If the surface temperature has decreased over geologic times, the depth range of liquid water would have moved a little lower. The surface itself and a thin layer below are cold, so that any water coming up from deeper levels would generally not spill over the surface, but freeze in the rocks. Very little would reach the surface; in contrast to the circumstances on the Earth, where a surface temperature above the freezing point of water allowed all the ocean water to come up and spill over the surface. Small amounts of water vapor have indeed been detected in the Martian atmosphere.

The surface materials will have had a very different chemical history on the Earth as on Mars; but below the surface there will be somewhat similar materials on the two bodies, as represented by a mix of the meteorites, the left-over debris of planetary formation.

A comparison of the Martian meteorites with terrestrial sub-surface materials may then be meaningful. Temperatures and pressures will generally increase with depth, but at different rates on the different bodies, rates that are not yet known for Mars.

The Widespread Presence of Hydrocarbons in the Solar System and in the Universe

The stability of hydrocarbon molecules against thermal dissociation is greatly increased by pressure, an effect frequently ignored in the Western petroleum literature. This has been studied by several thermodynamicists in the USSR., and the conclusion they reached was that on Earth there could be hydrocarbon molecules at a depth of as much as 300 km, at a temperature of 1,000°C and a pressure of 100 kilobar. In the Western literature no oils are expected to exist at deeper levels than 10 km, and hence a supply of petroleum from below seemed impossible.

Of all the materials in the crust, the hydrocarbons (natural petroleum liquids and gases) appear to be the carriers of a large fraction of the element carbon, percolating to the surface in thousands of locations. Once in our oxidizing atmosphere they would rapidly be converted to CO2. Atmospheric-oceanic CO2, which the plants use for their carbon, would be depleted in a small fraction of geologic time, chiefly by the deposition of carbonate rocks. A source of carbon must be provided by the interior of the Earth, throughout all of the time that carbonates have been laid down, and the geologic record shows this to have occurred in all geologic epochs.

Similar outgassing processes seem to have occurred on many other planetary bodies. Jupiter, Saturn, Uranus, and Neptune have hydrocarbons in their massive atmospheres. Titan, a satellite of Saturn, has a substantial atmosphere in which the hydrocarbons methane and ethane seem to play a role similar to that of water on Earth, forming clouds and probably rain, and as with water here, there must be evaporation from lakes or oceans on Titan to resupply the clouds. In addition to methane and ethane, a number of other hydrocarbon molecules are identified spectroscopically, and they are quite similar to the range of molecules in terrestrial natural petroleum.

Many of the asteroids, the small planetary objects in orbits between Mars and Jupiter, have a surface reflectance resembling that of solid hydrocarbons. Also, interplanetary dust grains have been captured and analyzed with great skill and have shown the larger hydrocarbon molecules [polycyclic aromatic hydrocarbons] to be present in them. Also, the molecular clouds in the galaxy, out of which solar systems like ours will have formed, contain carbon, the fourth most abundant element, largely as hydrocarbons. The meteorites show us a group called carbonaceous chondrites, containing a few percent of their mass in heavy hydrocarbon molecules.

The Hydrocarbon Association with Helium and with Biological Materials

When it was widely believed that natural petroleum had derived from very large deposits of plant and animal debris in the sedimentary cover of the crust, this seemed to provide the explanation for the existence of many specifically biological molecules found in all the oils. But not only biological molecules show a strong association with hydrocarbons, the noble gas helium is also seen closely associated with hydrocarbons all over the world. All commercial helium is produced from oil and gas wells. Although the literature contains hundreds of examples of this association, no mechanism has been suggested that would explain how it could have arisen.

Helium, being chemically quite inert, could not have been concentrated by plants or by any chemical action. This association of hydrocarbons with helium and with biological molecules is seen not only in major oil and gas fields, but also in the seepage of gases in many locations on the Earth's surface. Why would helium come up preferentially in petroleum- bearing zones?

The only possibility for concentrating helium is a purely mechanical action, a pump. Some pumping action must have driven helium specifically to the hydrocarbons area. But why and how?

The only solution to this puzzle that I have been able to see, would require a very deep origin of the hydrocarbons, a depth of 100 kilometers or more, where the temperature and pressure would liquefy some components of the solid hydrocarbons that were present in the building materials of the Earth. Buoyancy forces relative to the higher density rocks would drive these liquids upwards. On their long pathways through the fractures in the rocks, caused and held open by the fluid pressure, they would force up helium atoms that constantly accumulate from the radioactive decay of the widely distributed radioactive elements uranium and thorium. This pumping action enriches the hydrocarbons with helium. If hydrocarbons are the most abundant fluids coming up from great depths, then they would be the ones that pump up the most helium.

But if the hydrocarbons come from great depth, they will not be of biological origin (just as they are not of biological origin on the other planetary bodies mentioned). The explanation of the biological molecules as coming from plant debris is then not valid. How then can the presence of biological molecules found in all oils be explained?

The only way I could see of solving this puzzle was to suggest that a widespread microbiology exists down to moderate depths, including the depths of all oil wells (a depth of about 8 km). Such microbiology could provide the oils with all the biological molecules that are seen; in fact several of them can only be produced by microbiology.

The viewpoint that the main components of petroleum formed at depth and without the intervention of biology, from materials incorporated in the Earth at its formation, has been vigorously pursued in Russia [Soviet Union] since the days of Mendeleev, who wrote an important paper on the analysis of petroleum and concluded that it all came from deep down in the Earth.

Several hundred publications exist that support this viewpoint, some indeed present strong evidence for it. Sir Robert Robinson, a Nobel Laureate, made detailed studies of natural petroleum, and he concluded:

Actually it cannot be too strongly emphasized that petroleum does not present the composition picture expected from modified biogenic products, and all the arguments from the constituents of ancient oils fit equally well, or better, with the conception of a primordial hydrocarbon mixture to which bio-products have been added. (1963)

"A primordial hydrocarbon mixture to which bio-products have been added" is a good summary of the position presented here. If there was much microbial life below, and a good food supply for it, then this might have far-reaching consequences, not only for petroleum geology but also for many aspects of the evolution of the crust, and possibly for biology and the evolution of life.

These considerations prompted me to write the paper: "THE DEEP, HOT BIOSPHERE", (Proc Nat. Ac. Sci. July 1992). The microbial life forms involved must then be hyperthermophilic, living at temperatures up to 120°C, possibly as much as 150°C. And the quantities, in terms of mass or volume, would have to be comparable with all the surface life we know. This would solve the sharp paradox that had split petroleum geology into two camps and had stymied progress of the discussion of the origin of petroleum for many decades.

What Energy Sources Would There be for Such Life?

Microbial life could only flourish if there was a supply of the element carbon and a chemical energy source, a "food" for them. The heat that surrounds each microbe can supply no energy; energy can be derived only from the flow of heat from a hot body to a colder one, and the microbes in the rocks are far too small for any temperature differences across their bodies to arise. ("You can sit in a hot tub as much as you like, but you will still need to eat.") Hydrocarbons are a chemical energy source, but only in the presence of oxygen, so that it becomes possible for the microbiology to mediate the energy-giving process of oxidizing them. On the surface of the Earth this is easy, the atmosphere provides virtually unlimited amounts of free oxygen. But where is the oxygen deep down in the pores of the rocks where we find oil?

The rocks contain oxygen in abundance, only most of it is bound too tightly, and it would take more energy to free this oxygen than could be obtained by the oxidation of the hydrocarbons. There are just a few commonly occurring substances in the rocks that have sufficiently loosely bound oxygen to allow the oxidation of hydrocarbons to be an energy source. Highly oxidized iron is one of them, sulfates (oxidized sulfur compounds) are another. Microorganisms can then feed on the combination of hydrocarbons with some oxygen they can take off these substances. One must then expect to see the accumulation at least of the solid end- products of some or all of these processes in hydrocarbon-rich areas.

Search for Life on Other Planetary Bodies

The search for sub-surface life on other planetary solid bodies such as the Moon, Mars, and many asteroids and satellites of the major planets, will now become a high priority item in planetary research. The surface conditions on the other solid planetary bodies are all quite different from those we have here, where the conditions are remarkably favorable for the development of surface life. But the sub-surface conditions will be similar to ours on most of these bodies, though depth dependence of pressure and temperature will be different. The possibility of developing life in them may then be not too different from the circumstances here. Hydrocarbons on them are known, and sub-surface liquid water can be expected on many of them. The rocks will contain some oxidized components that will serve as oxygen donors. The scene would be set for the existence of microbiology there. The recommendations I made specifically for Mars (in the paper mentioned above) included the search for evidence of microbial life in the carbonaceous Martian meteorites that had been found in Antarctica (a search that is still in progress now). For future interplanetary missions that could return a sample back to Earth, I thought that it would be best to go to locations where material is exposed now, that must once have been at some depth. The outstanding case is the floor of the deep "Vallis Marineris," where massive landslides have exposed material that must once have been at a depth well into the liquid water domain.

What are the Solid Products of this Microbial Activity?

The liquid or gaseous products will generally escape in short times and would not be maintained in a small meteorite on a long space flight. Where iron oxides served as the oxygen donors, the end product will be iron in a less oxidized state in which it is magnetic. Magnetite is the most common form. A further removal of oxygen, such as the step to metallic iron, requires more energy than is available in the reaction. Where sulfur oxides were the oxygen donors, one must expect to see just sulfur or unoxidized sulfur compounds such as hydrogen sulfide or metal sulfides. The product of the oxidation of the hydrocarbons will be carbon dioxide and water, and in many rocks this will react with oxides of calcium or magnesium to make solid carbonates. Those are the carbonate cements that fill up small pore spaces, and must have been transported by a liquid before precipitating.

Hydrocarbon-rich Areas on Earth

Magnetite and sulfur or metal sulfides are often seen in great concentration in hydrocarbon-rich areas on Earth, as are carbonate cements that fills cracks and pore spaces in the rocks. The isotopic composition of their carbon suggests that the ultimate derivation was from the oxidation of methane. The clearest example of this of which I am aware (but not the first) was the discovery of many tons of highly concentrated grains of magnetite, together with isotopically anomalous carbonate cements and with crude oil, all at great depth in two boreholes in Sweden. From these same boreholes and depths, previously unknown microbes were sampled and successfully cultured by the Swedish National Bacteriological Laboratory. These microbes could be cultured only in the circumstances that prevailed at the depth from which they were collected, namely a temperature of around 60°C and an absence of free oxygen, making a contamination by surface microbes very improbable. By now many locations are known in which oil, magnetic iron compounds, sulfides, and carbonate cements are found together. In regions not bearing hydrocarbons, a close association of these three solids is not common.

Sub-surface Life on Mars Discovered?

Microbial life on Mars could be dependent on the same processes as we have discussed for sub-surface life here. Highly oxidized iron is abundant on Mars, and very small-grained magnetite can then be expected to be one of the accumulated residues of microbial processes; so can iron sulfide and methane-derived carbonates. Polycyclic aromatic hydrocarbons are the large molecules that might remain in a rock that originally contained crude oil but then was exposed for millions of years to the high vacuum of space. All these substances have been found in the discovery meteorite, closely packaged to each other, and this by itself would make a strong case for the microbial interpretation. In addition, there are small objects seen under scanning electron microscopy that may well be fossils of microbes. While the last item by itself would not be conclusive evidence, the combination of this together with oil and the three residue products make a strong case for the microbial explanation. It is true that each step can occur without biological intervention, but the chance of finding by chance the evidence for all three solids in a small volume, together with hydrocarbons, seems to be very low. Many terrestrial oil and gas wells show just such an association (but an association with helium also, which the meteorite could not have transported through space).

Past Life Fed by Photosynthesis on Mars?

A planetary surface without photosynthesis is in any case inhospitable for life. It is only the immense energy supply that photosynthesis provides here that may favor surface life over chemically fed life at depth. In all other respects such as radiation environment, temperature variation, and evaporation of liquids, the surface is less hospitable than the sub-surface.

It does not seem probable that Mars ever had surface life based on the energy supply of photosynthesis. Not only would a temperature regime be required that would maintain liquid water on the surface, but also a sufficient atmospheric pressure would be needed to prevent rapid evaporation of water and subsequent deposition as ice at the poles. The atmosphere would also have to be such as to prevent the continual loss of water, through dissociation by sunlight and the subsequent loss of hydrogen to space. A substantial atmosphere would also be required to protect the surface from the destructive ionizing radiations from the Sun and from space, more so because of the absence of a protective magnetic field. The small force of gravity on Mars is not likely to have maintained a sufficiently massive atmosphere that would satisfy all these requirements.

Origin of Life: Many Independent Beginnings or Panspermia?

Does microbial life evolve spontaneously in all locations that are favorable (reminiscent of pre-Pasteur views, but with an enormously longer evolutionary time scale)? Have all such independent origins of life a similar basic chemistry? Is panspermia, the transportation of living systems between different host bodies, a real possibility? These will be the important questions.

If on another planetary body we were to find a type of biology that used quite different basic steps of chemistry, outside the range of the variants we have observed here, then we would judge this to represent an independent origin (though even then not with complete certainty). We would then be led to believe that some variants of life arise with high probability in many other favorable locations. But if we saw life forms with a similar basic chemistry, could we then make a distinction between panspermia and a very closely parallel evolution? Perhaps our chemistry is the only one that could work to make functional organisms, so no other would be found; or perhaps ours is one of a small number of possible ones, and for this reason would be likely to be discovered elsewhere.

The Significance of Chirality or "Mirror Symmetry"

But even in the cases of a similar chemistry, there would still be a possibility of deciding between parallel evolution from independent beginnings, and a distribution of life from one source, such as panspermia would provide. This arises from the property of "chirality," the symmetry that the right hand has to the left hand, or that a right-handed screw has to a left-handed one. Chirality implies that an object is different from its mirror image, no matter from which side you look at it. (Remember, a right-handed screw is a right-handed screw from whichever side you look at it; but it is seen as a left-handed screw in a mirror.) Two-dimensional objects do not posses chirality; the outline of the right hand drawn on a sheet of paper will become the outline of the left hand if observed from the other side of the paper.

In chemistry, molecules can possess chirality if they are composed of four or more atoms. To visualize this, consider first three atoms, positioned at the corners of a triangle of three unequal sides. This is necessarily a two dimensional object and cannot possess chirality; it will look like its mirror image when it is turned over. But if a fourth point is added, out of the plane of the triangle, and identified by being (say) farther from any of the three points than these are from each other, then the object possesses chirality: No direction of viewing can make it look like its mirror image.

Chirality assumes a particular importance in relation to biology. While there are many chiral molecules in inanimate matter, in each case the two forms are present in equal numbers to within random statistical expectation. Inanimate chemistry has no preference for the right-handed or the left-handed form of any molecule. All chemical processes will be accurately the same in any grouping of different chiral molecules, as they would be for another such grouping of the same molecules, but with each of the first set replaced by its chiral opposite. Now, it is a remarkable fact that in all terrestrial biology the molecules that are concerned with the basic steps of genetics and that determine the construction of next generation of the organism, represent a choice of one chirality over the opposite one. For example, if you were to select any one of the chiral amino acid molecules that make up proteins, it will show the same chirality, whether it comes from a microbe, an insect, a fish, a plant or an elephant. The usual explanation for this is that there is a common origin of all terrestrial biology; the first beginnings involved an even chance for the choice of the chirality, but after that all that followed in all of evolution continued in that same pattern. Possibly this is the right explanation, but many scientists, including the great chemist Linus Pauling, have expressed doubts whether a single beginning could have enforced such a strict rule throughout all the diverse branches of evolution that followed. Perhaps genetic material is transferred occasionally between different species, so that there is much more interaction and more coherency in the evolution of the different species than we have yet recognized. If such interaction is beneficial to one or other of the species, this would tend to enforce a common pattern.

But whatever the correct explanation may be for this remarkable fact, we clearly have a large example in front of us. For this reason we will be inclined to attribute any observation of a large asymmetry (non-racemic chiral substances) of this nature that we might find on another planetary body as arising also from living systems. The search for such an effect will be one aspect of the search for life on other bodies. Transparent liquids like water or oils have been very useful for finding biological materials, even in small concentrations, since any such asymmetry causes plane polarized light to suffer a rotation of its plane of polarization, with the sense of this rotation depending on the sense of the chiral molecule involved. In the absence of biological materials no such rotation has been found. Liquids, or liquids derived from their frozen forms such as ices or bitumens, can be examined for any asymmetry in the content of chiral molecules. Possibly the massive ice covers of several satellites of major planets are good candidates for such an examination.

But the examination of chirality also offers the possibility of distinguishing between an origin of life that is common with ours and one that derived from an independent beginning.

If we found the same basic chemistry in biological molecules of another planetary body as the one we have here, we would investigate whether the molecules there had the same chirality as ours. If they had the opposite one, we would immediately know a lot more: We would then conclude that life, using the same basic chemistry, had a good probability of arising independently on other bodies that had similar sub-surface conditions as our planet. If, however, we found the same chirality there, all we could say is that they might derive from the same evolution as ours, or that an independent origin favoring the same basic chemistry, had hit (with a 50-50 chance) on the same chirality as ours. Panspermia could be responsible, but we could not know for sure.

If we repeated such observations on yet another planetary body and obtained the same result, we would conclude that the probability was beginning to point towards a common origin, since an independent origin would have given a chance of only one in four of providing the same sense in three independent cases. The investigations of yet more planetary bodies would then become essential for resolving the issue.

Galactic Panspermia?

Are there bodies of planetary sizes that exist in abundance in the spaces between the stars? We would not have discovered them even if they were so numerous that their combined masses were an appreciable fraction of the total masses of all the stars. Molecular clouds may well be forming such objects constantly , and only a fraction would come to be associated with a star. Perhaps the frequent motion of such objects through the outer reaches of our solar system are the causes of the large perturbations that comets seem to suffer, and that bring them occasionally into the inner part of the solar system where they become evident to us. Such objects could contain and maintain for billions of years an active internal microbial life, just as seems to be the case on the Earth. Panspermia across galactic distances would then be a possibility, through impacts spalling off pieces like our Martian meteorite, when such an object had come, perchance, into the vicinity of a planetary system. In this case there would be no dependence on dormant life for long periods, nor on any long term resistance to the damage of cosmic rays, two problems that have made other galactic scale panspermia proposals seem improbable.

The Origin of Life

From the investigation of microbial life on other bodies of our solar system we may then be able to come closer to an answer to the basic questions of the origin of life. The microbes that are able to withstand the highest temperatures, and that therefore can live at the deepest levels, are found to be a very early type, judged by their genetic make-up. This may suggest that their early appearance and the evolution following them occurred underground, in the favorable circumstances of having a constant food supply, no problems of temperature changes, no radiation hazards, and minimal difficulty resulting from the evaporation of water. The deep life seems to be the best candidate for the early evolution.

It has been said that "nature abhors a vacuum." But what nature also abhors is free energy. All of biology is just a device for degrading energy available from chemical sources, and on the surface from the great temperature differential between the hot surface of the Sun and the cold of space. Perhaps biology is just a branch of thermodynamics, and there is no sudden beginning of life, but a gradual systematic development towards more and more efficient ways of degrading energy. The step to photosynthesis was no doubt a difficult one to achieve, and much evolution must have preceded it. The chemical energy available in a planetary body is then most likely to have been the first energy source, and surface creatures like the elephants and the tigers and humans and all, feeding indirectly on solar energy, are just a specific adaptation of that life to the strangely favorable circumstances on the surface of our planet.

Bibliography

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Gold, T. 1992. The deep, hot biosphere. Proc. Natl. Acad. Sci. USA 89:6045-6049.

McKay, D.S, et al. 1996. Search for past life on Mars: Possible relic biogenic activity in martian meteorite ALH84001. Science 273:924-930.

Nikonov, V.F. 1969. Relation of helium to petroleum hydrocarbons. Dokl. Akad. Nauk SSSR, Earth Sci. Sect. 188:199-201.

Wright, I.P., M.M. Grady, and C.T. Pillinger. 1989. Organic materials in a martian meteorite. Nature 340:220-222.