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Saturday, February 26, 2011

The Value of Astrobiology with or without Specimens

Chapter 6D

The Value of Astrobiology with or without Specimens

The lack of extraterrestrial specimens is an objection to the pursuit of astrobiology only if we accept a narrow definition of the field. Astrobiology goes beyond the search for extraterrestrial life: It is largely the application of space science and technology to understand how life may originate and evolve anywhere in the cosmos. As a practical matter, astrobiology often devotes itself to investigating how life originated and evolved on this planet. Astrobiology tries to determine, for example, what the Earth was like 3.5 - 4.5 billion years ago — what was the ultraviolet flux? What were the volcanic and other tectonic activity? How much molecular oxygen was in the atmosphere? And how much ozone? How much carbon, hydrogen, and nitrogen were "recycled" through the Earth's crust and how much were brought to the Earth by asteroids and comets? To decide these issues we must go away from the Earth to study the older surfaces of the Moon and Mars, the presumably still primordial atmosphere of Titan, and the largely untouched chemistry of comets.[1] Astrobiology is thus inseparable from comparative planetology. (Figure 3).

This connection is all the more evident when we remember that to understand the nature of an environment well we need to understand its origin and evolution. In the global environment of the Earth life has played a crucial part (e.g., in the density, temperature, and composition of the atmosphere, cf. Ch. 4). How life originated is thus a question of great importance if we are to understand how our global environment came to be as it is. At the same time, we cannot begin to settle that question without making some critical determinations about how the planet was formed, how its atmosphere was created, how much energy it received from the sun in its early evolution, and in general all those questions that form integral part of comparative planetology.

In trying to answer the question of the origin of life, however, there are great difficulties of substance and of method. For example, some investigators would not be satisfied with an answer unless it demonstrated that life was somehow inevitable or at least very likely. They would require that we specify the early conditions in the planet (e.g., a reducing atmosphere and later a primordial soup of organic materials), and then show that by processes that should be expected (e.g., radiation, lightening) organic evolution towards life was highly probable. Another school of thought would have a series of extraordinary coincidences bring life about. Thus even if organic matter was abundant on the early Earth, it would have taken an accident, or accidents, to get organic evolution on the road to life. To illustrate the sort of disputes involved let me consider a hypothesis that requires the presence of a large moon for life to begin. Of course, if that hypothesis is right, we should conclude that life is probably very rare in the galaxy and not to be found in the solar system at all, except for our own kind.

According to this hypothesis, clays served as the templates for the amino acids to combine into the first complex organic molecules. Presumably the amino acids would be floating in shallow waters over clay surfaces, but still required a mechanism that could make them more complex. That mechanism is provided by having two amino acids form a peptide bond — a bond through which carbon and nitrogen in separate organic chains become linked. This bond can form when, for example, a nitrogen atom loses its bond with a hydrogen atom (H) and a carbon nearby loses its hydroxide bond (OH). Since the H and the OH combine to form water (H20), the formation of the peptide bond requires a loss of water. The problem is how to lose water in the presence of all that water in which the amino acids are suspended.

The tides created by the Moon provide the solution to the problem. When the tides go out a residue of amino acids is left on the clays, and the heavier concentration in drier surroundings allows the peptide bond to form. Once formed, the peptide bond is stable in water. After many repetitions of this process, organic molecules of an increasing complexity can be formed. The function of the clays is to provide a mechanism for replication, but that matter will wait a few paragraphs. For the time being, the issue is the role of the Moon in the origin of life.

The sun produces tides, too, but they are much smaller and perhaps not sufficient to bring about the needed peptide bonds. If a planet were closer to the star it would enjoy greater solar tides, but unfortunately it may also become locked into a very slow rotation (the Moon always offers the same face to the Earth, Mercury rotates every 58.6 Earth days, and Venus' day is longer than its year). Having a day-night cycle seems important because the opportunity to move away from equilibrium gives the pre-biological molecules a chance to vary, and this opportunity for variation is an essential characteristic of evolution. Cyclical events are in general favorable because they permit the molecules to reach a state of equilibrium to consolidate their gains before having to change again. Apart from the tides and the day-night cycle we have the concomitant temperature fluctuations. And we have seasons. Now, the Earth has seasons because its axis is tilted in just the right way. Then we must also take into account the gravitational influence of other planets, the role of the magnetosphere and many other factors whose possible relevance or even their very existence may escape the experts at this time.

Insofar as any of these are large factors in allowing life to gain a foothold on Earth, the origin of life becomes an improbable event. But we simply do not know. As plausible as hypotheses such as this may seem today, they may sound very quaint in two or three decades, let alone in a few centuries. And even if the events in question were indeed factors in bringing life into our world, on further examination they may turn out to be just some among the many alternative mechanisms that could have provided for the evolution of ever-more complex molecules in a variety of other worlds. It happens all too often that when a mechanism cannot be immediately proposed to explain a particular step on the way to life, people who ought to know better jump to the conclusion that life on Earth was an extraordinary coincidence.

Many biochemists, for example, have felt that the problem of the origin of macromolecules is insoluble. At the most basic level, life consists of nucleic acids (such as DNA and RNA) that contain the genetic information, and of functional proteins (such as enzymes). If we imagine that DNA or RNA was the original macromolecule we have to explain how it could replicate in the absence of enzymes, which are essential in modern living systems. On the other hand if we imagine that the proteins came first, how could they have built around themselves the nucleic acids that would carry the information necessary for future coding of the same proteins?

For years none of the mechanisms proposed seemed satisfactory, not even co-evolutionary mechanisms because the chemical association of nucleotides (the building blocks of nucleic acids) and amino acids (the building blocks of proteins) was just too problematic. On the face of this situation some people thought that life was a stroke of luck. And some others even suggested that life had probably come from elsewhere to the Earth, as if removing the problem of the origin of life a few light years amounted to a solution.

Nevertheless some plausible mechanisms were proposed in the late 1960s and have been refined ever since. One of them, suggested by A.G. Cairns-Smith, is that microscopic crystals in clays can serve to replicate molecules. Such clays have a large capacity for adsorption, which causes tiny bits of proteins to stick to them, just as particles of meat do to the surface of a frying pan. The crystals in these clays would then grow and reproduce the patterns of the amino acids adsorbed in the clays.

These processes can be repeated millions of times, until with the development of enzymes, as J.D. Bernal notes, we would also see the appearance of co-enzymes, some of which are identical to the nucleotides of RNA. As the co-enzymes are adsorbed, their efficiency in chemical energy transfer would give clear reproductive advantages to their associated enzymes. Under these conditions, a co-evolution of functional proteins and nucleic acids becomes possible. And this result presumably paves the road to the eventual origin of the first cells. This view has been buttressed by the work of the space scientist James Lawless, who has shown that clays do select precisely the amino acids that can form biologically active proteins.

There are many other hypotheses, many of them buttressed by experimental work that fill in some of the steps deemed necessary to take us from atmospheric gases to living cells (e.g., proteins that can "make" their own RNA) But what is necessary and what is not depends on the approach one takes to explain the origin of life. First, there are different starting points. Some want, even demand, a reducing atmosphere (poor in oxygen, rich in hydrogen and other gases like methane). Others think that the original atmosphere was composed largely of carbon dioxide. Second, then comes a story of the evolution of organic matter, a story that may involve thousands of steps, of which only some are specified. And of course, there could be alternative plausible stories. What makes them plausible is that some of the steps that may have seemed baffling at one time can be produced in the laboratory now, while others can be explained theoretically. For example, the first generally accepted story gained its plausibility from the Miller-Urey experiment, in which a reducing atmosphere in a flask was subjected to electrical discharges. Presumably the result of the experiment was a soup containing the building blocks of life. Apparently, however, only very few interesting organic molecules were actually produced in such an experiment, and those were of very little complexity. The steps from there to, say, a self-replicating molecule, let alone a cell, are truly gigantic.

Since there are so many ways to tell the story, many equally unconstrained by the scant evidence, it is not surprising that the intuitions of different investigators differ on what is crucial and what is not. And even if most of the apparently necessary steps of a particular story can be accounted for by experiments, there remains the difficulty that the answer to one part of the puzzle is often at odds with the proposed answer to the next part (e.g., a molecule used as a building block for a more complex molecule is produced in an alkaline solution, but the more complex molecule has to be produced in its opposite, an acidic environment; this is not a fatal setback, since in living things the product of a reaction can be transported to a different internal environment to be used to build something else, and in general we find that natural processes have co-evolved in the living world to accomplish just this transport, cf. the example about elephants and their environments given above). The problem is that it all seems just too convenient. What we want to know now is not just how it could have been, but how it was -- we want the "real" story.

To go from just-so stories to compelling hypotheses we need a better understanding of the initial conditions on the Earth, and as we develop our hypotheses accordingly, we will get ideas of what sorts of evidence about the subsequent evolution of the global environment we may want to look for. One helpful way to proceed is to examine those worlds where according to some approaches life might have started, or at least where we should expect some small amount of organic evolution. To the extent that organic evolution has taken place we learn much about our own, once we factor in the relevant differences. To the extent to which organic evolution has not taken place, we also learn much about the failure of some forms of reasoning about the origin of life and perhaps get some clues about more appropriate forms of reasoning.

Astrobiology and comparative planetology will merge in many other contexts. Take, for example, the search for the origin of the organic carbon on the Earth, surely a needed background to make a definitive determination of how life started on the Earth. To have organic compounds, we first need to trace the carbon and the other relevant elements (hydrogen is normally easy, since it is almost everywhere, with the glaring exception of the Moon and Mercury). We begin the search for carbon in the solar system and then see how it was apportioned to the Earth. If the Earth had a disproportionate amount of carbon, we must deal with a certain set of scenarios in which the Earth comes to occupy a privileged position. If carbon is very common in the solar system, as indeed it is, our scenarios are of a different sort, but we still want to know how the Earth came by the amounts that it has: Did it happen during the initial accretion of the planet, or was most of the carbon brought in by the subsequent cometary and asteroidal bombardment?

How do we trace the carbon in the solar system? We will look for clues in the comets and asteroids, as well as in the inner planets and all the other rocky objects of respectable size in the solar system, particularly the large moons of Jupiter and Saturn. If we wish, space exploration will make them all available to us. In the meantime we may also examine the meteorites that we do have. In some of these meteorites we find that carbon is associated with two sets of rare gases — apparently the carbon serves as a casing that keeps the rare gases trapped. When the carbon is released from the meteorite it comes out in two installments. In the first it also frees amounts of argon, krypton and xenon in relative ratios that are pretty much the same as those found in terrestrial planets. Other carbon in some meteorites, however, encases a completely different mix of rare gases (of neon, xenon, and krypton) that cannot be accounted for by any process known in the solar system. Further investigation reveals that such is the mixture that can be expected in the process by which stars become red giants. This presumably leads to the conclusion that the stellar cloud from which the solar system formed was already seeded with carbon from the death of another star.

This interplay between biology and astrophysics takes place on many fronts. As another example, we may consider the matter of extinction, which has been described earlier. The 26 million-year period for large extinctions was calculated by David Raup from the fossil records of marine animals. It was this periodicity together with the asteroid hypothesis of Luis Alvarez that led to the further astrophysical hypothesis that the sun had a companion star. Incidentally, the computer program used to estimate the effects of the Alvarez asteroid was based on a program originally developed by Brian Toon and others to study the dust clouds of Mars, and in turn served as the basis for the so-called "nuclear winter" study — which some observers still consider a reliable model of what would happen to the Earth in case of nuclear war.[2]

If such cycles of extinction are indeed determined by astrophysical events, posterity will have much reason to thank the day these combined biological and astrophysical studies were undertaken; even if the astrophysical causes turn out to be very different from those now proposed. But quite apart from preventing great disasters, the study of extinction cycles will contribute greatly to our understanding of the forces that affect the environment of the Earth.

These and other investigations underscore the intimate connections between astrobiology and those aspects of space science that deal with the formation and evolution of planets. Since the role of life has been of crucial importance for the Earth, and since we need to know specifically how terrestrial life may not only survive but also prosper, this study of origins is highly justified. To put the point differently, the biota and many of the other elements of the global environment co-evolved. Thus to understand the evolution of one of those elements we need to understand it in its relationship to the evolution of the others. Moreover, the very role of the imagination in trying to determine the range within which life can be born and the possible forms life may take provides a fruitful context in which to discuss questions of origin and evolution. For by the consideration of likely scenarios for life, and by the comparative examination of the planets in our solar system and of other planetary systems, we will be better able to understand not only how life came about but also why it took the paths that it did when it apparently had others available.

[1]. Although the emphasis has been mine all along, in making these remarks I find myself paraphrasing Harold P. Kline's many comments on earlier drafts of this essay.

[2] R.P. Turco, O.B. Toon, T.P. Ackerman, J.B. Pollack, C. Sagan, "Nuclear Winter: Global Consequences of Multiple Nuclear Explosions," Science, 23 December 1983, Vol.222, P.1293. See also O.B. Toon et al, "Evolution of an Impact-Generated Dust Cloud and its Effects on the Atmosphere," Geological Society of America, Special Paper 190, 1982, p.187.

Saturday, February 19, 2011

ALH84001 and Occam’s Razor

Chapter 6C

ALH84001 and Occam’s Razor

In the case of Mars, the hypothesis has been much strengthened by David McKay’s team’s analysis of the Martian meteorite known as ALH84001.[1] According to this analysis, the meteorite contains globules of carbonate, polycycle aromatic hydrocarbons (PAHs), magnetite and iron sulfides, and some intriguing structures that some believe might be the fossils of ancient Martian bacteria. We know that the meteorite, which was found in Antarctica, came from Mars because the air trapped inside exhibit the same mix of rare gasses that the Martian atmosphere has. Great care was taken to rule out the possibility that the organic materials found could be the result of contamination (a claim buttressed by the fact that the proportion of those materials increases towards the center of the meteorite). Nevertheless the reaction to McKay’s results, particularly by those scientists considered experts on meteorites, was extremely hostile.[2]

The motivation for the hostility was in no small part the fear of subjecting to ridicule the subdiscipline of planetary science devoted to the study of meteorites, to let it fall into the pit of another “cold fusion,” the big scientific embarrassment of a few years previously. The main argument against McKay’s analysis was methodological. It was based on Occam’s razor, a principle named after William of Occam, the medieval philosopher who insisted that we should accept the simplest explanation available:

All the compounds and structures found in ALH84001 could have been produced by inorganic processes. Therefore, by Occam’s razor, we should eliminate the unnecessary conclusion that we have found evidence of alien life.

As it was often repeated during the debate about ALH84001, extraordinary claims require extraordinary evidence, but hydrocarbons, the magnetite and the minute worm-like features could be explained by ordinary inorganic processes. There is presumably, then, no need to conclude that Martian life caused the phenomena found in the meteorite.

Occam’s razor, however, does not rule against McKay’s analysis. For what we have is a collection of three things in an extremely confined space (a few nanometers across): (1) typical bacterial food (hydrocarbons), (2) structures that look like typical bacteria, and (3) typical excreta of bacteria (magnetite and iron sulfides). One simple hypothesis, life, accounts for all these phenomena and the fact that they are closely packed together: Martian bugs ate the hydrocarbons and left the droppings behind. The inorganic-origins hypothesis requires at least three separate mechanisms and has little to say about why they are together in such a small space.

Some of the strongest criticisms have been made against the claim that the worm-like structures are fossils. Critics have argued, for example, that those structures formed under too much heat (but other studies support McKay on this point). But the most telling criticism was that the structures were much too small to be able to carry out many important organic functions— they came in at less than one-hundredth the size of terrestrial bacteria. This leads to a very important point I will discuss below. For now let me say that, even if those structures are not fossils, the life hypothesis is still more economical: Martian bugs ate the hydrocarbons, left their droppings, but then failed to fossilize — which is perfectly understandable for soft cells. Moreover, inorganic magnetite forms at a temperature about three times higher than that apparently experienced by the Martian meteorite. And to make matters worse, the magnetite in the sample, unlike that produced by inorganic processes, is of an extremely pure form, which on Earth is normally produced only by bacteria. It is far from obvious then that the inorganic-origins hypothesis is simpler. Occam seems to smile on the life hypothesis instead.

This is not to say that I favor the life hypothesis. For I do not believe that a matter of such significance can be decided by one single methodological point -- Occam’s razor. In any event, there seems to be little doubt now that Mars has had organic carbon, and that is a great find after the discouraging results from the Viking experiments. The sensible way to proceed -- and both sides of the controversy seem to agree on this point -- is to go back to Mars and look for evidence of fossils in places naturally protected from ultraviolet radiation.

The prospects for Europa also look good, now that the favored view is that all the origin of life requires is an environment with plenty of liquid water, organic carbon and a source of energy. Europa is ten percent water, much of it apparently in an ocean under the ice; it should have plenty of organic carbon because of its location in an area of the solar system rich in organics; and it obviously has a reliable source of energy, otherwise it could not continue to smooth out its icy surface. Let us be cautions, though, about the prospects for life on the watery moons of the outer planets, Europa for example.[3] It is not enough to have around liquid water, simple organic compounds, and a source of energy. Metals such as iron, zinc, copper, nickel, cobalt, magnesium, and manganese performed catalytic and other crucial roles in terrestrial organic evolution, as they do in the normal functioning of cells today. Without most of those metals the prospects for life in any of the outer moons would be very dim. And so we must ask whether Europa is likely to have them. The answer is yes. Europa is one of the few outer moons with a density (3.0) close to that of our Moon (3.3). Indeed the rocky core of Europa under the watersphere would have a density even closer to the Moon’s. It is a good bet, then, that it would offer a variety of metals similar to that of the Moon — and this is enough to keep Europa as one of the leading candidates in the solar system to harbor alien life.

If we never find life, or fossils of life, in Mars, Europa, of anywhere else in the solar system, astrobiology will have to look at other solar systems for specimens. Unfortunately a mission to the stars would last tens of thousands of years with the technology we can muster today. And even with the best space technology that we can plausibly envision for the next century, it would still take hundreds of years to travel to another solar system. Perhaps fancier technologies (to be described in the next chapter) can get us there faster, but those technologies will not be around in the near future. There is a shortcut available to astrobiology, and that is to make contact with intelligent extraterrestrial life. But that is a long shot, as we will see in Chapter 8.[4]

Nevertheless, even if we find no alien life in the solar system, the pursuit of astrobiology will continue to be worth our while, as we will see in a soon-to-come posting.

[1] D.S.McKay et al, “Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite AL84001,” Science, Vol. 273, 16 August, 1996, pp. 924-929.

[2] See R.A. Kerr, “Ancient Life on Mars,” ibid., pp. 864-866. The controversy spread to public arguments in the newspapers; see for example the front page article “Life on Mars: Scientists ‘thrilled’ by prospect,” Seattle Times, August 7, 1996.

[3] Other rocky moons with possible oceans include the gigantic Jovian moon Ganymede: R. Cowen, “Ganymede May Have Vast Hidden Ocean,” Science News, Vol. 158, December 23 & 30, 2000, p. 404.

[4]. For a fuller treatment of this issue see Chapter 8.

Sunday, February 6, 2011

The (so far) Unsuccessful Search for Life in the Solar System

Chapter 6B

The (so far) Unsuccessful Search for Life in the Solar System

Given this apparent potential of astrobiology, then, it is not surprising, that the Viking missions to Mars in the 1970's created great hopes of a scientific bonanza. In those missions we could for the first time examine a world where life might exist. Moreover, the harsh Martian environment offered the possibility that if life had evolved there it would be radically different from Earth's. Thus when the first experiments carried out by the robot landers seemed to indicate that photosynthesis was going on, which presumably only living things can perform, the excitement was deservedly extraordinary. But the excitement soon led to puzzlement when a chemical experiment determined that the soil possessed no organic carbon (C-H bonds). The fundamental chemistry of life simply was not there. And therefore neither was life. Now this is the way the story was told at the time and for many years after. Today, several developments have changed the picture considerably. To see the importance of those developments, let me present the picture of the post-Viking landings as it stood for almost 30 years.

The result of the Viking chemistry experiment was very disheartening to many. We find organic matter in meteorites, in comets, even in deep interstellar space. How can Mars be so utterly deprived? Experiments by Stanley Miller and others have shown that some organic molecules form easily in the presence of gases that might have existed on both the Earth and Mars shortly after their formation (essentially, Miller mixed methane, ammonia, and hydrogen in a jar, subjected the mixture to electrical discharges, and then analyzed the goo that grew in the water, which yielded some amino acids and other organic compounds).[1] It is possible that in Mars those gases did not meet in any convenient ratios, but it must be said that the nature of the Earth’s primordial atmosphere is a matter of dispute. In some scenarios, that atmosphere was dominated by CO2, which would not respond to a Miller type of experiment by producing organic compounds. The terrestrial air of today, to mention another illustration, when subjected to discharges of electrical energy, produces smog, not the organic soup of the classical Miller experiment. Even so, if we drastically reduce the free oxygen and vastly increase the concentration of hydrogen in today's air, organic compounds will still form in a Miller type of experiment. Why was life so unlucky on Mars?

At the time many suspected that the Viking missions did not offer such a grand opportunity for exobiology after all. The density of the Martian atmosphere is so low that liquid water could not now exist on it. This presents at least two problems for life. The most obvious one is that it is difficult for life to get by without liquid water — although perhaps the permafrost detected on the surface of Mars might suffice to sustain certain kinds of microbial life under the rocks.[2] The second is that when the permafrost is heated by sunlight it becomes water vapor. As the water vapor rises in the atmosphere, it is subjected to ultraviolet radiation, which disassociates the molecules of water. The freed light hydrogen atoms are eventually lost to space (Mars is surrounded by a gigantic envelope of hydrogen). The heavier oxygen could be expected to react with the substances in the soil. As we saw in Chapter 4, oxygen was a poison to earlier forms of life, precisely because it reacts so easily to form compounds. The oxidizing Martian soil would make it very difficult for organic evolution, let alone life. To make matters worse, Mars has no ozone layer and thus the ground is constantly bathed by ultraviolet radiation. All in all, some even claimed, the surface of Mars is far more antiseptic than the most fastidious operating room on Earth.

Furthermore, the Martian atmosphere shows no signs that its chemistry is being altered by the presence of living organisms. The Earth's atmosphere, by contrast, has far less carbon dioxide and far more oxygen that can be accounted for by physics and chemistry alone. Indeed, as we saw in Chapter 4, the composition of the Earth's atmosphere indicates that it is not in chemical equilibrium. And according to Margulis and Lovelock, the atmospheric gases can be kept so far from chemical equilibrium only through the action of living organisms, especially that of bacteria.[3] On the basis of this reasoning, by the mid-sixties Lovelock had predicted that no life would be found on Mars.

But is life on Mars truly impossible? Some exobiologists argued at the time that the results of the Viking experiments do not warrant such a conclusion. They tried to meet the objections drawn from the chemical experiments in the following way. First, they pointed out that life would not alter the Martian atmosphere substantially if it only existed on the margins, so to speak. Small colonies of small organisms might sustain themselves without exerting pressures on the rest of the Martian environment large enough for detection with telescopes and the Viking state of the art. Second, the unprotected surface is not the place to look. There are sites in Antarctica devoid of life on the surface, but if we care to dig we may find it in porous rocks below. Third, these exobiologists brought up numerous examples of life surviving under extreme conditions: in the core of nuclear reactors, in underground streams with temperatures of hundreds of degrees Fahrenheit, under incredible pressures at the bottom of the ocean. Organisms have been found even deep in the Earth's crust. No extreme habitat, though, is as challenging as the Don Juan Pond in Antarctica, where the salinity is so great that a random sample is likely to fail the Viking test for the presence of carbon compounds. But living organisms exist there! [4]

The claim of these exobiologists was that the Viking experiments were designed to detect average life, whereas it is clear that if any life exists on Mars it should be in extreme forms. Mars is an extreme habitat, if it is a habitat at all; experiments should be designed and interpreted accordingly. Furthermore, if life cannot be entirely ruled out on Mars, we can hope, however dimly, to find it where organic matter is plentiful: in the outer solar system. We may imagine that if life can adapt to such inhospitable habitats on Earth, it might be able to make a stand in the organic clouds of Jupiter or perhaps in some underground caves in active Io. Another moon of Jupiter, Europa, is covered by smooth ice, which indicates a good amount of internal heat. This makes Europa particularly interesting to exobiologists because under the ice there appears to be an ocean of water. Another notable prospect is Titan, the large moon of Saturn with a dense atmosphere and at least traces of organic compounds. Unfortunately Titan is too cold for life as we know it — cold enough (-288 F°) that the argument about the ability of life to adapt to extreme habitats begins to wear thin.

In any event there is a serious problem with this argument. The evidence presented can show only that once life begins it can adapt to very hostile conditions. But it does not show that life could begin in such conditions. These are two very different things. Let me illustrate this point by means of an analogy. During pregnancy, many substances can be lethal to the developing embryo, e.g., alcohol, tobacco, and hallucinogenic drugs. The chances for the new life are greatly hampered under those conditions. Once the baby is born, the situation begins to change. Eventually it may grow into an adult who smokes, drinks and abuses drugs, none of which are conducive to a healthy life, but none of which need be immediately lethal either, as they could be to the embryo. Also, life might not be able to make a start on a planet that would otherwise be exactly like today's Earth; but it surely has no trouble flourishing in it now. And as far as I know, most of the scenarios considered favorable to the birth of life are at odds with the extreme habitats of these examples. It is difficult enough to figure out how life started on Earth, as we will see presently, without the complications posed by such extreme environments. On this point we require further argument from these exobiologists of old and astrobiologists of new.

As far as Mars is concerned, however, the angels are with them. For if the Martian atmosphere was at one time much denser, life might have indeed begun; and then it might have survived, gone hail or high water. These hopes are sustained to a large extent by the tantalizing possibility that, once upon a time, Mars was far warmer and wetter, a possibility indicated by surface features that resemble river deltas and by dendritic channels also similar in appearance to river systems on our planet. These channels presumably constitute evidence of running water. As we saw earlier, in a thin atmosphere, such as the present Martian atmosphere, water goes from solid to vapor without first becoming liquid. Therefore, this evidence of running water is in turn evidence that the atmosphere was much denser once upon a time.

There are alternative hypotheses on the dendritic channels, though. According to one of them, for instance, occasional but pronounced tilts in Mars' axis of rotation would expose one of the poles to the full action of the sun. If that were the case, the melting polar cap would provide enough pressure to permit water to run off and presumably form those surface features -- all without the benefit of a dense atmosphere.[5] Photographs by the Mars Reconnaissance Orbiter, with ten times better resolution than any taken before, indicate that lava and wind-driven dust have run through those presumed river channels and gullies far more recently than water, even though catastrophic floods might have carved them once upon a time.[6]

Nevertheless the preponderance of new evidence indicates that the Martian atmosphere was far denser once upon a time and that liquid water ran on the Martian surface. The most striking findings are those of Opportunity, a Mars Exploration Rover. Opportunity found, for example, salt deposits in a region called Meridiani Planum. According to Mike Carr, the man who wrote the book on water on Mars, it is clear that a large body of water existed in that region. It is also clear that the “water had to pass through the ground to pick up the dissolved ions that ultimately were precipitated out as salts.”[7] This means that the water in that region (a lake or a sea) could not have been mere runoff from melted polar ice.

If we assume that planets similar to the Earth have similar beginnings -- in this case similar distributions of organic materials, atmospheric gases, and sources of energy -- and if we keep in mind that our own earliest fossils are about 3.5 billion years old, it seems plausible to suppose that life made a start in Mars. If that is so, the possibility exists that in some regions of Mars we may find fossils of organisms that thrived in days when the atmosphere was denser and warmer.

Although such Martian fossils would perhaps not be as exciting as living organisms, they still would be invaluable in that they would permit us to compare our form of life with an alien one. We may also be able to draw some interesting lessons from a failed interaction between life and a planetary environment.

As luck would have it, though, liquid water is likely to exist in underground deposits. The permafrost is presumed to exist to a depth of hundreds of meters, which suggests that some of water will be found in temperatures high enough to keep it liquid, if nothing else because of proximity to magma deposits and other sources of thermal energy. Martian life forms, if any exist, need not be quite as extreme after all.

Incidentally, it seems to me that the reasoning that would lead us to hope for fossils in Mars might also lead us to hope for fossils in Venus, as long as we assume that it took the greenhouse effect several hundred million years to run away and vaporize the oceans. Such Venusian fossils (or perhaps even living organisms!) would have to be found well below the surface, away from the poisonous atmosphere, just as their counterparts on Earth to whom oxygen is a poison. Most likely, though, the active geophysics of Venus, as well as its high internal temperature, would have destroyed any fossils in the rocks. 600 million years ago, the entire surface of Venus is supposed to have melted by internal processes that surely would have obliterated even the hardiest of bacteria. At any rate, looking for fossils in Mars should be far more pleasant than in a place where lead would run liquid and the clouds rain sulfuric acid.

These arguments do not establish that extraterrestrial life in the solar system is likely. All they establish is that we cannot rule it out completely. But we should remind ourselves that in order to make a case for the possibility of life in Mars or Europa, we must assume either that an extreme environment is not barrier to the origin of life or that organic evolution is likely on an environment similar to that of the early Earth. This second assumption, taken as a testable hypothesis, is what makes the search for life in Mars and Europa a reasonable enterprise.

In the case of Mars, however, such an enterprise changed from reasonable to exciting when we were lucky to receive a gift from the heavens, which will be the subject of the next posting.

[1]. For a very accessible account of the classic Miller-Urey experiment see D. Goldsmith and T. Owen, The Search for Life in the Universe, Benjamin/ Cummings Publishing Co., 1980, pp. 174- 177.

[2]. Several alternative liquids have been proposed, especially ammonia and methyl alcohol. Goldsmith and Owen offer a very accessible account of this matter also (ibid. pp. 212-216). For more technical literature the reader may consult the appropriate titles in Note 8.

[3]. L. Margulis and J.E. Lovelock, "Atmospheres and Evolution," Life in the Universe, John Billingham, ed., NASA cp.2156, 1981, p.79.

[4]. For this point of view see p. 5024. S.M. Siegel, "Experimental Biology of Extreme Environments and Its Significance for Space Bioscience," Spaceflight, ____, p. 128; Siegel et al "Experimental Biology of Ammonia-Rich Environments: Optical and Isotopic Evidence for Vital Activity in Pennicillium in Liquid Ammonia-Glycerol Media at -40 C," Proceedings of the National Academy of Sciences, Vol. 60, No. 2 (1968), p. 505; S.M. Siegel and T. W. Spettel, "Life and the Outer Planets: II. Enzyme Activity in Ammonia-Water Systems and Other Exotic Media at Various Temperatures," Life Science and Space Research, 15 (1977), p. 76; B. Z. Siegel and S.M. Siegel, "Further Studies on the Environmental Capabilities of Fungi: Interactions of Salinity, Ultraviolet Irradiation, and Temperature in Penicillium," Gospar Life Sciences and Space Research, Vol. 8, R. Holmquist, ed., Pergamos Press (1980), p. 59.

[5]. Science on radial shift as cause of dendritic channels

[6] Science, Vol 317, September 21, 2007. “Special Section: Mars Reconnaissance Orbiter”, pp. 1705-1719.

[7] M. Carr, “The Proof is in: Ancient Water on Mars,” The Planetary Report, Vol. XXIV, No. 3, May/June, 2004, p.11.