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). 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. 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. 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! 
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. 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.
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.” 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.
. 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.
. 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.
. L. Margulis and J.E. Lovelock, "Atmospheres and Evolution," Life in the Universe, John Billingham, ed., NASA cp.2156, 1981, p.79.
. 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.
. Science on radial shift as cause of dendritic channels
 Science, Vol 317, September 21, 2007. “Special Section: Mars Reconnaissance Orbiter”, pp. 1705-1719.
 M. Carr, “The Proof is in: Ancient Water on Mars,” The Planetary Report, Vol. XXIV, No. 3, May/June, 2004, p.11.