Detecting a second genesis on Mars

There are several ways to search for life. First we can search for life as a collective general phenomenon. However, life might also be a single isolated organism. And that organism might be dead. Finally, signs of life may be fossils, artefacts or other inorganic structures. In the search for life on Mars, any of these would be of interest. Definitions of life typically focus on the nature of the collective phenomenon. In general, such definitions are not useful in an operational search for life on other worlds. The one exception is the proposal by Chao (2000) to modify the Viking LR experiment to allow the detection of organisms that improve their capacity to utilize the provided nutrients. This would, in principle, provide a direct detection of Darwinian evolution and could unambiguously distinguish between biological metabolism and chemical reaction. Chao (2000) argues that Darwinian evolution is the fundamental property of life and other observables associated with life result from evolutionary selection. His method for searching for evolution would be practical if the right medium can be selected to promote the growth of alien microbes. Unfortunately, we now know that only a fraction of microorganisms from an environmental sample grow in culture.

Of course growth experiments of any kind do not detect dead organisms. Yet the remains of dead organisms are potentially important evidence of life on another planet. And so are fossils. However, there is an important distinction between dead organisms and fossils. A fossil is evidence of past life but it does not reveal anything about the biochemical or genetic nature of that life. If we are searching for a second example of life then we need to be able to compare the nature of that life to Earth life. For this, an organism is needed, either dead or alive, but a fossil is not sufficient.

As discussed above, a promising target for the search for biological remains of past life is the subsurface. The deep permafrost on Mars may hold remnants of past life (Smith and McKay, 2005). The organisms in the ground ice are likely to be dead from the accumulated radiation dose but their organic remains could be analysed and compared to the biochemistry of Earth life.

I have argued previously (McKay, 2004) that one way to determine if a collection of organic material is of biological origin is to look for a selective pattern of organic molecules similar to, but not necessarily identical with, the selective pattern of biochemistry in life on Earth. Pace (2001) has argued that life everywhere will be life as we know it. He contends that the biochemical system used by life on Earth is the optimal one and therefore evolutionary pressure will cause life everywhere to adopt this same biochemical system. It is instructive to consider this argument in the context of a conceptual organic phase space. If we imagine all possible organic molecules as the dimensions of a phase space, then any possible arrangement of organic molecules is a point in that phase space. We can define biochemistries as those points in phase space that allow life. The biochemistry of Earth life - life as we know it - represents one point in the organic phase space. We know that this one point represents a viable biochemistry. Pace's (2001) contention that biochemistry is universal is equivalent to stating that in the region of phase space of all possible biochemistries there is only one optimum biochemistry and thus any initial set of biochemical reactions constituting a system of living organisms will move toward that optimum as a result of selective pressure. If Pace (2001) is correct, then the only variation between life forms that we can expect is that associated with chirality. As far as is known, the left and right forms of chiral organic molecules (such as amino acids and sugars) have no differences in their biochemical function. Life is possible that is exactly similar in all biochemical respects to life on Earth except that it has right instead of left amino acids in its proteins and left instead of right sugars in its polysaccharides.

The question of the number of possible biochemistries consistent with life is an empirical one and can only be answered by observations of other life forms on other worlds, or by the construction of other life forms in the laboratory. The observation or construction of even one radically alien life form would suffice to show that biochemistry as we know it is not universal.

The pattern of biochemistry of Earth life follows what I have called the 'Lego principle' (McKay, 2004). This is the unremarkable observation that life on Earth uses a small set of molecules to construct the diverse structures that it needs. This is similar to the children's play blocks known as Lego in which a few different units, repeated over and over again, are used to construct complex structures. The

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Fig. 12.3. Comparison of biogenic with non-biogenic distributions of organic material. Non-biological processes produce smooth distributions of organic material, illustrated here by the curve. Biology, in contrast, selects and uses only a few distinct molecules, shown here as spikes (e.g., the 20 left-handed amino acids on Earth). Analysis of a sample of organic material from Mars or Europa may indicate a biological origin if it shows such selectivity. Figure from McKay (2004).

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Fig. 12.3. Comparison of biogenic with non-biogenic distributions of organic material. Non-biological processes produce smooth distributions of organic material, illustrated here by the curve. Biology, in contrast, selects and uses only a few distinct molecules, shown here as spikes (e.g., the 20 left-handed amino acids on Earth). Analysis of a sample of organic material from Mars or Europa may indicate a biological origin if it shows such selectivity. Figure from McKay (2004).

biological polymers that construct life on Earth are the proteins, the nucleic acids, and the polysaccharides. These are built from repeated units of the 20 left-handed amino acids, the five nucleotide bases, and the right-handed sugars. The use of only certain basic molecules allows life to be more efficient and selective. Evolutionary selection on life anywhere is likely to result in the same selective use of a restricted set of organic molecules. As discussed above, I believe it is premature to conclude that all life anywhere will use the same set of basic biomolecules. Thus I suggest that life will always use some basic set but it may not be the same basic set used by life on Earth. This characteristic biogenic pattern of organic molecules would persist even after the organism is dead. Given our present state of understanding of biochemistry, we are not able to propose alternative and different biochemical systems that could be the basis for life, but that may reflect a failure of our understanding and imagination rather than a restriction on the possibilities for alien life.

A sample from the deep permafrost in the southern hemisphere of Mars could be analysed for organic material with a fairly simple detection system. If organic material was detected, then it would be of interest to characterize any patterns in that organic material that would indicate a 'Lego principle' pattern. Clearly one such pattern is the identical pattern of all Earth life; 20 amino acids, the five nucleotide bases, A, T, C, G, and U, etc. However, more interesting would be a clear pattern different from the pattern known from Earth life. Figure 12.3 shows a schematic diagram of how a biological pattern would be different from a non-biological pattern.

Implementing this search in practical terms in near term missions will require a sophisticated ability to separate and characterize organic molecules. Currently the instrument best suited for this task is a GC-MS with solvent extraction. However, new methods of fluorescence and Raman spectroscopy could provide similar information and may have a role in future mission applications.

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