Why Is Studying Mars Relevant to Astrobiology

The question of a past and/or present Life on Mars remains under discussion. The first significant point to note is that in more than forty years of exploration of the Mars planet, only one mission had an astrobiological goal: the Viking mission in the middle of the 70s.6-7

The different measurements performed onboard Viking landers to gather evidence for biological activity gave positive answers at the two landing sites and thus seemed to prove the existence of a martian life. However, all these results were subsequently explained by chemical processes of oxidation due to the presence of oxidant materials at the atmosphere/surface interface ofthe planet.8-15 More strikingly, the nondetection of any organic molecule (a type of molecule required for life as we know it) by the Gas Chromatograph— Mass Spectrometer experiment seems to be incompatible with the existence of life.11 The conclusion of the mission at the end of the

1970s was that no life was—or has been—present on Mars and the search for biological activity was abandoned. Much later in the 1990s, scientists studied a meteorite supposed to have originated from Mars: ALH84001. Different molecules related to biological processes were detected and assigned to possible martian life,16 even if their presence could also be explained by chemical processes or terrestrial contamination. These discoveries suddenly once again raised the interest in searching for life on Mars. The phase ofintense exploration of Mars that started from these research studies, which still goes on to-day, has revealed that the history of Mars was compatible with the emergence of life, or at least with the existence of prebiotic chemical activity (see Section 4.3).

Then, coming back to the observations of astrobiological interest collected by Viking, which could not give any positive evidence for past or present biological activity on Mars, scientists determined that there was also no firm evidence for the absence of life on Mars. Why was organic material not observed by Viking in the pyrolysis-GC/MS experiment?8,17,18 This could be due to experimental conditions not being adapted for amino acid (destroyed by heating) detection and having a relatively high detection limit for their decomposition products (amines). Furthermore, the Viking landers sampled only the first centimetres in depth of martian soil, where the organic material would have been oxidized/destroyed by atmospheric H2O2,19,20 atomic oxygen21 and superoxide ions.22 Benner23 showed that it might be degraded into salts of benzen-ecarboxylic acids that are highly refractory. Since Viking's ovens would be unable to pyrolyse such molecules, no signal could be expected. The "biological" experiments onboard the Viking landers that were intended to answer the question of the presence or absence of some form of current life on Mars thus provided partial results that are still under debate.24-27 However, the most pessimistic verdict of these experiments is that there was no—at the end of 1970s—biological activity of the terrestrial type in the first ten centimeters of the soil within the few square meters of the explored surface on Mars.

Recently, methane was detected in the Mars atmosphere.28,29 The presence of methane is at first surprising for a body the atmosphere of which was considered to be in thermodynamic equilibrium.30 For example, methane in the Earth's atmosphere, which is considered to be off equilibrium, directly reflects biological activity. Although many inorganic sources could be the origin of methane on Mars, the presence of ecological niches can still be a logical possibility to consider. Anyway, it was pointed out recently that methane could also be produced by a nonbiological process called

Corresponding Author: Patrice Coll—LISA, Universités Paris 12 et Paris 7, CNRS, 61 Av. du Général de Gaulle, 94010, Créteil, France. Email: [email protected]

serpentinization, involving water, carbon dioxide and the mineral olivine that is known to be common on Mars.31

On the other hand, it is noteworthy that Mars might have possessed key ingredients for the emergence of Life according to our knowledge about the process. While the evolution pathways of Mars and of the Earth seem to have been separated very early in their histories, it is completely possible that similar conditions were present on the two planets during the first hundreds of millions of years following their formation. We know that these conditions had led to the emergence of the first terrestrial forms of life, which are suggested by the oldest sedimentary records to be more than 3.5 billion years old.32-35 So, if the birth of Life on Earth was the result ofreproducible physicochemical processes, there is no reason why these same processes could not have allowed the appearance of Life on Mars. Conversely, if it is demonstrated that Life did not appear on Mars even though all the conditions there were favorable, it would be of great interest to ask: why had Life emerged on the Earth but not on Mars?

Is our list of the essential parameters for Life incomplete ? Is it erroneous ? If so, what are the missing elements ?

We have to keep in mind that Mars offers geological records that are much better preserved than the Earth's: half of its surface is more than 3,8 billion years old,36-39 whereas only 1% of the Earth surface is more than 2 billion years old on account ofplate tectonics that have irremediably erased the prebiotic records of the origin of Life. It has even been suggested that Life could have appeared first on Mars for two principal reasons:

1. The small size of Mars (Mars' diameter is 6800 km, approximately 50% that of the Earth) would have been an advantage in the beginning, because its surface would have cooled down faster than the Earth's, thereby supporting an earlier appearance of liquid water.

2. Mars did not see a gigantic impact comparable with the one supposed to have caused the birth of the Moon and blown away the primitive atmosphere of the Earth.40,41

Consequently, while the question of potential biological activity on Mars is based on certain assumptions, it is also accompanied by solid arguments that justify our continued interest. Clearly Mars remains one ofthe main astrobiological targets in our Solar System: it is the only place apart from the Earth where it appears possible to discover traces of past or present Life.

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