It is quite unlikely that Martian life, if it ever existed, could have evolved beyond the simplest single-celled beings, equivalent to our terrestrial prokaryotes (bacteria or archaea). The exploration of Mars in search of evidences of the emergence of life remains nonetheless, a very exciting goal that deserves we devote significant resources to it.

If the bacterial-type monocellular life probably started on Earth very soon after the Earth's crust hardened, just after the Hadean, some 4 billion years ago (first clues of “mix” of typical chemical elements and light isotopic content favored by life, around -3.8 billion), the metazoan life (organized multicellular life) did not appear before only 600 million years ago (Ediacara fauna – “vendobiontes”, before the Cambrian life-explosion) and the process that led up to that point was fraught with difficulties and accidents. More specifically, it was necessary that molecular oxygen produced by the first life-forms on the surface of the planet (cyanobacteria), had enough time to accumulate in the atmosphere down to the Great Oxidation Event and the first “Snowball Earth” episode, some 2 billion years ago, so that single cell eukaryotes (improbable chimeras of bacteria and archaea) could use it to proliferate despite the dangers that this very powerful oxidizer made them run (but fostered to do so by its huge energetic advantage). Afterwards, much later, 700 to 600 million years ago, on the occasion of a new planetary disequilibrium (a new series of Snowball Earth episodes), the newly reached levels of oxygen (still below but close to ours) allowed the population of these eukaryotes to structure themselves into multicellular beings, specialized by organs, each consisting of many identical individuals (metazoans).

But molecular oxygen, indispensable deadly elixir, could only be produced by cyanobacteria using photosynthesis on the surface of the oceans. On Mars, the disappearance of liquid water on the surface, except cataclysmic episodes afterwards, occurred much too early (around -3.6 billion years), at a period corresponding to that of our first fossils of the prokaryotic type not practicing photosynthesis. And the Martian surface poorly protected by an atmosphere insufficiently thick and containing no ozone, has been from that time on, probably too hostile for life. Therefore, let us not count on an evolved Martian life beyond the equivalent of the simplest anoxic Earth bacteria; but let's hope, anyway. It is indeed not impossible that organic molecules present in a region of space close to ours, had evolved within the Martian planetary "bioreactor" until reaching a primitive form of monocellular life using materials and sources of energy drawn from an environment originally as rich as ours. NB: all the chemical elements of which we are made are present on Mars (Carbon, Hydrogen, Oxygen, Nitrogen + Phosphorus, Calcium + Sulfur, Sodium, Potassium, Manganese, Iron & Chlorine).

On Earth, it is more and more likely that life began, before the cyanobacteria proliferated on the surface of the ocean, within the geothermal chimneys formed at the bottom of an acidic ocean, at the limits of the tectonic plates, by fluxes of basic water loaded with mineral effluents, heated, but not too much (around 60° C) by the magma (“Lost-City” type gray smokers, on faults parallel to the mid-oceanic magmatic resurgences). On Mars, some places (the bottom of the Eridania Sea for instance) could have known the same conditions and the same phenomenon could have occurred. But it is not impossible that the same favorable conditions have also existed in Yellowstone-type environments (hot water, percolating through faults of a soil rich in alkaline minerals, meeting acidic fluids descending from the surface, rich in sulfur and carbon dioxide, with a significant pH differential). It is therefore at this very primitive level, before photosynthesis, that we must consider the most likely possibility of life on Mars.

The process could then have developed and extended itself in the underground environment, stable, warm (under the layer of permafrost) and protected from radiations, but moderately and slowly on account of the environmental conditions (notably the lack of oxygen and also the difficulties of contacts between individuals or populations). This is the thesis developed by astrophysicist Joseph Michalski in a scientific paper published in January 2018. The icing on the cake is that Mars habitable underground should be much deeper than Earth’s because of the low mass of the planet (1/10th of that of Earth), and therefore of its lower gravity, which induces a much deeper potential rock porosity zone (more than double that of Earth, down to 10 km). In fact the limit of habitability should be rather set before this depth is reached, by the rise of temperature as the distance to the surface increases, 120°C being regarded as a maximum bearable (3 km on Earth, 6 km on Mars).

But, as Joseph Michalski points out, the interest of Mars is not only to offer the possibility of observing a form of life elsewhere than on Earth, it is also to offer the possibility of observing the path of a pre-biotic evolution towards life. On Earth, only 0.001% of the surface is old enough to bear traces of the time when this process should have taken place, because the rest is completely inaccessible under sediments or deeply transformed by plate tectonics, the “steamroller” of which has been crushing, churning and transforming rocks for more than 2 billion years. On Mars, unprocessed surfaces older than 3.5 billion years represent more than half of the total and the immediate underground transformed by lava spilling of the Hesperian and Amazonian eons, is even more important.

Do you see the issue? Going to Mars is not only “going for a sightseeing tour” in an exotic place, it is also looking for the sources of life, which we cannot hope to find, in the near future, anywhere else. What man would remain insensitive to this argument that challenges us in various forms since we became aware ?!

Référence :
“The Martian subsurface as a potential window into the origin of Life” par Joseph Michalski et al. in Nature Geoscience, janvier 2018; doi.org/10.1038/s41561-017-0015-2.