Reacción a "Reactions to study claiming Mars was habitable for micro-organisms"

The paper by Sauterey et al. presents a very interesting discussion of the habitability of early Mars at a time when, in parallel, life on Earth emerged more than 3.7 billion years ago. Using geochemical and ecological models, the authors evaluate the possibility that ecosystems formed by organisms with hydrogenotrophic and methanogenic metabolism, i.e. organisms that use molecular hydrogen as an energy source and generate methane, may have been the basis of primitive biospheres after the origin of life. It is not correct to call them bacteria, as they arose during biological evolution on Earth some time after the origin of life, so there is no reason to think that they are biologically comparable to the organisms on Mars, beyond the type of metabolism they use.

Although the work is based on theoretical climate and ecological models, which is always a limitation and will need to be corroborated by future exploration, it is consistent with what we know about life on Earth and provides some interesting insights.

First, that early Mars may have been habitable - from the point of view of methanogenic organisms; we could never have breathed there - and that there is a relatively high probability that H₂-based methanogenic ecosystems would have arisen in some areas of the planet. This could be comparable to the situation on Earth. But, on the other hand, differences in climate, geochemistry (such as salinity) and, above all, in the characteristics and composition of the atmosphere, conditioned the evolution of these ecosystems. Thus, while on Earth the activity of methanogens may have favoured habitability and the evolution of life, on Mars it may have been the opposite, with these organisms causing a global cooling of the planet.

Therefore, life on Mars must have continued deeper and deeper beneath the surface, using geochemical processes as a source of hydrogen. 

Although, at first glance, the paper appears to tell us that Mars was habitable and that there was a high probability that life originated there, the authors suggest that the planet lost habitability early on at the surface and in the areas closest to it, so in the unlikely event that life on Mars had continued or that there is life today, it must be found deep beneath the surface.

The proposed model is also interesting from the point of view of the exploration of the planet, as it is calculated that the areas with the highest probability that a methanogenic ecosystem would have emerged or that it would have remained closer to the surface for longer are the Jezero crater, Hellas Planitia and Isidis Planitia, making them the ideal places to look for biomarkers, that is, traces that could have been left by those ancestral ecosystems and that would be definitive to know what really happened.