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During the Noachian, Mars' crust may have provided a favorable environmentfor microbial life. The porous brine-saturated regolith would have created aphysical space sheltered from UV and cosmic radiations and provided a solvent,while the below-ground temperature and diffusion of a dense reduced atmospheremay have supported simple microbial organisms that consume H2 and CO2 as energyand carbon sources and produce methane as a waste. On Earth, hydrogenotrophicmethanogenesis was among the earliest metabolisms but its viability on earlyMars has never been quantitatively evaluated. Here we present a probabilisticassessment of Mars' Noachian habitability to H2-based methanogens, and quantifytheir biological feedback on Mars' atmosphere and climate. We find thatsubsurface habitability was very likely, and limited mainly by the extent ofsurface ice coverage. Biomass productivity could have been as high as in earlyEarth's ocean. However, the predicted atmospheric composition shift caused bymethanogenesis would have triggered a global cooling event, ending potentialearly warm conditions, compromising surface habitability and forcing thebiosphere deep into the Martian crust. Spatial projections of our predictionspoint to lowland sites at low-to-medium latitudes as good candidates to uncovertraces of this early life at or near the surface.

Early Mars' habitability and global cooling by H2-based methanogens