The subsurface of Mars during the Noachian period was likely to have been habitable for microorganisms that feed on hydrogen and produce methane, according to new research.
The young Mars would have had enough water to cover its entire surface in a liquid layer about 140 m deep, but it is more likely that the liquid would have pooled to form an ocean occupying almost half of Mars’s northern hemisphere, and in some regions reaching depths greater than 1.6 km. Image credit: M. Kornmesser / ESO / N. Risinger, skysurvey.org.
The potential habitability of early Mars, more than 3.7 billion years ago, has been extensively debated.
Evidence suggests that the planet hosted — at least for part of its history — potentially favorable conditions for the development of life.
“During the Noachian, Mars’ crust may have provided a favorable environment for microbial life,” said Dr. Boris Sauterey, a researcher in the Department of Ecology & Evolutionary Biology at the University of Arizona and the Institut de Biologie de l’Ecole Normale Supérieure at the Université Paris Sciences et Lettres, and his colleagues.
“The porous brine-saturated regolith would have created a physical space sheltered from ultraviolet and cosmic radiation and provided a solvent, whereas the below-ground temperature and diffusion of a dense, reduced atmosphere may have supported simple microbial organisms that consumed hydrogen and carbon dioxide as energy and carbon sources and produced methane as a waste.”
“On Earth, hydrogenotrophic methanogenesis was among the earliest metabolisms, but its viability on early Mars has never been quantitatively evaluated.”
In their study, the authors modeled the interaction between the early Martian environment and an ecosystem of methanogenic hydrogenotrophs — microorganisms that survive by consuming hydrogen and producing methane — which are considered to be among the earliest forms of life on Earth.
Their simulations predict that the Martian crust was a viable place for this ecosystem — provided that the surface was not fully covered with ice — and could have produced biomass similar to that of the early ocean of Earth.
They predict that this ecosystem would have triggered a feedback event with the climate on Mars, cooling it globally by up to 40 Kelvin and creating less habitable conditions closer to the surface.
This would have forced the microbes to move progressively deeper within the planet’s crust.
Looking forward, the researchers identified three sites — Hellas Planitia, Isidis Planitia and Jezero crater — as the best places to look for signs of this early methanogenic life near the surface of Mars.
“We find that subsurface habitability was very likely, and limited mainly by the extent of surface ice coverage,” they said.
“Biomass productivity could have been as high as in the early Earth’s ocean.”
“However, the predicted atmospheric composition shift caused by methanogenesis would have triggered a global cooling event, ending potential early warm conditions, compromising surface habitability and forcing the biosphere deep into the Martian crust.”
“Spatial projections of our predictions point to lowland sites at low-to-medium latitudes as good candidates to uncover traces of this early life at or near the surface.”
A paper on the findings was published in the journal Nature Astronomy.
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B. Sauterey et al. Early Mars habitability and global cooling by H2-based methanogens. Nat Astron, published online October 10, 2022; doi: 10.1038/s41550-022-01786-w
