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Early Mars was habitable for microbes, at least for a while

The relatively warm waters of early Mars may have initially created conditions for microorganisms to thrive, but those same methane-producing organisms could have self-destructed as they altered the planet enough to lead to their eventual decline.

(CN) — Researchers may be able to answer David Bowie’s famous question: Is there life on Mars? A new study indicates that Mars’ surface may have been habitable for microbes over 3.7 billion years ago.

In a study published Monday in Nature Astronomy, scientists describe how they ran simulations combining models for the environmental conditions on early Mars and ecological processes based on microorganisms on early Earth.

On Earth, hydrogenotrophic methanogens, microbes that consumed hydrogen and produced methane, were some of the earliest forms of life, and it has long been theorized that the Noachian period on Mars would have been a likely time period when surface and atmospheric conditions would have aligned to create the ideal circumstances for life. The new study is the first to quantitively measure the viability of these simple microbial organisms.

Early Mars’ comparatively dense atmosphere was possibly warmer than it is today, and the planet’s surface is hypothesized to have been favorable to the presence of large bodies of liquid water, allowing for the possible emergence of microbial lifeforms similar to early Earth’s.

The study used a three-prong approach to model the development of these microbial lifeforms, according to Boris Sauterey, the study's lead author who works with both the University of Arizona and the Institut de Biologie de l’Ecole Normale Supérieure in France. Sauterey and his colleagues had previously studied methanogens on their own planet.

The simulation combined a model of the chemical composition of early Martian atmosphere and climate, along with a model of the planet’s crust and surface temperature — which was dependent on the diffusion of atmospheric gases into the crust — and finally a biologic model of the microbes themselves to demonstrate how they interacted with both the atmosphere and crust of Mars.

“This modification of the crust chemistry then feeds back onto the gaseous exchanges between the crust and the atmosphere, which can in turn modify the atmospheric composition and the climate,” Sauterey told Courthouse News. “So our coupled model allow us to link the microscopic scale of the interaction of micro-organisms with their local environment in the crust with the planetary scale of the atmospheric composition and climate.”

While the model does show that Mars was indeed habitable for methane-producing microorganisms, the study also explains how these same microbes could have rendered Mars uninhabitable for themselves.

Sustainability of microbial life relied in some part on consistent surface temperatures. The interaction between the planet’s climate and biological activity, combined with a thinning of the atmosphere and loss of liquid water, may have contributed to the eventual decline of these methanogenic microorganisms on Mars’ surface.

According to the study, as these microbes fed on hydrogen and proliferated, they were also releasing methane as waste, disrupting the equilibrium of the chemicals in the atmosphere. This disruption would have had drastic effects on Martian climate and surface temperatures. The model extrapolates that the methane-heavy biosphere would cause a global cooling event across the planet.

“Assuming that methanogens indeed colonized the crust of Mars, they would have dramatically cooled the climate of Mars down by removing from the atmosphere one of its most potent warming gas, H2. By doing so, they would have driven the expansion of the ice coverage of the planet, ultimately decreasing the fraction of the surface of Mars that would have been habitable to them,” Sauterey said.

The study also draws a contrast between the effect of microbial life on Mars and on Earth.

“Although hydrogenotrophic methanogens may have contributed to maintaining temperate conditions on Earth, they would have cooled the early Martian surface, with a reduction of the maximum possible temperature by 33–45 K." the study states. "Such divergence in climate evolution is the consequence of different prebiotic atmospheric compositions.”

The same cooling event and ice coverage that would have led to the demise of the microorganisms, however, may provide the highest chance of preservation to be discovered by those searching Mars for hints of life. The study identifies the Hellas Planitia, Isidis Planitia, and Jezero crater —all lowland locations where the cooling atmosphere would have driven the early Mars biosphere down into — as sites to find potential fossilized biomarkers.

Ultimately, Sauterey says that this kind of feedback loop between life and its environment raises questions about the habitability and sustainability of life in general.

“I think it is interesting to see that even a very primitive biosphere could accelerate that process, which raises the question: could it be a common thing in the universe? Could the commonality of life in the universe actually be limited by a tendency of life to cause its own demise?” he said.

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