Plausible microbial metabolisms on Mars

The surface of Mars is currently inimical to life, being both oxidizing and exposed to high levels of ultraviolet and ionizing radiation, and has been so for a significant part of its history. Oxygen exists in the martian atmosphere at trace levels only, meaning that the planet is anoxic. Anaerobic redox-driven metabolisms, which use thermodynamically favourable reduction–oxidation reactions to generate energy, operating within the deepand near-subsurface, are therefore the most plausible energy pathways for life on Mars, past and present. Some of these redox reactions can operate independently of photo synthesis, meaning they can persist deep underground, while some may have once used geochemical disequilibria within near-surface hydrothermal systems. Here, we provide an overview of both the availability of geologically derived redox couples on Mars, and the known (and unknown) microbial metabolisms that could feasibly exploit them.

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