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Abstract EANA2025-76


Assessing Microbial Survival on Basalt Under Early Mars CO₂–H₂ Atmospheres With and Without UVC Exposure

Asena Kuzucan (1,2), Nina Zeyen (2,3), Emeline Bolmont (1,2), Luca Caricchi(2,3)
(1) Department of Astronomy, University of Geneva, Switzerland (2) CVU – Center for Life in the Universe, University of Geneva, Switzerland (3) Section of Earth and Environmental Sciences of the University of Geneva, Switzerland


Recent 3D climate simulations suggest that early Mars, under a dense CO₂–H₂ atmosphere enhanced by H₂ collision-induced absorption, could have supported conditions suitable for sustained liquid water. Despite the elevated UVC radiation from the young Sun, a dense CO₂–H₂ atmosphere may have provided suitable surface UVC conditions through photochemical processes. Our 3D climate simulations show that, under such an atmosphere, early Mars could have maintained surface conditions favorable to life, considering water availability, surface temperatures, and radiation levels (Kuzucan et al. 2025 in prep). Together, these factors may have offered transiently habitable conditions for primitive life. To experimentally assess the biological implications of these environments, we exposed hydrated basalt samples inoculated with Methanococcus maripaludis to simulated early Martian conditions. Experiments were conducted under a 2 bar atmosphere composed of an 85–15 % CO₂–H₂ gas mixture, both with and without UVC irradiation. UVC exposure levels were based on diurnally averaged surface fluxes (~0.084 W/m²) and cumulative daily doses (~1366 mJ/cm²) derived from 3D climate-photochemistry simulations, and replicated in a custom-designed exposure chamber. Methane production was monitored via gas chromatography, and scanning electron microscopy was used to examine mineral surface alteration and biofilm formation. Comparison between irradiated and non-irradiated samples showed that M. maripaludis remains metabolically active under intermittent UVC exposure, producing methane at similar rates to the non-irradiated controls. Furthermore, the organism successfully colonizes the basalt surface in both conditions, inducing comparable microscale dissolution features and biofilm development. These findings demonstrate that UV shielding by mineral matrices, coupled with dense greenhouse atmospheres, could have supported microbial survival on early Mars. The experimental evidence confirms the plausibility of  habitable surface environments inferred from climate models and offers constraints for the interpretation of biosignatures in upcoming Mars missions.