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Early Mars habitability and global cooling by H2-based methanogens

Abstract

During the Noachian, Mars’ crust may have provided a favourable environment for microbial life1,2. The porous brine-saturated regolith3,4,5 would have created a physical space sheltered from ultraviolet and cosmic radiation and provided a solvent, whereas the below-ground temperature2 and diffusion6,7 of a dense, reduced atmosphere8,9 may have supported simple microbial organisms that consumed H2 and CO2 as energy and carbon sources and produced methane as a waste. On Earth, hydrogenotrophic methanogenesis was among the earliest metabolisms10,11, but its viability on early Mars has never been quantitatively evaluated. Here we present a probabilistic assessment of Mars’ Noachian habitability to H2-based methanogens and quantify their biological feedback on Mars’ atmosphere and climate. We find that subsurface habitability was very likely, and limited mainly by the extent of surface ice coverage. 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.

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Fig. 1: Modelled photochemistry and climate of early Mars.
Fig. 2: Initial and steady-state characteristics of Noachian Mars under the influence of hydrogenotrophic methanogens, for brines freezing at 203, 252 and 273 K.
Fig. 3: Median evolution of the ice coverage of Noachian Mars under the influence of hydrogenotrophic methanogens.
Fig. 4: Steady-state distribution of habitable conditions and ice on the surface of Noachian Mars.

Data availability

The datasets produced and analysed in this study are available in the following repository: https://github.com/bsauterey/MarsEcosys (ref. 38 https://doi.org/10.5281/zenodo.6963348).

Code availability

The planetary ecosystem model coupling climate, atmosphere, ice coverage and below-ground ecosystem and the datasets produced with it are available in the following repository: https://github.com/bsauterey/MarsEcosys (https://doi.org/10.5281/zenodo.6963348; ref. 38). The photochemical and climate models are accessible on the Virtual Planet Laboratory’s gitlab (https://github.com/VirtualPlanetaryLaboratory/atmos; ref. 32); the adapted versions used in this study are available upon request.

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Acknowledgements

We are grateful for discussion with D. Apai, A. Bixel, Z. Grochau-Wright, B. Kacar, C. Lineweaver, S. Rafkin, A. Soto, V. Thouzeau and members of the OCAV Project at PSL University and of NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network. We thank J. Kasting and his student J. Liu for their help adapting and running the VPL’s photochemical and climatic models. B.S. is grateful to E. Lutz for her open-access codes of beautiful Martian maps (https://github.com/eleanorlutz/topography_atlas_of_space). This work is supported by France Investissements d’Avenir programme (grant numbers ANR-10-LABX-54 MemoLife and ANR-10-IDEX-0001-02 PSL) through PSL IRIS OCAV and PSL–University of Arizona Mobility Program. R.F. acknowledges support from the US National Science Foundation, Dimensions of Biodiversity (DEB-1831493), Biology Integration Institute-Implementation (DBI-2022070), Growing Convergence in Research (OIA-2121155) and National Research Traineeship (DGE-2022055) programmes; and from the United States National Aeronautics and Space Administration, Interdisciplinary Consortium for Astrobiology Research program (award number 80NSSC21K059).

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Authors and Affiliations

Authors

Contributions

B.S., B.C., R.F. and S.M. conceptualized the study. B.S., A.A., R.F. and S.M. were responsible for the methodology. B.S. carried out the investigation and performed the formal analysis. B.S. and R.F. carried out the visualization. B.S. and S.M. wrote the software. R.F. and S.M. supervised the study. B.S. wrote the original draft of the manuscript. B.S., B.C., A.A., R.F. and S.M. reviewed and edited the manuscript.

Corresponding author

Correspondence to Boris Sauterey.

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Nature Astronomy thanks Michael Wong and Owen Lehmer for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Simulated depth profiles of (A) temperature and (B) diffusivity in Mars’ Noachian regolith.

Gray areas bounded by dashed lines represent the entire space in which the depth profiles can exist. Each line (here 2,000 in total) represents one specific profile simulated for one set of parameters drawn from the ranges given in Supplementary Table 1.

Extended Data Fig. 2 Ice-free surface fraction, ρ, (A) and average temperature in the corresponding region (B).

Ice coverage and average surface temperature are evaluated across the spatial projection of Mars average temperature distribution (see Methods). The black dotted line in B is the first diagonal corresponding to the planetary averaged surface temperature \(\bar T_{surface}\).

Extended Data Fig. 3 Surface and vertical distribution of a putative hydrogenotrophic methanogenic biosphere on Noachian Mars.

Spatial projection of the median minimum depth of this biomass occurrence for three values of brines’ freezing point of 203 K (A), 252 K (B), and 273 K (C). The white shaded areas correspond to the probability (from 50% to 90% by steps of 10%) of ice-coverage superimposed to the maps by transparency. Open circles indicate the Noachian lakes distributed along the South-North dichotomy. See Methods for more detail.

Supplementary information

Supplementary Information

Supplementary Tables 1 and 2 and refs. 39–47.

Supplementary Video 1

Median evolution through time of of the ice coverage of Noachian Mars under the influence of hydrogenotrophic methanogens assuming that brines freeze at 252 K.

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Sauterey, B., Charnay, B., Affholder, A. et al. Early Mars habitability and global cooling by H2-based methanogens. Nat Astron 6, 1263–1271 (2022). https://doi.org/10.1038/s41550-022-01786-w

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