Earth-like Habitable Environments in the Subsurface of Mars

Abstract:

In Earth’s deep subsurface, many microbial communities are sustained by water-rock reactions, including radiolysis, which produce a closed cycle of oxidant and reductant production in groundwaters. With strong relevance to Mars subsurface habitability, some communities are found in fracture waters that have been isolated for millions to billions of years. When rock-hosted radionuclides and liquid water are present, radiolysis produces H₂ and complementary oxidants, including H₂O₂. Where sulfides exist within the rock matrix, they can be oxidized by H₂O₂, forming sulfate. Sulfate-reducing bacteria that couple H₂ and sulfate to drive their metabolisms are common in many ancient deep groundwaters on Earth, as radiolysis alone generates a closed loop of redox nutrient production. Sulfide produced by sulfate-reducing bacteria can also be reoxidized via radiolysis, generating more nutrients. The present study demonstrates that the martian meteorite lithologies could sustain sulfate-reducing bacteria via these same radiolytic reactions wherever groundwater is present. There is significant variability in the supportable sulfate-reducing bacteria cell densities between the regolith breccia ([7 × 10²]-[1 × 10⁶] cells per kg rock), shergottite ([2 × 10^-1]-[3 × 10⁴] cells per kg rock), nakhlite ([3]-[3 × 10⁴] cells per kg rock), and chassignite ([4 × 10^-1]-[1 × 10³] cells per kg rock) martian meteorite lithologies. The regolith breccia lithology, which comes from the southern highlands, could sustain the highest sulfate-reducing bacteria cell densities—comparable to cell densities measured in Earth’s deep subsurface—because of the higher concentrations of sulfides in regolith breccias compared to the other martian meteorites. The results presented here demonstrate that martian groundwaters would likely be habitable from a redox energy perspective. This adds to a growing body of evidence that the martian subsurface is likely the largest, longest-lived, and most consistently habitable environment on Mars, and has the potential to still be habitable today.