Earth-like Habitable Environments in the Subsurface of Mars

Abstract

In Earth's deep continental subsurface, where groundwaters are often isolated for >10 6 to 10 9 years, energy released by radionuclides within rock produces oxidants and reductants that drive metabolisms of nonphotosynthetic microorganisms. Similar processes could support past and present life in the martian subsurface. Sulfate-reducing microorganisms are common in Earth's deep subsurface, often using hydrogen derived directly from radiolysis of pore water and sulfate derived from oxidation of rock-matrix-hosted sulfides by radiolytically derived oxidants. Radiolysis thus produces redox energy to support a deep biosphere in groundwaters isolated from surface substrate input for millions to billions of years on Earth. Here, we demonstrate that radiolysis by itself could produce sufficient redox energy to sustain a habitable environment in the subsurface of present-day Mars, one in which Earth-like microorganisms could survive wherever groundwater exists. We show that the source localities for many martian meteorites are capable of producing sufficient redox nutrients to sustain up to millions of sulfate-reducing microbial cells per kilogram rock via radiolysis alone, comparable to cell densities observed in many regions of Earth's deep subsurface. Additionally, we calculate variability in supportable sulfate-reducing cell densities between the martian meteorite source regions. Our results demonstrate that martian subsurface groundwaters, where present, would largely be habitable for sulfate-reducing bacteria from a redox energy perspective via radiolysis alone. We present evidence for crustal regions that could support especially high cell densities, including zones with high sulfide concentrations, which could be targeted by future subsurface exploration missions.

The Effect of Grain Size on Porewater Radiolysis

“…Dzaugis et al (2018) showed that this fracture-based model yields radiolytic H2 production rates for Martian basaltic crust broadly comparable to those predicted for fine-grained Martian sediments by the model of Blair et al (2017). One implication of these studies, in agreement with other considerations, is that the best place to seek extant life on Mars today is probably the subsurface (e.g., Tarnas et al, 2021).…”

“…Assuming fully water-saturated basement, H2 production rates were found to be comparable to crystalline basement on Earth because of the higher porosity of Mars' crust, despite the lower concentrations of uranium and thorium. Updated (higher) radionuclide concentrations were used by Tarnas et al (2018Tarnas et al ( , 2021 to estimate an even greater rate of radiolytic H2 production on Mars, using the same model. Bouquet et al (2017) applied a similar model to Jupiter's moon, Europa.…”

The Effect of Grain Size on Porewater Radiolysis

“…(2007). One implication of these studies, in agreement with other considerations, is that the best place to seek extant life on Mars today is probably the subsurface (e.g., Tarnas et al., 2021).…”

“…Updated (higher) radionuclide concentrations were used by Tarnas et al. (2018, 2021) to estimate an even greater rate of radiolytic H 2 production on Mars, using the same model. Bouquet et al.…”

86Kr excess and other noble gases identify a billion-year-old radiogenically-enriched groundwater system

“…The findings of this study underscore the need to expand understanding of key radiogenic processes in the crust, both continental and marine, to validate quantitative models of fluid age and transport critical to understanding the degree of isolation, or degree of connection, of the subsurface to surface processes. Constraining radiogenic processes and their associated fluxes will provide refined habitability models of the subsurface, both for the Earth and on other solar system bodies, where habitability driven by radiolysis a major driver of current investigation 12,44,[50][51][52] .…”