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Whether extant life exists in the martian subsurface is an open question.High concentrations of photochemically produced CO and H2 in the otherwiseoxidizing martian atmosphere represent untapped sources of biologically usefulfree energy. These out-of-equilibrium species diffuse into the regolith, sosubsurface microbes could use them as a source of energy and carbon. Indeed, COoxidation and methanogenesis are relatively simple and evolutionarily ancientmetabolisms on Earth. Consequently, assuming CO- or H2- consuming metabolismswould evolve on Mars, the persistence of CO and H2 in the martian atmosphereset limits on subsurface metabolic activity. Here, we constrain such maximumsubsurface metabolic activity on Mars using a 1-D photochemical model with ahypothetical global biological sink on atmospheric CO and H2. We increase thebiological sink until the model atmospheric composition diverges from observedabundances. We find maximum biological downward subsurface sinks of 1.5e8molecules/cm^2/s for CO and 1.9e8 molecules/cm^2/s for H2. These covert to amaximum metabolizing biomass of <10^27 cells or <2e11 kg, equivalent to <1e-4to 1e-5 of Earth's biomass, depending on the terrestrial estimate. Diffusioncalculations suggest this upper biomass limit applies to the top few kilometersof the martian crust in communication with the atmosphere at low to midlatitudes. This biomass limit is more robust than previous estimates because wetest multiple possible chemoautotrophic ecosystems over a broad parameter spaceof tunable model variables using an updated photochemical model with preciseatmospheric concentrations and uncertainties from Curiosity. Our results ofsparse or absent life in the martian subsurface also demonstrate how theatmospheric redox pairs of CO-O2 and H2-O2 may constitute antibiosignatures,which may be relevant to excluding life on exoplanets.

A Maximum Subsurface Biomass on Mars from Untapped Free Energy: CO and H2 as Potential Antibiosignatures