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

Whether extant life exists in the martian subsurface is an open question. High concentrations of photochemically produced CO and H2 in the otherwise oxidizing martian atmosphere represent untapped sources of biologically useful free energy. These out-of-equilibrium species diffuse into the regolith, so subsurface microbes could use them as a source of energy and carbon. Indeed, CO oxidation and methanogenesis are relatively simple and evolutionarily ancient metabolisms on Earth. Consequently, assuming CO- or H2- consuming metabolisms would evolve on Mars, the persistence of CO and H2 in the martian atmosphere set limits on subsurface metabolic activity. Here, we constrain such maximum subsurface metabolic activity on Mars using a 1-D photochemical model with a hypothetical global biological sink on atmospheric CO and H2. We increase the biological sink until the model atmospheric composition diverges from observed abundances. We find maximum biological downward subsurface sinks of 1.5e8 molecules/cm^2/s for CO and 1.9e8 molecules/cm^2/s for H2. These covert to a maximum metabolizing biomass of