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

Abstract

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 <10^27 cells or <2e11 kg, equivalent to <1e-4 to 1e-5 of Earth's biomass, depending on the terrestrial estimate. Diffusion calculations suggest this upper biomass limit applies to the top few kilometers of the martian crust in communication with the atmosphere at low to mid latitudes. This biomass limit is more robust than previous estimates because we test multiple possible chemoautotrophic ecosystems over a broad parameter space of tunable model variables using an updated photochemical model with precise atmospheric concentrations and uncertainties from Curiosity. Our results of sparse or absent life in the martian subsurface also demonstrate how the atmospheric redox pairs of CO-O2 and H2-O2 may constitute antibiosignatures, which may be relevant to excluding life on exoplanets.

arXiv comment30 pages, 4 figures, 3 tables. Accepted for publication in Astrobiology

References in corpus (4)

• Radar evidence of subglacial liquid water on Mars
• Exoplanet Biosignatures: A Framework for Their Assessment
• On detecting biospheres from chemical thermodynamic disequilibrium in planetary atmospheres
• Anoxic Atmospheres on Mars Driven by Volcanism: Implications for Past Environments and Life

Cited by in corpus (9)

• The Case and Context for Atmospheric Methane as an Exoplanet Biosignature
• Abundant atmospheric methane from volcanism on terrestrial planets is unlikely and strengthens the case for methane as a biosignature
• Habitability Models for Astrobiology
• When is chemical disequilibrium in Earth-like planetary atmospheres a biosignature versus an anti-biosignature? Disequilibria from dead to living worlds
• Understanding planetary context to enable life detection on exoplanets and test the Copernican principle
• Early Mars' habitability and global cooling by H2-based methanogens
• Atmospheric processes affecting methane on Mars
• Potential For Liquid Water Biochemistry Deep Under The Surfaces Of The Moon, Mars And Beyond
• Physical Constraints on Motility with Applications to Possible Life on Mars and Enceladus