On terrestrial planets, the abundances of key trace gases such as methane CH4 are controlled by photochemistry and source fluxes such as the rate of volcanic outgassing, water-rock reactions, or biological production. The interpretation of CH4 as a biosignature is thus ultimately dependent on the production flux inferred from its abundance and the likelihood that this flux could be produced by geological sources alone. Prior work has shown that the buildup of CH4 in the atmosphere at a given flux is highly favored for planets with oxygen-rich atmospheres orbiting K and M dwarfs, relative to Sun-like stars. However, relatively limited attention has been given to anoxic, Archean-like atmospheres and their flux-abundance relationships. We use a photochemical model to predict the atmospheric CH4 mixing ratio as a function of its production rate for anoxic planets in the habitable zones of FGKM stars. We then compare the fluxes to those produced by primitive bacterial biospheres and geologic processes to evaluate what levels of CH4 would suggest biological activity. We find that the flux-abundance relationships, photochemical destruction pathways, and ultimate detectability of CH4 in anoxic exoplanets are highly dependent on the host star spectrum (Figure 1). For example, at low abundances CH4 destruction by the OH radical dominates for anoxic planets orbiting all stellar types but is much less efficient for anoxic planets orbiting M dwarfs, which may challenge biosignature interpretations based on CO2-CH4 disequilibrium. In contrast, at high methane fluxes/abundances, direct photolysis of CH4 is the dominant loss channel for MGK stars and at sufficiently high fluxes their flux-abundance relationship converges. The surface flux at which this convergence occurs is dependent on the distribution of NUV and FUV photons. We find that a biosphere similarly productive to that of Archean-Earth would likely produce similar methane mixing ratios on habitable anoxic planets with MGK stellar hosts (in contrast to oxygen-rich cases where the mixing ratios can be enhanced by a factor of ~1,000 when comparing an M dwarf host to a G dwarf host). For F dwarf stars, the dominant loss channel for methane is always OH, which would limit methane accumulation but also strongly reduces the potential for biosignature false positives.
This research was supported by Exobiology grant 18-EXO18-0005.