When is Atmospheric Methane a Good Exoplanet Biosignature?

The next phase of exoplanet science will focus on characterizing exoplanet atmospheres, including those of terrestrial planets. A comprehensive understanding of possible biosignatures that may be detected with the next generation of ground and space telescopes is needed. While some biosignature gases, such as oxygen, phosphine, isoprene, and ammonia have recently been reviewed in depth (Meadows et al. 2018, Sousa-Silva et al. 2020, Zhan et al. 2021, Huang et al. 2021), these will likely be extremely difficult to detect in planets with high mean molecular weight atmospheres with JWST. In contrast, methane at Earth-like biogenic fluxes is one of the only biosignatures that may be readily detectable with JWST. In fact, an early Earth-like, methane-rich atmosphere would be easier to detect with JWST than modern Earth’s oxygen-rich atmosphere (Krissansen-Totton et al. 2018a). Here, we present the case for methane as a biosignature and highlight areas in need of further work. We show that the photochemistry of terrestrial planet atmospheres implies that large CH₄ surface fluxes are necessary to sustain significant levels of atmospheric methane. There are a variety of abiotic sources that can produce atmospheric methane including high-temperature magmatic outgassing, low-temperature water-rock and metamorphic reactions and impact events. However, we investigated these sources and found that even under diverse planetary conditions, they are unlikely to generate significant abiotic CH₄ fluxes comparable to Earth’s biogenic flux without also generating observable contextual clues that would signify a false positive. In particular, abiotic sources cannot easily produce atmospheres rich in CH₄ and CO₂ with negligible CO due to the strong redox disequilibrium between CO₂ and CH₄ and the fact that CO is expected to be readily consumed by life. The atmospheric CO/CH₄ ratio is a promising diagnostic tool for distinguishing lifeless and inhabited, anoxic worlds; although further work is required to investigate plausible CO/CH₄ ratios for a variety of host star types and planetary ecosystems. Beyond producing atmospheric methane on Earth-like planets, we explored whether exoplanets with large volatile inventories like Saturn’s moon Titan could sustain long atmospheric lifetimes for CH₄. Taken as a whole, this work provides a preliminary framework for identifying methane biosignatures that accounts for the broader planetary and astrophysical context.