When Is Atmospheric Methane a Good Exoplanet Biosignature?

With the recent launch of JWST, we are entering an exciting new phase of exoplanet science that will focus on characterizing exoplanetary atmospheres, including those of potentially habitable, terrestrial worlds. Therefore, 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, these gases likely will be extremely difficult to detect with JWST in high mean molecular weight atmospheres. 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. Although there is a patchwork of prior studies on methane biosignatures, an up-to-date, dedicated assessment of the planetary conditions needed for methane to be a good exoplanet biosignature has been lacking. Here we present our work on understanding the necessary planetary context for methane biosignatures and the potential false positive scenarios that arise from its abiotic sources. Methane has been invoked as a potential biosignature due to its short photochemical lifetime (≤1 Myr) on habitable-zone, rocky planets orbiting solar-type stars, which requires substantial replenishment fluxes to sustain large atmospheric abundances. On Earth, life is the only source that can generate such large CH₄ replenishment fluxes. Although methane can be produced by various abiotic mechanisms including magmatic outgassing, water-rock and metamorphic reactions, and impact events, we find that known abiotic processes cannot easily produce atmospheres with abundant CH₄ and CO₂ with comparatively little CO due to the strong redox disequilibrium between CH₄ and CO₂. We also explore whether temperate, terrestrial planets with large volatile inventories like Titan could have long lifetimes of atmospheric CH4 and determine that the photochemical lifetime of such CH₄ is still short. We conclude that methane is more likely to be biogenic for terrestrial planets with 1) a high mean molecular weight and anoxic atmosphere, 2) an atmospheric CH₄ abundance that implies surface fluxes exceeding what could be generated by known abiotic processes, and 3) atmospheric CO₂ with comparatively little CO.