Abundant Atmospheric Methane from Volcanism on Terrestrial Planets Is Unlikely and Strengthens the Case for Methane as a Biosignature
Nicholas F. Wogan, Joshua Krissansen‐Totton, David C. Catling
Abstract
Abstract The disequilibrium combination of abundant methane and carbon dioxide has been proposed as a promising exoplanet biosignature that is readily detectable with upcoming telescopes such as the James Webb Space Telescope. However, few studies have explored the possibility of nonbiological CH 4 and CO 2 and related contextual clues. Here we investigate whether magmatic volcanic outgassing on terrestrial planets can produce atmospheric CH 4 and CO 2 with a thermodynamic model. Our model suggests that volcanoes are unlikely to produce CH 4 fluxes comparable to biological fluxes. Improbable cases where volcanoes produce biological amounts of CH 4 also produce ample carbon monoxide. We show, using a photochemical model, that high abiotic CH 4 abundances produced by volcanoes would be accompanied by high CO abundances, which could be a detectable false-positive diagnostic. Overall, when considering known mechanisms for generating abiotic CH 4 on terrestrial planets, we conclude that observations of atmospheric CH 4 with CO 2 are difficult to explain without the presence of biology when the CH 4 abundance implies a surface flux comparable to modern Earth’s biological CH 4 flux. A small or negligible CO abundance strengthens the CH 4 +CO 2 biosignature because life readily consumes atmospheric CO, while reducing volcanic gases likely cause CO to build up in a planet’s atmosphere. Furthermore, the difficulty of volcanically generated CH 4 -rich atmospheres suitable for an origin of life may favor alternatives such as impact-induced reducing atmospheres.