Abundant atmospheric methane from volcanism on terrestrial planets is unlikely and strengthens the case for methane as a biosignature
Nicholas Wogan, Joshua Krissansen-Totton, David C. Catling
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 non-biological CH4 and CO2 and related
contextual clues. Here, we investigate whether magmatic volcanic outgassing on
terrestrial planets can produce atmospheric CH4 and CO2 with a thermodynamic
model. Our model suggests that volcanoes are unlikely to produce CH4 fluxes
comparable to biological fluxes. Improbable cases where volcanoes produce
biological amounts of CH4 also produce ample carbon monoxide. We show, using a
photochemical model, that high abiotic CH4 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 CH4 on terrestrial planets, we conclude that observations of
atmospheric CH4 with CO2 are difficult to explain without the presence of
biology when the CH4 abundance implies a surface flux comparable to modern
Earth's biological CH4 flux. A small or negligible CO abundance strengthens the
CH4+CO2 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 CH4-rich atmospheres
suitable for an origin of life may favor alternatives such as impact-induced
reducing atmospheres.