Disequilibrium biosignatures in exoplanet atmospheres and their detectability with the James Webb Space Telescope
Friday 6 July, 12:10
New ground- and space-based telescopes will soon characterize habitable exoplanets and look for atmospheric gases produced by life. Oxygen is a promising biosignature gas, but several hypothetical scenarios have been proposed for producing abiotic oxygen. Furthermore, even if these scenarios can be ruled out by other observations, oxygenic photosynthesis may be an uncommon metabolism. Atmospheric chemical disequilibrium is potentially a more general biosignature. We have developed a quantitative metric for atmospheric disequilibrium and applied it to the Solar System planets. The biogenic disequilibrium in the modern Earth's atmosphere, which is mostly attributable to the coexistence of O2, N2, and liquid water, far exceeds the photochemically-produced disequilibria of the other Solar System atmospheres. We have also applied our disequilibrium metric to the Earth through time, and discovered that on the anoxic Archean Earth (4-2.5 billion years ago), life maintained a sizeable disequilibrium between CO2, CH4, N2, and liquid water. Such a combination of gases would not persist without substantial replenishment of CH4 from the surface, which we have shown to be unlikely without life. We therefore propose the coexistence of CO2 and CH4 in the atmosphere of an ostensibly habitable exoplanet as a potential biosignature. Biogenicity would be strengthened by the absence of atmospheric CO, which would be present if abiotic CH4 were outgassed and if no life were consuming CO. Finally, we performed simulated Bayesian retrievals to show this new biosignature is potentially detectable with the James Webb Space Telescope for nearby transiting planets such as the TRAPPIST-1 system.