General Circulation Model Constraints on the Detectability of the CO2-CH4 Biosignature Pair on TRAPPIST-1e with JWST

Terrestrial exoplanets such as TRAPPIST-1e will be observed in a new capacity with the JWST/Near Infrared Spectrograph (NIRSpec), which is expected to be able to detect CO _2 , CH _4 , and O _2 signals, if present, with multiple coadded transit observations. The CO _2 -CH _4 pair in particular is theorized to be a potential biosignature when inferred to be in chemical disequilibrium. Here, we simulate TRAPPIST-1e’s atmosphere using the ExoCAM general circulation model, assuming an optimistic haze-free, tidally locked planet with an aquaplanet surface, with varying atmospheric compositions from 10 ^−4 bar to 1 bar of partial CO _2 pressure with 1 bar of background N _2 . We investigate cases both with and without a modern Earth-like CH _4 mixing ratio to examine the effect of CO _2 and CH _4 on the transmission spectrum and climate state of the planet. We demonstrate that in the optimistic haze-free cloudy case, H _2 O, CO _2 , and CH _4 could all be detectable in less than 50 transits within an atmosphere of 1 bar N _2 and 10 mbar CO _2 during JWST’s lifespan with NIRSpec as long as the noise floor is ≲10 ppm. We find that in these optimistic cases, JWST may be able to detect potential biosignature pairs such as CO _2 -CH _4 in TRAPPIST-1e’s atmosphere across a variety of atmospheric CO _2 content, and that temporal climate variability does not significantly affect spectral feature variability for NIRSpec PRISM.