GCM Constraints on the Detectability of the CO${}_2$-CH${}_4$ Biosignature Pair on TRAPPIST-1e with JWST

Abstract

Terrestrial exoplanets such as TRAPPIST-1e will be observed in a new capacity with JWST/NIRSpec, which is expected to be able to detect CO${}_2$, CH${}_4$, and O${}_2$ signals, if present, with multiple co-added 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 (GCM), 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 $\lesssim$ 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.

11 pages, 4 figures, 1 table, accepted for publication in ApJL

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