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

TL;DR

This study uses climate modeling and simulated JWST observations to assess the detectability of CO2 and CH4 biosignatures on TRAPPIST-1e, highlighting conditions under which these gases could be identified within JWST's mission lifetime.

Contribution

It demonstrates the potential for JWST to detect CO2-CH4 biosignature pairs on TRAPPIST-1e using GCM simulations and spectral analysis, considering various atmospheric compositions.

Findings

Detectability of CO2 and CH4 in less than 50 transits under optimistic conditions.

JWST can identify biosignature pairs across different CO2 levels.

Temporal climate variability does not significantly impact spectral feature detection.

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.