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

Abstract

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 CO2, CH4, and O2 signals, if present, with multiple coadded transit observations. The CO2-CH4 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 CO2 pressure with 1 bar of background N2. We investigate cases both with and without a modern Earth-like CH4 mixing ratio to examine the effect of CO2 and CH4 on the transmission spectrum and climate state of the planet. We demonstrate that in the optimistic haze-free cloudy case, H2O, CO2, and CH4 could all be detectable in less than 50 transits within an atmosphere of 1 bar N2 and 10 mbar CO2 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 CO2-CH4 in TRAPPIST-1e’s atmosphere across a variety of atmospheric CO2 content, and that temporal climate variability does not significantly affect spectral feature variability for NIRSpec PRISM.

Analogous response of temperate terrestrial exoplanets and Earth’s climate dynamics to greenhouse gas supplement

“…Note that these GCM simulations implicitly assume an atmospheric composition that would allow for habitable surface conditions, but it is possible that the UV emission during the bright pre-main-sequence phase of M dwarf stars causes atmospheric loss along with water photolysis and resulting escape to space 8 – 10 . These GCMs have predicted that TRAPPIST-1e will likely have a temperate climate for a wide range of possible background carbon dioxide supplements e.g., 11 – 13 . Additionally, the TRAPPIST-1 Habitable Atmosphere Intercomparison protocol has compared the predictions of four GCMs (ExoCAM, LMD-Generic/PCM, ROCKE-3D, UK Met Office Unified Model) and found broad agreement, with ExoCAM having the most humid and cloudy atmosphere in the protocol experiments 11 .…”

Double Trouble: Two Transits of the Super-Earth GJ 1132 b Observed with JWST NIRSpec G395H

“…The impact of atmospheric variability on the transmission spectra of tidally locked terrestrial planets has been explored by numerous teams (e.g., May et al 2021;Song & Yang 2021;Cohen et al 2022;Rotman et al 2023), with the general conclusion being that such variability is below the detectability limit of JWST instruments. This is because, while the cloud cover of any one region of the terminator can be highly variable, we are observing limb averaged spectra that simultaneously probe cloudy and cloudfree regions.…”

Tropical Cyclones on Tidally Locked Rocky Planets: Dependence on Rotation Period

“…On Earth, tropical cyclones may have large-scale impacts on the mean climate, most notably by drying surrounding regions (Schenkel & Hart 2015) and thereby enabling dry regions to more efficiently radiatively cool to space (Pierrehumbert 1995;Wing 2019). Tropical cyclones are also expected to form on tidally locked terrestrial exoplanets (Komacek et al 2020;Yan & Yang 2020) and may have potential consequences for observable properties, especially the amplitude of water vapor features in transmission spectra (Yan & Yang 2020) as well as their time variability (May et al 2021;Song & Yang 2021;Rotman et al 2023).…”