Potential Atmospheric Compositions of TRAPPIST-1c Constrained by JWST/MIRI Observations at 15 m

The first James Webb Space Telescope observations of TRAPPIST-1 c showed a secondary eclipse depth of 421 +/- 94 ppm at 15 mu m, which is consistent with a bare rock surface or a thin, O-2-dominated, low-CO2 atmosphere. Here we further explore potential atmospheres for TRAPPIST-1 c by comparing the observed secondary eclipse depth to synthetic spectra of a broader range of plausible environments. To self-consistently incorporate the impact of photochemistry and atmospheric composition on atmospheric thermal structure and predicted eclipse depth, we use a two-column climate model coupled to a photochemical model and simulate O-2-dominated, Venus-like, and steam atmospheres. We find that a broader suite of plausible atmospheric compositions are also consistent with the data. For lower-pressure atmospheres (0.1 bar), our O-2-CO2 atmospheres produce eclipse depths within 1 sigma of the data, consistent with the modeling results of Zieba et al. However, for higher-pressure atmospheres, our models produce different temperature-pressure profiles and are less pessimistic, with 1-10 bar O-2, 100 ppm CO2 models within 2.0 sigma-2.2 sigma of the measured secondary eclipse depth and up to 0.5% CO2 within 2.9 sigma. Venus-like atmospheres are still unlikely. For thin O-2 atmospheres of 0.1 bar with a low abundance of CO2 (similar to 100 ppm), up to 10% water vapor can be present and still provide an eclipse depth within 1 sigma of the data. We compared the TRAPPIST-1 c data to modeled steam atmospheres of <= 3 bars, which are 1.7 sigma-1.8 sigma from the data and not conclusively ruled out. More data will be required to discriminate between possible atmospheres or more definitively support the bare rock hypothesis.