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

The first James Webb Space Telescope observations of TRAPPIST-1 c showed a secondary eclipse depth of 421 ± 94 ppm at 15 μ m, which is consistent with a bare rock surface or a thin, O _2 -dominated, low-CO _2 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 –CO _2 atmospheres produce eclipse depths within 1 σ 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 CO _2 models within 2.0 σ –2.2 σ of the measured secondary eclipse depth and up to 0.5% CO _2 within 2.9 σ . Venus-like atmospheres are still unlikely. For thin O _2 atmospheres of 0.1 bar with a low abundance of CO _2 (∼100 ppm), up to 10% water vapor can be present and still provide an eclipse depth within 1 σ of the data. We compared the TRAPPIST-1 c data to modeled steam atmospheres of ≤3 bars, which are 1.7 σ –1.8 σ 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.