2023ApJ...955L...7L

The first James Webb Space Telescope observations of TRAPPIST-1 c showed a secondary eclipse depth of 421 ± 94 ppm at 15 μm, which is consistent with a bare rock surface or a thin, O₂-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 O₂-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 O₂-CO₂ atmospheres produce eclipse depths within 1σ 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 O₂, 100 ppm CO₂ models within 2.0σ-2.2σ of the measured secondary eclipse depth and up to 0.5% CO₂ within 2.9σ. Venus-like atmospheres are still unlikely. For thin O₂ atmospheres of 0.1 bar with a low abundance of CO₂ (∼100 ppm), up to 10% water vapor can be present and still provide an eclipse depth within 1σ of the data. We compared the TRAPPIST-1 c data to modeled steam atmospheres of ≤3 bars, which are 1.7σ-1.8σ 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.