Coupled Photochemical-Climate Modeling of Plausible Tenuous Outgassed Atmospheres on the TRAPPIST-1 Planets

Abstract

Available JWST observations TRAPPIST-1 system have suggested that several of the planets are likely airless, or possess a very tenuous atmosphere. However, the high atmospheric escape rates expected for these planets suggest that any tenuous atmosphere must be replenished by constant outgassing, and past studies on modeling potential atmospheres for the planets have not widely considered surface pressures <1 bar. Here, we show that tenuous atmospheres on the TRAPPIST-1 planets are likely possible, supported by constant plausible rates of water and/or CO$_{2}$ outgassing against assumed high escape rates (up to ~10$^{30}$ s$^{-1}$). We use a coupled photochemical-climate model and sample from a broad phase space of outgassing, surface deposition, and top-of-atmosphere escape rates to test hundreds of atmospheres per planet. Critically, our model also allows surface pressure to vary based on the balance of sources and sinks. We find that 6 different compositional archetypes are generated via H$_{2}$O and/or CO$_{2}$ outgassing across our phase space, and atmospheres commonly fall between 10$^{-4}$ -- 1 bar. We find that potentially habitable surface environments are possible for TRAPPIST-1d and e at pressures between 0.05 -- 2 bar and 0.5 -- 1 bar, respectively. Where possible, we compare our models to JWST observational data for TRAPPIST-1b, c, d, and e; all atmospheres found in this study for these planets match available transmission data to <3$\sigma$. However, emission data are consistent with atmospheric outcomes constrained to thin O$_{2}$-dominated compositions for TRAPPIST-1b ($\lesssim$0.01 bars) and c ($\lesssim$0.2 bars), which may or may not contain trace SO$_{2}$.