Constraints on water outgassing rates on the TRAPPIST-1 planets from interior modeling

The TRAPPIST-1 exoplanet system consists of an ultracool M dwarf star with 7 transiting terrestrial planets, several of which are remarkably similar to Earth in mass, radius, and received stellar radiation – making it a natural laboratory to study planetary habitability. Recent observations with JWST are consistent with atmospheric water vapor on TRAPPIST-1c. Interpretation of the first 15 μm secondary eclipse measurement from Zieba et al. (2023) indicates that several types of atmospheres are possible on TRAPPIST-1c, including ones that contain up to 3 bars of water vapor (Lincowski et al. 2023). Given that TRAPPIST-1 planets are highly susceptible to atmospheric escape from stellar XUV radiation, a large water source would be required to sustain a steady-state atmosphere that contains water vapor. So, is it possible that such a large water source exists? Here, we place theoretical constraints on one possible water source: volcanic outgassing. We model the physics and chemistry of magma formation and outgassing based on our knowledge of Earth processes, and we explore a wide range of parameters applicable to TRAPPIST-1. We also incorporate observational constraints on the TRAPPIST-1 planets to narrow the range of plausible outgassing rates, e.g. we eliminate model runs that result in extended, H-dominated atmospheres, which have not been observed. This work was supported by the Virtual Planetary Laboratory (VPL). From this analysis we find that (1) water outgassing rates can plausibly be up to two orders of magnitude higher on the TRAPPIST-1 planets than on modern Earth. It is thus possible that outgassing could sustain steady-state water vapor atmospheres, but future work is required to determine their steady-state abundances. We also find (2) relationships between the predicted outgassing rates and the properties of the planets’ interiors (e.g., mantle water content) which can be probed with future observations. Ultimately, this work provides theoretical justification for the presence of a large water source on the TRAPPIST-1 planets, which could explain the TRAPPIST-1c data, and motivates the search for water – and potentially habitable conditions – on the other planets.