Predicting the observable atmospheres of Trappist-1 planets from an atmosphere-interior evolution model

The seven planets of the Trappist-1 system provide a unique opportunity to test current understanding of terrestrial planet evolution. JWST is expected to characterize the bulk atmospheres of these planets, potentially detecting constituents such as CO₂, CO, H₂O, CH₄, or abundant, abiotic O₂ left over from the photodissociation of water and subsequent H escape. The presence or absence of such oxygen has important astrobiological implications for the interpretation of atmospheric oxygen as a biosignature. Here, we apply a fully coupled atmosphere-interior evolution model to the Trappist-1 planets to anticipate their modern atmospheres. This model—which has previously been validated against Earth and Venus evolution—connects magma ocean crystallization to subsequent, temperate geochemical cycling. Mantle convection, magmatic outgassing, thermal and non-thermal escape, crustal oxidation, a radiative-convective climate model, and deep carbon and hydrologic cycling are all explicitly coupled to anticipate bulk atmosphere composition and planetary redox evolution over 8 Gyr. By adopting a Monte Carlo approach that samples a broad range of initial conditions and unknown parameters, we make some tentative predictions about current Trappist-1 atmospheres. We find that anoxic atmospheres are probable, but not guaranteed, for the outer planets (e, f, and g); oxygen produced via H loss during the pre-main sequence is typically consumed by crustal sinks. In contrast, oxygen accumulation on the inner planets (b and c) occurs in around half of all models runs. Complete atmospheric erosion would be unsurprising for the inner planets, but the outer planets retain significant surface volatiles in all but a few percent of model simulations. For all planets, in cases where substantial atmospheres are retained, CO₂-dominated or CO₂-O₂ atmospheres are expected; water vapor is unlikely to be a detectable atmospheric constituent in most cases. There are necessarily many limitations and caveats to these crude predictions, but the ways in which they misalign with upcoming JWST observations will highlight gaps in current knowledge of terrestrial planet evolution.