Volatile-bearing mineral atmospheres of hot rocky exoplanets as probes of interior state and composition

Context . Spectra collected by the James Webb Space Telescope (JWST) hint at a volatile-rich and perhaps CO/CO 2 -bearing atmosphere on the hot rocky exoplanet (HRE) 55 Cancri e. Such atmospheres, should they persist, are thought to be products of mass exchange with underlying magma oceans and thereby carry an imprint of the geochemical state of HREs. Aims . Here, we aim to identify diagnostic features in emission and transmission that can be used to infer the composition and geochemical state of HREs. Methods . We constructed a coupled atmosphere-interior model that computes the equilibrium gas speciation in the system Si–Mg–Fe–O–C–H–S–N–He. The model accounts for both the equilibrium vaporisation of mineral gases and the partitioning of volatile species between the magma ocean and atmosphere. Using a fiducial planet with the properties of 55 Cancri e, we explored a parameter space that spans volatile mass fractions from 0.1 to ten times that of the Earth, solar- to Earth-like metallicities, and oxygen fugacities ( f O 2 ) −6 to +6 log 10 units relative to the iron-wüstite (IW) buffer. Results . We find the f O 2 of the mantle to be the major control on emission and transmission spectra. At low f O 2 (ΔIW < −3), SiO and CO appear in both emission and transmission. Between −3 < ΔIW < +3, CO and CO 2 dominate infrared spectra. At high f O 2 (ΔIW ≥ +3), CO 2 in high-metallicity atmospheres is joined by SO 2 , producing strong absorption at 9 μm. H 2 O features appear in all ΔIW > −3 atmospheres, but are sensitive to H 2 content. Condensation of silicates and iron oxides is common in the upper atmospheres of oxidised planets and is driven by cooling from triatomic molecules. Observed mass–radius trends suggest that a substantial fraction of HREs, including 55 Cancri e, have modest atmospheres of mixed heritage that are degenerate in f O 2 , volatile mass, and metallicity. Combined with JWST Mid-Infrared Instrument observations, high metallicity (Earth-like) atmospheres as well as highly reduced solar-like atmospheres are precluded on 55 Cancri e, while the Near Infrared Camera data remain inconclusive. Future observations at wavelengths beyond 8 μm are key to discerning between potential scenarios.