Impact of oxygen fugacity on atmospheric spectra of hot rocky exoplanets

An essential aspect of understanding how rocky (exo-)planets form and evolve is unravelling their bulk composition. While mass and radius alone do not yield precise estimations of exoplanet compositions due to the degeneracy of interior models that can fit such observations, abundances of refractory elements in their host stars are often used as proxies to constrain terrestrial planet composition. However, oxygen, whose relative abundance governs how iron (and other siderophile elements) partition between the mantle and core, is both a volatile and a refractory element, preventing a straightforward determination from stellar abundances. Therefore, we require independent means to estimate exoplanet oxidation states through observations of their atmospheres and/or surfaces. To do so, observations of ultra-hot rocky exoplanets would be ideal, owing to the fact that their atmospheres are expected to be in thermodynamic equilibrium with their surfaces. To interpret such observations, we investigate the impact of oxygen fugacity (fO₂), temperature (T) and composition on the formation of atmospheres on ultra-hot rocky exoplanets. Our approach treats melt vaporisation and atmospheric gas speciation thermodynamically self-consistently, before using radiative transfer simulations to predict atmospheric structure and emission spectra. We find that compositional effects are minor within the range of plausible rocky compositions. However, the emission spectrum is particularly sensitive to fO₂, owing to its influence on the partial pressures of gas species in equilibrium with the silicate mantle. This effect is exacerbated when the atmosphere contains a volatile component such as H₂O, CO₂ or N₂. We show that observations made with the James Webb Space Telescope (JWST) hold the potential to distinguish between fO₂ scenarios, thereby paving the way for the first independent constraints on the chemistry of rocky exoplanets.