Interior redox state effects on the stability of secondary atmospheres and observational manifestations: LP 791-18 d as a case study for outgassing rocky exoplanets

TL;DR

This study models how the interior redox state influences the atmospheric stability and observational signatures of rocky exoplanets like LP 791-18 d, highlighting the importance of oxidation in atmospheric retention and detectability.

Contribution

It introduces a coupled interior-atmosphere model that links redox state to atmospheric stability and observational features for rocky exoplanets.

Findings

Highly oxidized interiors lead to stable atmospheres.

Reduced interiors tend to undergo hydrodynamic escape.

Color-color diagrams can distinguish between bare rocks and atmospheres.

Abstract

Recent advances in space and ground-based facilities now enable atmospheric characterization of a selected sample of rocky exoplanets. These atmospheres offer key insights into planetary formation and evolution, but their interpretation requires models that couple atmospheric processes with both the planetary interior and the surrounding space environment. This work focuses on the Earth-size planet LP791 18d, which is estimated to receive continuous tidal heating due to the orbital configuration of the system; thus, it is expected to exhibit volcanic activity. We estimate the mantle temperature of 1680-1880 K. Our results show that the atmospheric mean molecular weight gradient is controlled by oxygen fugacity rather than bulk metallicity. Furthermore, we use the atmospheric steady-state solutions produced from the interior redox state versus surface pressure parameter space and explore their atmospheric stability. We find that stability is achieved only in highly oxidized scenarios while reduced interior states fall into the hydrodynamic escape regime with mass loss rates on the order of 10^5-10^8 kg/s. We argue that scenarios with reduced interior states are likely to have exhausted their volatile budget during the planets lifetime. Furthermore, we predict the atmospheric footprint of the planets interior based on its oxidation state and assess its detectability using current or forthcoming tools to constrain the internal and atmospheric composition. We show that the degeneracy between bare rock surfaces and thick atmospheres can be resolved by using three photometric bands to construct a color-color diagram that accounts for potential effects from photochemical hazes and clouds. Our modeling approach connects interior and atmospheric processes, providing a basis to explore volatile evolution and potential habitability.