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arXiv · 2509.13610

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

Fabian L. Seidler·Paolo A. Sossi·Dan J. Bower·Brice-Olivier Demory
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Abstract

The atmospheres of hot rocky exoplanets (HREs), should they persist, are products of interactions with underlying magma oceans. Spectra collected by the James Webb Space Telescope (JWST) hint at a CO/CO$_2$-rich atmosphere on the HRE 55 Cancri e, indicative of such a process. Here, we aim to identify diagnostic features that can be used to infer the composition and geochemical state of HREs. We construct a coupled atmosphere-interior model that computes the equilibrium gas speciation in the atmosphere 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 explore a parameter space that spans volatile mass fractions from 0.1 to 10 times that of the Earth, solar- to Earth-like metallicities, and 12 orders of magnitude in oxygen fugacity fO$_2$. We find fO$_2$ to be the major control of the shape of emission and transmission features. The presence of species such as SO$_2$ and the relative intensities of H$_2$O and CO$_2$ features allow to distinguish the origin of the gas, accreted or outgassed, while the atmospheric mass is more challenging to constrain. Inflated HREs, whose densities are compatible with a nebular atmosphere are rare, but a viable explanation for the planet TOI-1408 c. The majority of HREs, including 55 Cancri e, are too dense to be dominated by H$_2$-rich nebular gas yet too puffy for an Earth-like volatile budget, implying modest atmospheres of mixed heritage that are degenerate in fO$_2$, volatile mass and composition. The MIRI observation of 55 Cancri e disfavours oxidised Earth-like and reduced primoridal atmospheres alike, while the NIRCam data remain inconclusive. Future observations at wavelengths beyond 8 $μ$m are key to discerning between potential scenarios.

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Fabian L. Seidler, Paolo A. Sossi, Dan J. Bower, Brice-Olivier Demory. 2026-06-22. Volatile-bearing mineral atmospheres of hot rocky exoplanets as probes of interior state and composition. https://doi.org/10.1051/0004-6361%2F202557276

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