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

¹ ETH Zürich, Department of Earth and Planetary Sciences, Institute for Geochemistry and Petrology, Zürich, Switzerland
² Universität Bern, Center for Space and Habitability, Bern, Switzerland
³ Universität Bern, Physikalisches Institut, Weltraumforschung und Planetologie, Gesellschaftsstrasse 6, 3012 Bern, Switzerland

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 September 2025
Accepted: 30 March 2026

Abstract

Context. Spectra collected by the James Webb Space Telescope (JWST) hint at a volatile-rich and perhaps CO/CO₂-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 (fO₂) −6 to +6 log₁₀ units relative to the iron-wüstite (IW) buffer.

Results. We find the fO₂ of the mantle to be the major control on emission and transmission spectra. At low fO₂ (ΔIW < −3), SiO and CO appear in both emission and transmission. Between −3 < ΔIW < +3, CO and CO₂ dominate infrared spectra. At high fO₂ (ΔIW ≥ +3), CO₂ in high-metallicity atmospheres is joined by SO₂, producing strong absorption at 9 μm. H₂O features appear in all ΔIW > −3 atmospheres, but are sensitive to H₂ 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 fO₂, 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.