Inferring the interior oxygen fugacity of rocky exoplanets from observations: Assessing biases by atmospheric chemistry

¹ University of Paris Saclay, OVSQ, LATMOS, 11 Boulevard d’Alembert, 78280 Guyancourt, France
² Ludwig Maximilian University, Faculty of Physics, Observatory of Munich, Scheinerstrasse 1, Munich 81679, Germany
³ ENS Paris-Saclay, 4 avenue des Sciences, 91190 Gif-sur-Yvette, France
⁴ University of Bern, ARTORG Center for Biomedical Engineering Research, Murtenstrasse 50, 3008, Bern, Switzerland
⁵ University College London, Department of Physics & Astronomy, Gower St, London WC1E 6BT, UK
⁶ University of Warwick, Department of Physics, Astronomy & Astrophysics Group, Coventry CV4 7AL, UK

^★ Corresponding author: thomas.drant@latmos.ipsl.fr

Received: 27 August 2024
Accepted: 24 February 2025

Abstract

In the era of the James Webb Space Telescope, inferring the presence and bulk composition of temperate rocky exoplanet atmospheres is now possible. The primary targets typically have equilibrium temperatures ranging from 400 to 1500 K, for which a balance between geochemical outgassing and escape is required to maintain an atmosphere. The composition of these exoplanet atmospheres hold crucial information on the redox state of the planetary interior characterized by the oxygen fugacity (fo₂). The relative molecular abundances of volatile species with opposite redox states inferred from observations can help constrain an effective interior fo₂. Using different model complexities from 0D simulations of chemical equilibrium to 1D atmospheric simulations with outgassing and self-consistent iterations of atmospheric chemistry (photochemistry and thermochemistry) and radiative transfer, we assess the reliability of using relative abundances in a C−H−O system to infer fo₂. The CO₂/CO, previously suggested as the most reliable tracer of fo₂, is increased by atmospheric cooling (thermochemical cooling between melt and atmosphere) and photochemistry, which would cause a bias of approximately one to two orders of magnitude on the retrieved fo₂. Constraints on the atmospheric temperature can help correct the effect of atmospheric cooling and improve the retrieval of fo₂. The increase of CO₂/CO driven by photochemistry is dominant for thin atmospheres, although it occurs over long timescales (tens or hundreds of thousands of years) and therefore would be negligible if the atmosphere is continuously replenished by outgassing. The transition between a chemical regime dominated by atmospheric thermochemistry toward a regime dominated by photochemistry is controlled not only by surface pressure and temperature but also by oxygen fugacity itself (via O/H). Inferring CO₂/CO from the data might be challenging given the low contribution of CO in transit and emission spectra for objects with high CO₂ and H₂O abundances. We suggest CO₂/CH₄ as an alternative tracer of fo₂, although high methane abundances are only expected in reducing conditions (i.e., less than the iron–wustite buffer) and high pressure-temperature surface conditions favoring the buildup of CH₄ by atmospheric cooling.