The Effect of Oxygen Fugacity and Initial Volatile Abundance on the Atmospheres of the TRAPPIST-1 Planets

The TRAPPIST-1 planets have become prime targets for studying the atmospheric and geophysical properties of planets around M-dwarf stars.

To effectively identify their atmospheric composition, we first must understand their geological evolution. For this study, we focus on enhancing an existing atmosphere-interior exchange model by incorporating additional geological processes relevant to rocky planets.

We have extended the model to include the carbon cycle, which enables the model to track four key gas species – CO₂, CO, H₂O, and H₂ – across four planetary reservoirs: the mantle, plate, ocean, and atmosphere. Major features added include surface temperature calculations which are crucial for the carbon cycle, oxygen fugacity as a planetary interior parameter in the model, and oxidation reactions and diffusion-limited escape calculations to the atmosphere portion of the model.

We successfully validated the model for Earth and applied this model to study the effect of oxygen fugacity and initial water abundance on TRAPPIST-1 d, e and f. Our results for present-day abundances show that oxygen fugacity significantly affects the partial pressures of H₂ and CO₂ for all three planets with minor effects for CO on two of the planets. We also found that H₂ is strongly dependent on water mass fraction (WMF).

The addition of atmospheric processes produced a significant difference in the H₂ and CO abundances at present-day. These results highlight the importance of considering interior parameters to be able to further constrain the geological evolution of these planets and effectively put atmosphere observations into context.

What’s Inside Matters: The Effect of Oxygen Fugacity and Initial Volatile Abundance on the Atmospheres of the TRAPPIST-1 Planets
Junellie Perez, Laura K. Schaefer, Edward Schwieterman, Kevin B. Stevenson, Howard Chen, Jacob Lustig-Yaeger

Comments: 39 pages, 8 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2511.10801 [astro-ph.EP] (or arXiv:2511.10801v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2511.10801
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Submission history
From: Junellie Perez
[v1] Thu, 13 Nov 2025 21:10:01 UTC (1,752 KB)
https://arxiv.org/abs/2511.10801
Astrobiology, exoplanet,