Atmospheres, Climate, Weather

Diversity of Rocky Planet Atmospheres In The C-H-O-N-S-Cl System With Interior Dissolution, Non-ideality, and Condensation: Application to TRAPPIST-1e and Sub-Neptunes

By Keith Cowing
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astro-ph.EP
July 6, 2025
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Diversity of Rocky Planet Atmospheres In The C-H-O-N-S-Cl System With Interior Dissolution, Non-ideality, and Condensation: Application to TRAPPIST-1e and Sub-Neptunes
Atmospheric speciation of TRAPPIST-1e at 280 K above a planetary surface with stable condensates for atmospheres originally in equilibrium with a fully molten mantle. In all panels, the curve colours correspond to the gas species listed in the legend and condensate masses are relative to Earth oceans (EO). Upper panels show the percentage of models by dominant species (VMR > 50%) that satisfy the requirement of a minimum mass of (a) Water, (b) Graphite, (c) α-sulfur, and (d) Ammonium chloride. Lower panels illustrate the composition and total pressure of the atmosphere for (e) CO2-rich above a water ocean, (f) CH4-rich above graphite, (g) CO2-rich above α-sulfur, and (h) CH4-rich above ammonium chloride. Median values are indicated by lines and shaded regions bracket the first and third quartiles. Compare to the atmospheric speciation derived from a partially molten mantle in Figure A5. — astro-ph.EP

A quantitative understanding of the nature and composition of low-mass rocky exo(planet) atmospheres during their evolution is needed to interpret observations.

The magma ocean stage of terrestrial- and sub-Neptune planets permits mass exchange between their interiors and atmospheres, during which the mass and speciation of the atmosphere is dictated by the planet’s volatile budget, chemical equilibria, and gas/fluid solubility in molten rock.

As the atmosphere cools, it is modified by gas-phase reactions and condensation. We combine these processes into an open-source Python package built using JAX called Atmodeller, and perform calculations for planet sizes and conditions analogous to TRAPPIST-1e and K2-18b.

For TRAPPIST-1e-like planets, our simulations indicate that CO-dominated atmospheres are prevalent during the magma ocean stage, which, upon isochemical cooling, predominantly evolve into CO2-rich atmospheres of a few hundred bar at 280 K. Around 40% of our simulations predict the coexistence of liquid water, graphite, sulfur, and ammonium chloride-key ingredients for surface habitability.

For sub-Neptune gas dwarfs, pressures are sufficiently high (few GPa) to deviate the fugacities of gases from ideality, thereby drastically enhancing their solubilities. This buffers the total atmospheric pressure to lower values than for the ideal case. These effects conspire to produce CH4-rich sub-Neptune atmospheres for total pressures exceeding around 3.5 GPa, provided H/C is approximately 100x solar and fO2 moderately reducing (3 log10 units below the iron-wüstite buffer).

Otherwise, molecular hydrogen remains the predominant species at lower total pressures and/or higher H/C. For all planets at high temperature, solubility enriches C/H in the atmosphere relative to the initial composition.

Dan J. Bower, Maggie A. Thompson, Kaustubh Hakim, Meng Tian, Paolo A. Sossi

Comments: 41 pages, 10 figures in main text, 8 figures in appendices, submitted to ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2507.00499 [astro-ph.EP] (or arXiv:2507.00499v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2507.00499
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Submission history
From: Dan Bower
[v1] Tue, 1 Jul 2025 07:14:10 UTC (12,751 KB)
https://arxiv.org/abs/2507.00499
Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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