Coupling Magma-ocean And Atmospheres In Spectral Retrievals Of Sub-Neptunes

Recent high-precision atmospheric observations with JWST is enabling detailed characterization of sub-Neptune atmospheres and motivating efforts to understand and constrain their interiors.

Theoretical studies suggest that sub-Neptunes possibly host hydrogen-dominated atmospheres that are chemically coupled with an underlying magma ocean. However, a quantitative retrieval framework directly linking atmospheric spectra to magma ocean properties has yet to be established. Here we introduce MELTYQ, a coupled magma-atmosphere retrieval framework that links transmission spectra to the oxidation state and volatile inventory of underlying magma oceans.

MELTYQ combines a magma-atmosphere equilibrium model, which includes the solubility of H-/O-/C-/N-bearing species in the melt and redox reactions, with a Bayesian spectral retrieval scheme. Using simulated retrieval tests, we validate the approach and show that magma redox state and volatile content can be constrained under favorable observational conditions.

As a proof of concept, we apply MELTYQ to JWST transmission spectra of the benchmark sub-Neptunes K2-18 b and TOI-270 d. We find that coupled magma-atmosphere retrievals are generally capable of reproducing the observed spectra of these planets. However, we identify several key limitations in the current framework. Specifically: more flexible free-retrieval approaches remain statistically preferred; the CO/CO₂ absorption feature near 4.5 μm for TOI-270 d is not fully captured; and a number of underlying model assumptions may not be strictly valid.

Nevertheless, embedding coupled magma-atmosphere models directly within Bayesian retrievals enables quantitative assessment of degeneracies and sensitivities, establishing a pathway for directly connecting atmospheric spectra to magma composition in this underexplored exoplanet regime.

Yuichi Ito, Quentin Changeat

Comments: 22 pages, 9 figures, accepted in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2605.08752 [astro-ph.EP](or arXiv:2605.08752v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2605.08752
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Submission history
From: Yuichi Ito
[v1] Sat, 9 May 2026 07:27:18 UTC (9,668 KB)
https://arxiv.org/abs/2605.08752

Astrobiology, exoplanet,