Coupling magma-ocean and atmospheres in spectral retrievals of sub-Neptunes
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| Format: | Preprint |
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2026
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| _version_ | 1866917476558700544 |
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| author | Ito, Yuichi Changeat, Quentin |
| author_facet | Ito, Yuichi Changeat, Quentin |
| contents | 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/CO$_2$ absorption feature near 4.5 $μ$m 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2605_08752 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Coupling magma-ocean and atmospheres in spectral retrievals of sub-Neptunes Ito, Yuichi Changeat, Quentin Earth and Planetary Astrophysics 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/CO$_2$ absorption feature near 4.5 $μ$m 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. |
| title | Coupling magma-ocean and atmospheres in spectral retrievals of sub-Neptunes |
| topic | Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2605.08752 |