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Abstract The recent JWST detections of carbon-bearing molecules in a habitable-zone sub-Neptune have opened a new era in the study of low-mass exoplanets. The sub-Neptune regime spans a wide diversity of planetary interiors and atmospheres not witnessed in the solar system, including mini-Neptunes, super-Earths, and water worlds. Recent works have investigated the possibility of gas dwarfs, with rocky interiors and thick H₂-rich atmospheres, to explain aspects of the sub-Neptune population, including the radius valley. Interactions between the H₂-rich envelope and a potential magma ocean may lead to observable atmospheric signatures. We report a coupled interior-atmosphere modeling framework for gas dwarfs to investigate the plausibility of magma oceans on such planets and their observable diagnostics. We find that the surface–atmosphere interactions and atmospheric composition are sensitive to a wide range of parameters, including the atmospheric and internal structure, mineral composition, volatile solubility and atmospheric chemistry. While magma oceans are typically associated with high-temperature rocky planets, we assess if such conditions may be admissible and observable for temperate sub-Neptunes. We find that a holistic modeling approach is required for this purpose and to avoid unphysical model solutions. Using our model framework, we consider the habitable-zone sub-Neptune K2-18 b as a case study and find that its observed atmospheric composition is incompatible with a magma ocean scenario. We identify key atmospheric molecular and elemental diagnostics, including the abundances of CO₂, CO, NH₃, and, potentially, S-bearing species. Our study also underscores the need for fundamental material properties for accurate modeling of such planets. 

We provide estimates of atmospheric pressure and surface composition on short-period, rocky exoplanets with dayside magma pools and silicate-vapor atmospheres. Atmospheric pressure tends toward vapor-pressure equilibrium with surface magma, and magma-surface composition is set by the competing effects of fractional vaporization and surface-interior exchange. We use basic models to show how surface-interior exchange is controlled by the planet’s temperature, mass, and initial composition. We assume that mantle rock undergoes bulk melting to form the magma pool, and that winds flow radially away from the substellar point. With these assumptions, we find that: (1) atmosphere-interior exchange is fast when the planet’s bulk-silicate FeO concentration is low, and slow when the planet’s bulk-silicate FeO concentration is high; (2) magma pools are compositionally well mixed for substellar temperatures ≲2400 K, but compositionally variegated and rapidly variable for substellar temperatures ≳2400 K; (3) currents within the magma pool tend to cool the top of the solid mantle (“tectonic refrigeration”) (4) contrary to earlier work, many magma planets have time-variable surface compositions.