Geophysical and atmospheric implications of f O 2 -dependent melting on rocky exoplanets

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Abstract
The geochemical evolution of long-lived magma oceans is strongly regulated by the volatile exchange between the molten mantle and the atmosphere. For planets inside the runaway-greenhouse limit, this coupled evolution can persist for billions of years, governing bulk density, surface conditions, and long-term geodynamics. However, most existing studies assume Earth-like (oxidized) conditions and neglect the influence of the redox state on melt thermodynamics and volatile release. We quantified how experimentally derived, oxygen-fugacity-dependent melting curves implemented within the coupled interior-atmosphere framework PROTEUS propagate into the thermal structure, melt fraction, and rheological evolution of rocky exoplanet interiors, and applied this to the short-period super-Earth GJ 1132 b. We found strongly non-linear thermal responses to variations in melting curves. In volatile-poor systems, reduced melting curves ( f O 2 ≤ IW, where IW denotes the iron–wüstite buffer) promote earlier deep-mantle crystallization relative to oxidized (IW+2.0) and Earth-like (IW + 4.0) cases (range IW–4.0 to IW + 4.0), favouring late-stage surface magma oceans sustained by greenhouse warming, while oxidized melting curves maintain higher melt fractions and a vertically extended magma ocean. Reduced mantles produce massive H 2 –CO-rich atmospheres; oxidized mantles favour thinner H 2 O–CO 2 envelopes. In volatile-rich systems, the interior reaches radiative equilibrium at high melt fractions, sustaining a steady-state global magma ocean in which melting curve variations do not significantly influence solidification timing. This indicates a hierarchical control: volatile inventory and surface oxygen fugacity act as the primary regulators of thermal state, while oxygen-fugacity-dependent melting relations provide a secondary modulation. These contrasting regimes produce distinct atmospheric compositions and formation timescales, offering testable spectral predictions for close-in rocky exoplanets evaluable with forthcoming JWST observations.

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