Abstract DGP2026-34

Secondary atmospheres on terrestrial planets are thought to emerge mainly from volcanic outgassing following the crystallization of a global magma ocean. One of the central factors governing volcanic degassing behavior and the resulting atmospheric makeup is the oxygen fugacity (fO₂) of the melt. Oxygen fugacity controls both the solubility of volatile elements in silicate melts and their partitioning into reduced versus oxidized gaseous species, thereby exerting a strong influence on atmospheric composition and surface pressure.

In many geodynamical and atmospheric evolution models, oxygen fugacity is treated as a fixed parameter that is uniform across the planet and constant over time. While convenient, this assumption neglects a range of processes that can modify redox conditions in a planet’s interior and at its surface. These include interactions between the atmosphere and the interior, photochemical reactions, and the loss of atmospheric species to space, all of which can drive temporal changes in fO₂.

In this study, we investigate how volatile release in the CHOS (carbon–hydrogen–oxygen–sulfur) system affects the oxygen fugacity of rising magmas, and how this redox feedback alters the composition of volcanic gases relative to models that ignore it. We introduce a simplified model of mantle melting that accounts for initial redox state, volatile abundances, pressure, and temperature, as determined by mantle characteristics and melting depth. The model follows melt ascent toward the surface, simulating volatile exsolution, bubble formation, and gas–melt equilibrium while explicitly tracking volatile solubility and redox-sensitive reactions. We additionally explore how chemical equilibria within gas bubbles can modify melt fO₂ through oxygen consumption or release.

Extending the framework of Brachmann et al. (2025), we couple our degassing model to a time-dependent atmospheric evolution model that includes atmospheric chemistry, condensation of water, and hydrogen escape. This combined approach allows us to explore the long-term (up to 1 Gyr) consequences of redox evolution driven by magma degassing on planetary atmospheres.

Our results show that the release of reduced gases such as H₂ and CO generally leads to oxidation of the melt, whereas sulfur degassing in the form of SO₂ can either oxidize or reduce the melt depending on its initial oxygen fugacity. As a result, the redox state of the magma can change substantially during degassing, with the magnitude and direction of this evolution controlled by the volatile inventory and the iron content of the planet's mantle and hence its melt.

When integrated with atmospheric evolution (Brachmann et al., 2025), these redox feedbacks are found to significantly reshape atmospheric compositions, particularly for planets starting with highly reduced mantles (IW to IW–6). Rather than retaining strongly reduced atmospheres dominated by species such as NH₃, CH₄, and H₂O, these planets may transition toward more oxidized atmospheres enriched in CO₂ and H₂O. Owing to their higher molecular weights and stronger greenhouse effect, such atmospheres are likely to produce higher surface pressures and temperatures, with important consequences for planetary climate and potential habitability.