Redox evolution during magma degassing and its impact on planetary atmospheres

Abstract

Secondary atmospheres on rocky planets primarily form through volcanic degassing after the solidification of a global magma ocean. A key parameter controlling the nature of volcanic degassing and the resulting atmospheric composition is the oxygen fugacity (fO2) of the melt. Oxygen fugacity strongly influences both the solubility of volatile species and their speciation into reduced or oxidized gases, which in turn critically impacts the atmospheric pressure and composition.

In geodynamical modeling, it is common practice to assume a fixed oxygen fugacity for an entire planet and to keep it constant throughout the modeled time period. However, this is a significant simplification. The oxygen fugacity of a planet's interior and surface can evolve over time due to various processes, such as atmospheric escape and photochemical reactions.

Here, we focus on the effect of volatile degassing in the CHOS (carbon-hydrogen-oxygen-sulfur) system on the oxygen fugacity of ascending magma, and how accounting for this feedback modifies the composition of the gases released, compared to models that neglect it. We present a basic model that simulates melt generation in a planet’s mantle, incorporating initial oxygen fugacity, volatile content, pressure, and temperature as functions of mantle properties and melting depth. The model tracks melt ascent, bubble formation, and gas composition, taking into account volatile solubility and equilibrium gas-melt reactions. We further examine how equilibrium reactions within gas bubbles alter the melt’s oxygen fugacity by either consuming or releasing oxygen.

Building on the results presented in Brachmann et al. (2025), see https://doi.org/10.1016/j.icarus.2024.116450, we couple this melt degassing model with atmospheric evolution, including processes such as atmospheric chemistry, water condensation, and hydrogen escape, to study the long-term effects (up to 1 Gyr) of changing oxygen fugacity on planetary atmospheres.

Our results indicate that degassing of reduced species such as H2 and CO can oxidize the melt, while sulfur degassing as SO2 tends to reduce it. Consequently, the oxygen fugacity of the melt evolves significantly during degassing, depending on its volatile inventory. As shown in Figure 1, the redox state of the melt tends to converge towards more intermediate values after degassing, reducing the variation seen in the initial conditions.

When coupling this process with our planetary atmosphere model (Brachmann et al., 2025), we find that redox changes due to magma degassing can profoundly influence atmospheric composition, especially for planets with initially reduced mantles (IW to IW–6). Instead of maintaining reduced atmospheres dominated by species like NH3, CH4, and H2O, such planets may develop more oxidized atmospheres with higher CO2 and H2O abundances. Because CO2 and H2O have higher molecular weights and are more efficient greenhouse gases compared to their reduced counterparts, these changes could lead to significantly higher atmospheric pressures and surface temperatures, with major implications for planetary climate and habitability.

Caption Figure 1: change in oxygen fugacity during melt ascent and degassing depending on initial oxygen fugacity and water content. We tested 6 cases with 3 different initial oxygen fugacities (IW +0, IW + 2 and IW + 4) and varied water content (0.07 wt % and 0.4 wt%). Sulfur was set at 0.1 wt% and CO2 at 0.05 wt%.