Pressure-Dependent Mantle Outgassing and the Evolution of Venus’ atmosphere​​

Venus and Earth exemplify two divergent evolutionary pathways of rocky planets despite their similar sizes, bulk densities and orbital distances. Today, Earth maintains a thin N2- and O2-dominated atmosphere, regulated by efficient volatile and crustal recycling, while Venus hosts a CO2-dominated, 92 bar atmosphere that results in extreme surface temperatures. The origin of Venus’ massive atmosphere is likely a combination of processes including early magma ocean degassing, impact delivery and long-term mantle outgassing, with possibly one or more catastrophic outgassing events (Gillmann et al., 2022). The efficiency and composition of long-term mantle outgassing is still poorly constrained for Venus. Previous studies have shown that pressure is the primary control on volatile solubility, regulating whether degassing occurs at all and the elemental sequence of volatility e.g. C-bearing species degass predominantely at higher pressures, while the redox state of the mantle controls primary the chemical speciation of outgassed volatiles e.g. wheather CH4 or CO or CO2 is degassed (Gaillard and Scaillet, 2014, Gaillard et al. 2020).   In contrast to Ortenzi et al. (2020), who have focused on the redox-dependence of volatile partitioning and gas speciation for various planetary masses, this study investigates the pressure-dependence of volcanic outgassing for varying redox states and its implications for Venus’ atmospheric evolution. First, Atmodeller (Bower et al., 2025) is used to calculate the pressure-dependent volatile solubility and gas speciation for erupted lava. Then, the findings are linked to 2D mantle convection models of Venus with varying surface temperatures and rheological parameters. These models use the numerical mantle convection code StagYY (Tackley, 2008) including partial melting with prescribed intrusion efficiencies to quantify melt production, lithospheric thickness as well as the tectonic regime. By explicitly accounting for volatile degassing as a function of surface pressure, our study aims to constrain total outgassing and the role of pressure-buffered volatile retention in shaping Venus’ atmosphere.