Atmospheric compositional variations due to changes in mantle redox state

Abstract

Rocky exoplanets' internal constitution is inferred indirectly through their atmospheric composition. Confidence in this inference necessitates coupling interior and atmospheric models. In the past, various atmospheric redistribution models were developed to determine the composition of exoplanetary atmospheres by varying element abundance, temperature and pressure (Woitke et al., 2021).

However, these models neglect that present-day atmospheres were formed via volcanic degassing and, consequently, element abundances are limited by thermodynamic processes accompanying magma ascent and volatile release. Here we combine volcanic outgassing with an atmospheric chemistry model to simulate the evolution of C-H-O-N-S atmospheres in thermal equilibrium below 600 K. These volatiles can be stored in significant amounts in basaltic magmas and are the most commonly degassed species.

Our model calculates possible atmospheric compositions by varying oxygen fugacity, melt and surface temperature, and volatile abundances, considering phase solubility, atmospheric processes (e.g., water condensation, hydrogen escape), the change in redox conditions caused by volcanic activity and the influence of existing atmospheres on further degassing.

Our findings indicate that the prevailing atmospheric type below 600 K typically consists of CO2, N2, CH4, and, depending on temperature, H2O. Moreover, we illustrate that evolving atmospheric pressure and composition hinge significantly on the oxygen fugacity of the melt due to its impact on gas speciation and solubility. Reduced conditions yield atmospheres dominated by H2, NH3, CH4, and H2O, with exceedingly low atmospheric pressures. In contrast, oxidized conditions result in atmospheres comprising H2O, CO2, N2, and limited CH4, accompanied by high atmospheric pressures. Sulfur gases emerge predominantly at higher surface temperatures, manifesting as S2 or H2S under low mantle redox states and as SO2 under high mantle redox states. Notably, O2 is not generated abiotically, as sufficient carbon or hydrogen remains available to form H2O, CO, or CO2. Therefore, the formation of O2-dominated atmospheres would require excessive photodissociation of H2O or CO2 (Chang et al., 2021), a phenomenon likely common on planets orbiting M-dwarf stars.

In addition to highlighting the indirect inference of rocky exoplanets' internal constitution through their atmospheric composition, we demonstrate that reduced magmas can oxidize via H2 and CO degassing, whereas oxidized magmas may undergo reduction through SO2 degassing. Furthermore, we conclude that the depth of the magma source region and the planetary size significantly influence atmospheric compositions due to the varying pressure dependence of degassed species' solubilities.