Growth and Evolution of Secondary Volcanic Atmospheres: I. Identifying the Geological Character of Hot Rocky Planets

Abstract

AbstractThe geology of Earth and super‐Earth sized planets will, in many cases, only be observable via their atmospheres. Here, we investigate secondary volcanic atmospheres as a key base case of how atmospheres may reflect planetary geochemistry. We couple volcanic outgassing with atmospheric chemistry models to simulate the growth of C‐O‐H‐S‐N atmospheres in thermochemical equilibrium, focusing on what information about the planet's mantle fO2 and bulk silicate H/C ratio could be determined by atmospheric observation. 800 K volcanic atmospheres develop distinct compositional groups as the mantle fO2 is varied, which can be identified using sets of (often minor) indicator species: Class O, representing an oxidized mantle and containing SO2 and sulfur allotropes; Class I, formed by intermediate mantle fO2's and containing CO2, CH4, CO and COS; and Class R, produced by reduced mantles, containing H2, NH3 and CH4. These atmospheric classes are robust to a wide range of bulk silicate H/C ratios. However, the H/C ratio does affect the dominant atmospheric constituent, which can vary between H2, H2O and CO2 once the chemical composition has stabilized to a point where it no longer changes substantially with time. This final atmospheric state is dependent on the mantle fO2, the H/C ratio, and time since the onset of volcanism. The atmospheric classes we present are appropriate for the closed‐system growth of hot exoplanets, and may be used as a simple base for future research exploring the effects of other open‐system processes on secondary volcanic atmospheres.