Growth and Evolution of Secondary Volcanic Atmospheres: I. Identifying the Geological Character of Hot Rocky Planets
Philippa Liggins, Sean Jordan, Paul B. Rimmer, Oliver Shorttle
Abstract
Abstract The 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 f O 2 and bulk silicate H/C ratio could be determined by atmospheric observation. 800 K volcanic atmospheres develop distinct compositional groups as the mantle f O 2 is varied, which can be identified using sets of (often minor) indicator species: Class O, representing an oxidized mantle and containing SO 2 and sulfur allotropes; Class I, formed by intermediate mantle f O 2 's and containing CO 2 , CH 4 , CO and COS; and Class R, produced by reduced mantles, containing H 2 , NH 3 and CH 4 . 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 H 2 , H 2 O and CO 2 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 f O 2 , 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.