The formation of secondary exoplanet atmospheres via volcanic degassing in the C-H-O-N system

Kurzfassung

The internal constitution of rocky exoplanets can be inferred only indirectly via their atmospheric composition. To address this issue with confidence requires the coupling of interior and atmospheric models to each other. 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 1000 K. These volatiles can be stored in significant amounts in basaltic magmas and are the most commonly degassed species. For the present study, we built a basic model to calculate possible atmospheric compositions by varying oxygen fugacity, melt and surface temperature and volatile abundances. Furthermore, we consider the solubility of each phase, atmospheric processes such as water condensation, graphite and sulfate precipitation, hydrogen escape and the effect an already existing atmosphere may have on further degassing. Our model suggests that the most common atmospheric type below 600 K is composed of CO2, N2, CH4 and (dependent on temperature) H2O. Furthermore, we show that the evolving atmospheric pressure and composition are highly dependent on the oxygen fugacity of the melt because of its influence on gas speciation and solubility. Reduced conditions produce H2, NH3, CH4 and H2O dominated atmospheres with extremely low atmospheric pressures. Oxidized conditions lead to atmospheres consisting of H2O, CO2, N2 and small amount of CH4 with high atmospheric pressures. O2 is never produced since carbon or hydrogen are still available in sufficient amounts to form H2O, CO or CO2. Hence it is not possible to form abiotically O2 dominated atmospheres unless O2 is formed via excessive photodissociation of H2O or CO2 (Chang et al., 2021) which is likely to be common on planets orbiting around M-dwarf stars. Furthermore we conclude that also the depth of the source region of a magma as well as the size of the planet may have a significant effect on atmospheric compositions because of the different pressure dependence of the solubilities of the degassed species.