Thermodynamic modelling of the outgassing chemistry of Super Earths and sub-Neptunes: applications to secondary and hybrid atmospheres

Hydrogen- and helium-rich primordial atmospheres of small rocky planets, formed as a result of planetary accretion, are subject to subsequent modifications of geochemical outgassing. Two outcomes are possible: a secondary atmosphere forms if the outgassing completely replaces the primordial atmosphere, or a hybrid atmosphere results if the primordial atmosphere undergoes a partial loss with its leftover reacting with the newly outgassed species. We constructed a zero-dimensional thermodynamic model where both scenarios can be consistently simulated. The model assumes chemical equilibrium and admits input parameters of oxidation and sulfidation states of the mantle, melt temperature, atmospheric nitrogen content, surface pressure (for secondary atmosphere models), and hydrogen partial pressure (for hybrid atmosphere models). It computes the chemical compositions of outgassing, namely, the volume mixing ratios of various gaseous species. Non-ideal gas behaviors are accounted for in the model and the calculated secondary and hybrid atmospheres both exhibit a vast chemical diversity. For example, hydrogen-rich atmospheres, conventionally deemed of primordial origin, can also stem from interior outgassing. By Monte Carlo sampling in the possible ranges of the input parameters, we found that outgassed methane-dominated atmospheres, regardless of secondary or hybrid, require rather specific conditions: (1) a reduced rocky mantle; (2) relatively low melt temperatures in comparison to those of basaltic or peridotitic melts; (3) relatively high atmosphere pressures (> c.a. 10 bar) on the rocky surface. Moreover, we found that the abundance ratio of CO₂ and CO can serve as a powerful diagnostic of oxygen fugacity of rocky mantles, which could potentially be constrained by future James Webb Space Telescope spectra. The current model does not consider atmospheric escape, chemical kinetics or photochemistry, which awaits to be incorporated in future works.