Using atmospheric types of rocky exoplanets to constrain planetary surfaces

Rocky exoplanets are expected to show a large diversity of planetary interiors, surface conditions, and atmospheric compositions. While all of these are connected through geological processes, direct observations are challenging even with current and upcoming ground and space based instruments, showing the importance of models combining planetary atmospheres, surfaces, and interiors.

Our modelling approach for rocky exoplanets provides a bottom-to-top surface-atmosphere model. The crust-atmosphere interaction layer is the basis of the 1D atmospheric model which includes the effects of element depletion due to cloud formation. The atmospheric and cloud composition are therefore a result of the surface composition and the pressure-temperature structure.

Modelling the variety of different gas-phase compositions reveals the existence of three distinct atmospheric types (H₂ rich, O₂ rich, or the coexistence of CH₄ and CO₂), defined by their atmospheric composition. In addition to these distinct atmospheric types, the presence of some cloud condensates provides constraints on the planetary surface temperature and pressure. Furthermore, the composition of the planetary surface mineralogy in contact with the atmosphere provides distinct transitions linked to the atmospheric types.

Investigations of various elemental compositions based on different rock compositions reveal links of surface minerals to the atmospheric types. In particular, the sulphur chemistry can be constrained. While the sulphur cloud condensates of H₂S and H₂SO₄ only form for planets with high surface pressures and/or temperatures, the sulphur-bearing condensates at the planetary surface (including especially FeS, FeS₂, and CaSO₄) are directly linked to the atmospheric type. 

This work shows that in principle, spectroscopic investigations of rocky exoplanet atmospheres can constrain the atmospheric composition to a specific atmospheric type and therefore put some constraints on the expected surface mineralogy.