Observations as a window to the diversity of rocky exoplanet surfaces

With the current and upcoming generations of space and ground based instruments the characterisation of atmospheres and surfaces of rocky exoplanets is going to be possible. Even though the interpretation will remain challenging and include many degeneracies, they will open a new window to the understanding of the diversity of rocky exoplanets throughout our galaxy. Our capabilities of constraining the surface conditions and compositions will depend on our understanding of their connected observables.

Based on atmospheric loss calculations, rocky planets on close in orbits are expected to be bare rocks, rather than remaining their atmosphere. While this is likely detrimental for the search for habitable planets around M dwarfs, it might allow direct constrains of their surface composition by investigations of emitted and reflected lights.

For planets, which retrain their atmosphere, the observation of the planetary surface is only possible through observations of their atmosphere. Modelling of the planetary atmospheres reveals the presence of distinctly different compositions. Besides atmospheres with H2 or O2 as characterising species, the atmospheric types include atmospheres with the coexistence of redox pairs (especially CH4 and CO2) in the atmosphere and ones with substantial SO2. These compositional differences can in principle also be observed spectroscopically. 

Being able to constrain the atmospheric type for a given exoplanet does not only provide information on the atmospheric composition, but also the mineralogy of the surface in chemical contact with this atmosphere. Some minerals (CaSO4, FeS, and others) are indicative of the surface in contact with a given atmospheric type. Therefore the atmosphere can be used as a window to the composition of surfaces of rocky exoplanets.