Atmospheres of rocky exoplanets

Abstract

The increasing number of known rocky exoplanets motivates investigations of the diversity of atmospheric and surface composition of these planets. We investigate the link between the composition of the surface, near-crust atmosphere and the lower atmosphere, including the presence of different cloud condensates. This allows working towards inferring the surface composition from clouds and gas species present in the atmosphere. Understanding the diversity of the atmospheric composition provides a further step towards the characterisation of rocky exoplanets. In this thesis, a fast and simple atmospheric model for the lower atmospheres of rocky exoplanets is presented. A range of different sets of total element abundances is used to investigate the surface composition in contact with the near-crust atmosphere in chemical and phase equilibrium. The atmosphere based on this crust-atmosphere interaction layer is build from bottom-to-top. At every point in the atmosphere, chemical equilibrium is solved and all thermally stable condensates are removed, depleting the atmospheric layers above in the affected elements. In order to characterise the general atmospheric composition, atmospheric types based on the chemical state of carbon, hydrogen, oxygen, and nitrogen are introduced. In order to further constrain the potential of an atmospheric environment for habitability, different habitability levels are introduced. These take the stability of liquid water as well as the chemical states of carbon, nitrogen, and sulphur into account. The investigation of the atmosphere-crust interaction layer shows, that the thermal stability of liquid water is only given, if all phyllosilicates (minerals which incorporate OH groups into their lattice structure) have completely formed. The composition of the resulting atmosphere can be categorised into three different atmospheric types. Of special interest is the possibility of the coexistence of CO₂ and CH₄ in chemical equilibrium. The atmospheric type is intrinsic to an atmosphere, as it does not change with the removal of thermally stable condensates in one given atmospheric model. The atmospheric models reveal a large diversity in thermally stable cloud condensates, which constrain the surface conditions of rocky exoplanets. The presence of water clouds is an integral part of many planetary atmospheres and is independent of the stability of water condensates at the surface. At the water cloud base, we show that reduced gaseous forms of carbon, nitrogen, and sulphur are present, while phosphorus is absent.