Habitability constraints by nutrient availability in atmospheres of rocky exoplanets

Abstract

The diversity of exoplanets provides a vast range of potential environments in which liquid water can exist: atmospheres, surfaces, sub-surfaces. Therefore, many environments provide the most fundamental requirement for life as we know it from Earth and could in principle be habitable. However, the presence of water is not the only necessity for life to form, especially the presence of nutrients (C, H, N, O, P, and S) is crucial for the formation of life as we know it. In order to further understand and constrain potentially habitable environments of diverse planets, we introduce the concept of nutrient availability to constrain the habitability of a planet. This framework is based on the concentrations of nutrient bearing molecules in the condensate and gas phase in the presence of liquid water. Applying this concept to a diverse set of atmospheres  allows to provide constraints on the potential of surface and aerial biospheres. The atmospheric model used is a bottom-to-top equilibrium chemistry model, wich includes cloud formation. In order to cover the range of different atmospheric compositions, we investigate various different sets of element abundances and surface conditions. We find that reduced forms of C, N, and S are commonly found at the water cloud base for a range of different compositions of the planetary surface and atmosphere - even in overall oxidised atmospheres. In our model atmospheres, the only non-CHNOS elements in the atmosphere in the surrounding of liquid water clouds are F and Cl, which are present in the form of HF and HCl. Although the CHNOS elements are present, the absence of P and metals in the atmospheres could be a limiting factor on the formation and evolution of life in aerial biospheres.