The atmospheres of rocky exoplanets

II. Influence of surface composition on the diversity of cloud condensates

¹ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
² Centre for Exoplanet Science, University of St Andrews, North Haugh, St Andrews, KY169SS, UK
e-mail: oh35@st-andrews.ac.uk
³ SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, KY169SS, UK
⁴ School of Earth & Environmental Sciences, University of St Andrews, Irvine Building, St Andrews, KY16 9AL, UK
⁵ TU Graz, Fakultät für Mathematik, Physik und Geodäsie, Petersgasse 16, 8010 Graz, Austria

Received: 25 June 2021
Accepted: 26 November 2021

Abstract

Clouds are an integral part of planetary atmospheres, with most planets hosting clouds. Understanding not only the formation, but also the composition of clouds, is crucial to understand future observations. As observations of the planet’s surface will remain very difficult, it is essential to link the observable high atmosphere gas and cloud composition to the surface conditions. We present a fast and simple chemical equilibrium model for the troposphere of rocky exoplanets, which is in chemical and phase equilibrium with the crust. The hydrostatic equilibrium atmosphere is built from bottom to top. In each atmospheric layer, chemical equilibrium is solved and all thermally stable condensates are removed, depleting the atmosphere above in the effected elements. These removed condensates build an upper limit for cloud formation and can be separated into high and low temperature condensates. The most important cloud condensates for 1000 K ≳ T_gas ≳ 400 K are KCl[s], NaCl[s], FeS[s], FeS₂[s], FeO[s], Fe₂O₃[s], and Fe₃O₄[s]. For T_gas ≲ 400 K H₂O[l,s], C[s], NH₃[s], NH₄Cl[s], and NH₄SH[s] are thermally stable, while for even lower temperatures of T_gas ≲ 150 K CO₂[s], CH₄[s], NH₃[s], and H₂S[s] become stable. The inclusion of clouds with trace abundances results in the thermal stability of a total of 72 condensates for atmospheres with different surface conditions (300 K ≤ T_surf ≤ 1000 K and p_surf = 1 bar, 100 bar). The different cloud condensates are not independent of each other, but follow sequences of condensation, which are robust against changes in crust composition, surface pressure, and surface temperature. Independent of the existence of water as a crust condensate, H₂O[l,s] is a thermally stable cloud condensate for all investigated elemental abundances. However, the water cloud base depends on the hydration level of the crust. Therefore, the detection of water condensates alone does not necessarily imply stable water on the surface, even if the temperature could allow for water condensation.