The atmospheres of rocky exoplanets

III. Using atmospheric spectra to constrain surface rock composition

Institute for Astronomy (IfA), University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria

^★ Corresponding author: oliver.herbort@univie.ac.at

Received: 19 November 2024
Accepted: 12 May 2025

Abstract

Context. The crust composition of rocky exoplanets with substantial atmospheres cannot be observed directly. However, recent developments have enabled novel observations and characterisations of their atmospheres.

Aims. We aim to establish a link between observable spectroscopic atmospheric features and the mineralogical crust composition of exoplanets. This enables us to constrain the surface composition simply by observing the transit spectra.

Methods. We used a diverse set of total element abundances inspired by various rock compositions, Earth, Venus, and CI chondrite as a basis for our bottom-to-top atmospheric model. We assumed thermal and chemical equilibrium between the atmosphere and the planetary surface. Based on the atmospheric models in hydrostatic and chemical equilibrium, with the inclusion of element depletion due to cloud formation, we calculated the theoretical transit spectra.

Results. The atmospheric type classification allows us to constrain the surface mineralogy, especially with respect to sulphur compounds, iron oxides and hydroxides, feldspars, silicates, and carbon species. Spectral features offer an opportunity to differentiate among the atmospheric types, allowing for a number of constraints to be placed on the surface composition.