LavAtmos 2.0

Incorporating volatile species in vaporisation models

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Department of Earth Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
³ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands

^★ Corresponding author; vanbuchem@strw.leidenuniv.nl

Received: 4 June 2024
Accepted: 9 February 2025

Abstract

Context. Due to strong irradiation, hot rocky exoplanets are able to sustain lava oceans. Direct interaction between these oceans and overlying atmospheres can provide insight into planetary interiors. In order to fully understand how the composition of the atmosphere of such planets are affected by the properties of the oceans, comprehensive chemical equilibrium models are required. Thus far, most models have only taken non-volatile species into account when calculating lava vaporisation.

Aims. We investigate the effect of including C-, H-, N-, S-, and P-bearing species in the equilibrium lava vaporisation calculations on the overall atmospheric composition of hot rocky exoplanets by expanding our LavAtmos code.

Methods. We calculated the O₂ partial pressure which satisfies both the laws of mass action and mass conservation in a system that contains both melt species and volatile elements. We integrated the chemical equilibrium code FastChem to expand the number of considered gas phase species to 523. We applied this new approach to calculate the composition of “pure” atmospheres which contain only C, H, N, S, or P and of more complex atmospheres which contain all five aforementioned elements. We also tested two proposed compositions for the atmosphere of 55-Cnc e.

Results. We find that the inclusion of volatile elements in vaporisation calculations increases the partial pressures of vaporised species (SiO, SiO₂, Na, and K) compared to volatile-free vaporisation for all tested atmospheric compositions. Moreover, including volatile species in the vaporisation reactions leads to a significantly greater O abundance in the atmosphere than in the volatile-free vaporisation case, which influences partial pressures of key volatile species such as CO₂ and H₂O. When testing the compositions proposed for 55-Cnc e, we find that a low C/O ratio could potentially serve as an indication of the presence of a surface lava ocean on an ultra-shortperiod planet with a volatile atmosphere.

Conclusions. Volatile elements must be taken into account for comprehensive modelling of vaporisation from a surface lava ocean into a volatile atmosphere.