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Experimental and Numerical Constraints on the Masses and Compositions of Rocky Exoplanet Atmospheres

Presentation #203.06 in the session Star-Planet Interactions, Ultra-Hot Worlds.

Published onApr 03, 2024
Experimental and Numerical Constraints on the Masses and Compositions of Rocky Exoplanet Atmospheres

Magma worlds, due to their hot, extended atmospheres are readily characterized spectroscopically by ground- and space-based telescopes, such as JWST. As yet, the lack of direct observations means that the nature and composition of these planets’ atmospheres are poorly constrained. Because the atmospheres of these planets are thought to be the result of chemical equilibrium with their interiors, their mass and composition are modulated by the solubilities of major gases in the magma. Therefore, we require a theoretical framework, informed by experimental data, to determine how volatile elements partition between the interior and atmosphere for diverse planetary compositions. However, there is limited experimental data on the solubilities of major atmosphere-forming gases in a range of silicate liquids relevant for exoplanets, which may be unlike that of the present-day Earth. To fill this gap, we performed new volatile (e.g., H, C, O) solubility experiments on exoplanet melt analog materials at high temperatures (≥ 1400 °C) using a 1-bar H2-CO2 gas-mixing furnace and an aerodynamic laser levitation furnace coupled to an FTIR spectrometer. We incorporate these experimental results into a new Python package (atmodeller), which computes chemical equilibrium at the melt-atmosphere interface of low-mass planets, from rocky worlds to sub-Neptunes. Given a set of planetary parameters (e.g., surface temperature, planetary mass, radius, mantle melt fraction) and initial volatile inventory, atmodeller uses experimentally calibrated solubility laws (including those determined from our experiments), together with free energy data for condensed and gas species, to determine how volatiles partition between the atmosphere and interior. Within the H-He-C-N-O-S-Cl system, we investigate a range of plausible atmospheric compositions and the impact of volatile dissolution into the interior for a set of known rocky exoplanets (e.g., the Trappist-1 system, 55 Cancri e) given current observational constraints from JWST. We also apply atmodeller to understand super-Earth and sub-Neptune atmospheres, simulating the effects of both volatile solubilities and non-ideal gas conditions for exoplanets like K2-18 b.

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