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DIX Planetary Science Seminar

Tuesday, December 5, 2023
4:00pm to 5:00pm
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South Mudd 365
A window into sub-Neptune interiors: coupled chemistry and structure of hydrogen-silane-water atmospheres
William Misener, Graduate Student, Department of Earth, Planetary, and Space Sciences, UCLA,

Sub-Neptunes are among the most common classes of exoplanet discovered to date, but there is considerable uncertainty surrounding their interiors. One model consistent with observations is that these planets consist of silicate cores surrounded by hydrogen envelopes. At the conditions of the magma-atmosphere interface of sub-Neptune planets, substantial silicate vapor is expected to be in chemical equilibrium in the atmosphere. These species could greatly alter the atmospheric structure and evolution of these exoplanets, but previous models have neglected this compositional coupling. I present a coupled chemical equilibrium and atmospheric structure model, including silicate gas and its interactions with the background hydrogen. We find that silane, SiH4, and water, H2O, are the main products of atmosphere-interior interactions in sub-Neptune planets. These vapor products act as condensable species, decreasing in abundance with altitude. The resultant mean molecular weight gradient inhibits convection at temperatures above ∼2500 K, inducing a non-convective layer near the magma surface. This layer decreases the planet's radius compared to a planet with the same base temperature and a convective, pure H/He atmosphere. Therefore, we expect silicate vapor to have major effects on the inferred envelope mass fraction of sub-Neptune planets, and on their thermal and mass evolution. The presence of silicon species in the atmosphere may also be observable, allowing a window into the interiors of these planets.