2022-02-16

Atmospheres of sub-Neptune-sized exoplanets in contact with magma ocean

speaker: Chanoul Seo

Abstract:

Previous observational studies revealed the bimodal distribution of the radius for the planets with R<4R_⊕. The larger size group, sub-Neptune, is likely to have a substantial atmosphere under which the rocky interior may be molten (i.e., “magma”) depending on the temperature. As the rocky core still dominates the mass, direct contact between the magma and the atmosphere implies that magma affects the atmospheric composition significantly. Intending to understand the relation between the final atmospheric composition and the origin of volatile, we study the effect of magma on the volatile distribution in the sub-Neptune-sized planets. Our model is based on the model of a former study, but we introduce a more realistic behavior of the fluids, new solubility laws, and carbon-bearing species simultaneously. Our calculation shows a much more reduced atmosphere than the former study with the same amount of accreted H2 at higher pressure (>2GPa), mainly due to the newly introduced pressure dependence of H2 solubility. The C-bearing species are mainly present as methane in the atmosphere because the role of the magma on the distribution of C-bearing species is limited. We show how H2O and CH4 mixing ratios in the atmospheric composition as well as mass and radius can help us constrain the atmospheric origin and the initial magma composition. Within the current framework, the small H2O/H2 ratio (<1) and carbon depleted atmosphere cannot be attained simultaneously, implying that the sub-Neptunes like K2-18b which show small water fractions require a carbon-enriched atmosphere or additional carbon-erasing mechanism.