Abstract
Abstract We demonstrate that the deep volatile storage capacity of magma oceans has significant implications for the bulk composition, interior, and climate state inferred from exoplanet mass and radius data. Experimental petrology provides the fundamental properties of the ability of water and melt to mix. So far, these data have been largely neglected for exoplanet mass–radius modeling. Here we present an advanced interior model for water-rich rocky exoplanets. The new model allows us to test the effects of rock melting and the redistribution of water between magma ocean and atmosphere on calculated planet radii. Models with and without rock melting and water partitioning lead to deviations in planet radius of up to 16% for a fixed bulk composition and planet mass. This is within the current accuracy limits for individual systems and statistically testable on a population level. Unrecognized mantle melting and volatile redistribution in retrievals may thus underestimate the inferred planetary bulk water content by up to 1 order of magnitude.
Highlights
Many exoplanets discovered to date likely host globally molten mantles – magma oceans – because the distance to their stars or their thick atmospheres prohibit efficient cooling and solidification of their rocky mantles (Massol et al 2016; Grenfell et al 2020)
We explore the effect of water in the model scenarios A, B, and C on exoplanet transit radii
The majority of observed super-Earths and sub-Neptunes are subject to irradiation that promotes molten rocky interiors, i.e. magma oceans, either due to direct surface melting or greenhouse forcing from water or other volatiles
Summary
Many exoplanets discovered to date likely host globally molten mantles – magma oceans – because the distance to their stars or their thick atmospheres prohibit efficient cooling and solidification of their rocky mantles (Massol et al 2016; Grenfell et al 2020). We investigate the effect of water in molten rocky mantles on the total radius of the planet and the total volatile abundance that can be inferred from exoplanet observations. The volatile abundance of rocky planets alters their structural and dynamical properties (Dorn et al 2018), such as differentiation between core and mantle (Bonati et al 2021), geodynamic regime (Meier et al 2021), atmospheric com-
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