On the possibility of phase separation in the H-He-H2O system in the ice giants

Abstract

Structure models of the interior of gas and ice giants are designed to fit available observational constraints: mass, radius, gravitational field, rotation rate and atmospheric temperature. The outcome of models consists of several layers of different composition - gas, ices and rock [1-4]. Inferring the interior structure requires phase diagrams and equations of state (EoS) of assumed material and mixtures. Discrete layers satisfy constraints on magnetic field, and a sharp boundary between different layers yields the simplest solution to the gravity field [5] and the luminosity of the planet [6] . A possible explanation for discrete layered structures is the immiscibility of major constituents. Studies on Jupiter and Saturn [7-12] suggest H-He phase diagrams agree in predicting demixing of H and He at temperatures of thousand K and pressures of a few Mbar. Bailey & Stevenson 2021 [5] explore possibility of H2-H2O immisicibility and suggest different states of H2-H2O demixing can account for the difference in heat fluxes between Uranus and Neptune. Current experimental data on H2-H2O shows demixing at pressure of a few GPa, where structure models require a change in composition from H-rich to ice-rich. In this work we investigate the possibility of phase separation in the H-He-H2O system in Uranus and Neptune.