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

Some fundamental properties of the interiors of the Ice giants Uranus and Neptune are far from being understood. According to structure models which follow observed gravitational harmonics J2 and J4, the interiors are composed of a H/He-rich envelope transitioning into an ice-rock interior. Formation theories can explain their current structure given certain conditions related to the formation location within the protoplanetary disk nebula and the degree of gas depletion during said formation are met, conditions which themselves seek an explanation [1]. A rather sharp boundary yields the simplest solution to the gravity field [2] and the luminosity of the planets [3]. Recently, the phase separation of two major constituents (water and molecular hydrogen) in the evolved and cooled planets Uranus and Neptune has been proposed as an explanation for the presence of this sharp interface. Furthermore, different H2O/H2 demixing states may offer an explanation for the paradox between intrinsic heat flux of both planets [2]. On the other hand, evolution models guided by the observed luminosities suggest the existence of a compositional barrier inhibiting or slowing down convection. Assuming small differences in the structure of both planets, this mechanism can account for the faintness of Uranus and the brightness of Neptune [3]. However, their evolution could be affected by possible complementary processes such as phase separation [2] or condensation in their atmospheres. In this work, we follow up on the possibility of demixing between the major constituents in the H-He-H2O system. First, we show that interior models which adjust to the luminosity [3] lead to temperatures above the critical temperature for H2O/H2 demixing to occur, and thus would predict a protosolar atmospheric helium abundance. Second, we adopt the assumption of an initially more homogeneous interior which did cool sufficiently to allow for H2O/H2 demixing [2]. We find that for deep interior H/He phase separation, which occurs at higher temperatures, favourable interior conditions were met much earlier in the evolution [4]

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

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