Miscibility calculations for water and hydrogen in giant planets

TL;DR

This study uses ab initio simulations to investigate water-hydrogen mixtures in giant planet interiors, finding they are fully miscible and likely to be homogeneously mixed, impacting planetary composition models.

Contribution

First ab initio simulation study of water-hydrogen mixtures across giant planet interior conditions, demonstrating complete miscibility and ideal mixing behavior.

Findings

Hydrogen and water are fully miscible in giant planet conditions.

Mixtures can be accurately described by an ideal mixing approximation.

Water-hydrogen phase separation is unlikely during planetary evolution.

Abstract

We present results from ab initio simulations of liquid water-hydrogen mixtures in the range from 2 to 70 GPa and from 1000 to 6000 K, covering conditions in the interiors of ice giant planets and parts of the outer envelope of gas giant planets. In addition to computing the pressure and the internal energy, we derive the Gibbs free energy by performing a thermodynamic integration. For all conditions under consideration, our simulations predict hydrogen and water to mix in all proportions. The thermodynamic behavior of the mixture can be well described with an ideal mixing approximation. We suggest a substantial fraction of water and hydrogen in giant planets may occur in homogeneously mixed form rather than in separate layers. The extend of mixing depends on the planet's interior dynamics and its conditions of formation, in particular on how much hydrogen was present when icy planetesimals were delivered. Based on our results, we do not predict water-hydrogen mixtures to phase separate during any stage of the evolution of giant planets. We also show that the hydrogen content of an exoplanet is much higher if the mixed interior is assumed.