Phase separation in hydrogen-helium mixtures at Mbar pressures

TL;DR

This study uses first-principles simulations to explore hydrogen-helium mixtures at Mbar pressures, revealing conditions under which helium becomes insoluble in dense hydrogen, impacting planetary interior models.

Contribution

It provides new first-principles calculations of the equation of state and helium solubility in hydrogen at extreme pressures relevant to giant planets.

Findings

Helium becomes insoluble in dense hydrogen at around 5 Mbar.

Helium is partially miscible in Saturn's interior.

Results impact models of Jovian planet evolution.

Abstract

The properties of hydrogen-helium mixtures at Mbar pressures and intermediate temperatures (4000 to 10000 K) are calculated with first-principles molecular dynamics simulations. We determine the equation of state as a function of density, temperature, and composition and, using thermodynamic integration, we estimate the Gibbs free energy of mixing, thereby determining the temperature, at a given pressure, when helium becomes insoluble in dense metallic hydrogen. These results are directly relevant to models of the interior structure and evolution of Jovian planets. We find that the temperatures for the demixing of helium and hydrogen are sufficiently high to cross the planetary adiabat of Saturn at pressures around 5 Mbar; helium is partially miscible throughout a significant portion of the interior of Saturn, and to a lesser extent in Jupiter.