Hydrogen-Helium Mixtures in the Interiors of Giant Planets

TL;DR

This study uses first-principles simulations to analyze hydrogen-helium mixtures under giant planet interior conditions, revealing helium's significant impact on molecular stability and deviations from linear mixing assumptions.

Contribution

It provides the first ab initio investigation of hydrogen-helium mixtures, assessing the validity of the linear mixing approximation under planetary interior conditions.

Findings

Helium stabilizes hydrogen molecules, shortening and strengthening bonds.

Deviations up to 8% from linear mixing in energy and volume during molecular dissociation.

Helium significantly influences the ionic and electronic structure of the mixture.

Abstract

Equilibrium properties of hydrogen-helium mixtures under conditions similar to the interior of giant gas planets are studied by means of first principle density functional molecular dynamics simulations. We investigate the molecular and atomic fluid phase of hydrogen with and without the presence of helium for densities between $\rho=0.19$ g$ $cm$^{-3}$ and $\rho=0.66$ g$ $cm$^{-3}$ and temperatures from $T=500 $K to $T=8000 {K}$. Helium has a crucial influence on the ionic and electronic structure of the liquid. Hydrogen molecule bonds are shortened as well as strengthened which leads to more stable hydrogen molecules compared to pure hydrogen for the same thermodynamic conditions. The {\it ab initio} treatment of the mixture enables us to investigate the validity of the widely used linear mixing approximation. We find deviations of up to 8% in energy and volume from linear mixing at constant pressure in the region of molecular dissociation.