Inhibiting Conduction by He Mixing in Interiors of Jupiter and Saturn

TL;DR

This study introduces a new method to accurately determine the immiscibility boundary in hydrogen-helium mixtures under planetary interior conditions, revealing significant effects on electrical conductivity and planetary evolution models.

Contribution

The paper presents a novel approach to identify the immiscibility boundary without free energy calculations, supported by large-scale ab initio simulations of hydrogen-helium mixtures.

Findings

IMT of hydrogen is delayed by helium admixture at high temperatures.

Mixtures remain insulating even after hydrogen dissociation below 150 GPa.

Electrical and thermal conductivities are significantly reduced in mixtures.

Abstract

Accurate knowledge of the electrical and thermal conductivities and structural properties of hydrogen-helium mixtures under thermodynamic conditions within and beyond the immiscibility range is very important to predict the thermal evolution and internal structure of gas giant planets like Jupiter and Saturn. Here, we propose a novel method to determine the immiscibility boundary accurately without the need for free energy calculations, while providing consistent insights into structural and transport properties of mixtures. We show with direct large-scale ab initio simulations that the insulator-metal transition (IMT) of the hydrogen subsystem is strongly affected by an admixture with a small fraction of helium and occurs at temperatures significantly higher than those of pure hydrogen. At pressures below 150 GPa, the IMT boundary is not related anymore to the H2 subsystem dissociation, the system remains insulating even after the full dissociation of H2 molecules and its transition to an H-He mixture. The offset of the IMT in the H-He mixture relative to the dissociation region in the hydrogen subsystem and the significant reduction of static electrical and thermal conductivity by a factor between two and a few thousand relative to pure hydrogen found in mixtures have consequences for Jupiter and Saturn's thermal evolution, internal structure, and dynamo action, affecting a large fraction of the interior of both planets.