H-He plasma mixtures in planetary interiors

Hydrogen and helium are the main components of giant planets like Jupiter or Saturn. Although the properties of pure hydrogen and helium are reasonably well understood nowadays, the behavior of the mixture at high pressure remains poorly known. However, this is of fundamental importance to understand the structure and the evolution of these giant planets. For instance, the scenarios of cooling for Saturn are not compatible with its age, unless a demixing process of H-He occurs in the interior. But the existence of such a phase separation still needs to be confirmed by numerical simulations and/or laser-driven experiments. We will present quantum molecular dynamics (QMD) simulations of hydrogen and helium mixtures. Our results explore a range of densities (from 0.1 to a few gcc) and temperatures (from 1 to a few tens kK) relevant for planetary interiors where the mixture is a partially pressure-ionized plasma. First, we will present the thermodynamic properties and secondly explore the transport properties (electrical and thermal conductivities and reflectivity) of H-He mixtures. In both cases we will identify the deviations from ideal mixing volume laws. Finally, we will discuss the existence of a phase separation in H-He mixtures and the observables that can be used to identify a demixing process in numerical simulations and in dynamic experiments.