Evidence of hydrogen-helium immiscibility at Jupiter-interior conditions

The phase behaviour of warm dense hydrogen-helium (H-He) mixtures affects our understanding of the evolution of Jupiter and Saturn and their interior structures^1,2. For example, precipitation of He from a H-He atmosphere at about 1-10 megabar and a few thousand kelvin has been invoked to explain both the excess luminosity of Saturn^1,3, and the depletion of He and neon (Ne) in Jupiter's atmosphere as observed by the Galileo probe^4,5. But despite its importance, H-He phase behaviour under relevant planetary conditions remains poorly constrained because it is challenging to determine computationally and because the extremes of temperature and pressure are difficult to reach experimentally. Here we report that appropriate temperatures and pressures can be reached through laser-driven shock compression of H₂-He samples that have been pre-compressed in diamond-anvil cells. This allows us to probe the properties of H-He mixtures under Jovian interior conditions, revealing a region of immiscibility along the Hugoniot. A clear discontinuous change in sample reflectivity indicates that this region ends above 150 gigapascals at 10,200 kelvin and that a more subtle reflectivity change occurs above 93 gigapascals at 4,700 kelvin. Considering pressure-temperature profiles for Jupiter, these experimental immiscibility constraints for a near-protosolar mixture suggest that H-He phase separation affects a large fraction-we estimate about 15 per cent of the radius-of Jupiter's interior. This finding provides microphysical support for Jupiter models that invoke a layered interior to explain Juno and Galileo spacecraft observations^1,4,6-8.