By mass, the composition of giant gaseous planets such as Jupiter and Saturn consists of about 75–85% hydrogen and helium. In the 1970s, physicists first predicted that at the high temperatures and pressures inside gas giants, the phase separation of hydrogen and helium should form a region of immiscibility where the two elements fail to form a homogeneous mixture. The unmixed helium would then fall as rain toward the planet’s center.
Since then, the search to confirm the theoretical understanding of H–He immiscibility in planetary interiors has been hampered by unconstrained ab initio calculations and experimental difficulties. Fifteen years ago, several researchers—Stephanie Brygoo and Paul Loubeyre, working at the French Alternative Energies and Atomic Energy Commission; their US colleagues from Lawrence Livermore National Laboratory; and Raymond Jeanloz of the University of California, Berkeley—decided to join two types of high-pressure experiments. The researchers applied static and dynamic compression methods to H–He samples to reach the pressure–temperature phase space found in Jupiter’s interior.
To reach such high pressures and temperatures, the researchers first compressed samples of homogeneously mixed hydrogen and helium to 4 GPa in a diamond anvil cell at the University of Rochester’s Laboratory for Laser Energetics. They modified the apparatus—shown in the picture above—so that one diamond window was only 300 μm thick, far thinner than the typically millimeters-thick diamond anvil. The modifications prevented a laser-induced shock wave from deteriorating as it propagated through the diamond window to compress the sample further, to pressures 1 million to 4 million times that of Earth’s atmosphere.
During the experiment, Brygoo, Loubeyre, and their colleagues observed a sudden change in sample reflectivity. Ab initio calculations and theory predict that as the H–He mixture becomes conductive, insulating helium separates from metallic helium, which produces a reflectivity signature distinct from H2.
Provided that the helium distribution is constant, the researchers estimate that about 15% of Jupiter’s radius would form an immiscibility region. The laboratory results agree with recent spacecraft observations from the Galileo probe, which measured a depletion of helium in Jupiter’s atmosphere, and Juno’s data on the planet’s high-order gravitational moments—a measurement consistent with a H–He immiscibility region. (S. Brygoo et al., Nature 593, 517, 2021.)
Editor’s note, 15 July: The article was updated to mention that the experiment was performed at the University of Rochester’s Laboratory for Laser Energetics.
