Physicists at Lawrence Livermore National Laboratory have pinned down exactly how diamond behaves when crushed to pressures found deep inside ice giant planets, resolving a discrepancy that had puzzled the field for nearly 20 years. Using laser-driven shockwaves, the team compressed microscopic diamond samples past one terapascal — roughly three times the pressure at Earth's center — and watched, in real time, as the crystal structure held firm until it melted directly into liquid carbon, with no intermediate solid phase in between.
The results, published in Nature Physics, finally bring experimental melting temperatures into agreement with quantum-mechanical simulations, which had disagreed with lab measurements by roughly 20 percent — a gap traced to earlier temperature readings that were off by more than 1,000 degrees.
"We were able to take tiny diamond samples and shock compress them to temperatures hotter than the surface of the sun and to pressures higher than the center of Neptune and Uranus — and still measure atomic structure, temperature, density and optical reflectivity," said Marius Millot, the LLNL physicist who led the study, describing the shockwave technique used at the Omega Laser Facility at the University of Rochester.
The experiments also settled a question about density: liquid carbon turns out to be denser than solid diamond at these pressures. "Liquid water is denser than ice, which makes ice cubes float," Millot said. "Jon's finding means that diamond would float in liquid carbon at high pressures."
"While it was frustrating to discover that our original temperature measurements were off by more than 1,000 degrees, it is exciting to see such a dramatic improvement in data quality with our new diagnostics."
Jon Eggert, LLNL physicist
Fusion fuel and diamond rain
The findings have two practical targets. At the National Ignition Facility, fusion capsules are built from diamond shells compressed by lasers; understanding precisely when and how that diamond melts could let engineers use slower initial shocks that keep the fuel more compressible, a change the LLNL team estimates could triple energy gain in future fusion experiments.
The second target is hundreds of millions of miles away. Neptune and Uranus are thought to host "diamond rain" deep in their carbon-rich interiors, where extreme pressure squeezes carbon compounds into diamond crystals that sink toward the planets' cores. The new melting curve gives planetary scientists a firmer basis for modeling how that process unfolds and how it shaped the ice giants' interior structure over billions of years.
