Lab Recreates Ice-Giant 'Diamond Rain,' Ends 20-Year Melting-Point Mystery

Deep inside the ice giants Neptune and Uranus, scientists have long believed heat and pressure turn carbon into diamonds that rain toward the core. Physicists at Lawrence Livermore National Laboratory (LLNL) have now recreated that process for the first time. Their study, published in Nature Physics, resolves a 20-year-old mystery about diamond's melting point.

The team ran the experiment at the University of Rochester's Omega Laser Facility, vaporizing a diamond sample's outer layer with a laser. This sent a shock wave through the diamond, compressing it to pressures more than three times Earth's core, with temperatures matching the sun's surface, for a billionth of a second. Sensors tracked the process, including an ultrafast technique called X-ray diffraction.

Diamond had been melted before, but earlier measurements of its melting point disagreed with quantum-mechanics-based models by up to 20% — a gap of over 1,000 kelvins between predicted and observed values. New X-ray diffraction data showed the LLNL team's measured melting point matched modern models, closing that gap.

The experiment also showed that solid diamond is less dense than liquid metallic carbon — much as solid ice floats on water. This means diamonds can float on a sea of liquid carbon inside these planets, or melt and sink as diamond rain, whose friction and heat are believed to drive much of Neptune's excess radiated energy. The paper gives data on the pressures and temperatures this requires.

LLNL also runs the National Ignition Facility (NIF), where scientists use inertial confinement fusion — compressing fuel capsules with lasers to trigger fusion — pursuing a power-positive reaction. The diamond used resembled the fuel capsule "target" in NIF shots. The authors found a slower, gentler initial shock could still fully melt such a capsule, and calculate this could triple the fusion energy released by its fuel — though NIF would remain orders of magnitude short of "engineering breakeven," the threshold for use as a power plant.