Diamond's 20-Year Melting Mystery Solved at Neptune-Like Pressures

Physicists crushed diamond to pressures beyond Neptune's core and finally settled a two-decade dispute over how the hardest natural material melts.

The hardest natural material on Earth has been keeping a secret, and physicists just spent twenty years prying it loose.

Researchers at Lawrence Livermore National Laboratory fired shock waves through tiny diamond samples, crushing them to pressures three times greater than Earth's core — hotter than the surface of the sun and higher than the center of Neptune and Uranus. The result, published this week in Nature Physics, closes a mystery that has lingered since the mid-2000s.

Back then, LLNL scientist Jon Eggert noticed something counterintuitive: diamond became denser when it melted. Most materials do the opposite, expanding as they turn to liquid, so the observation clashed with computer models and triggered a long-running debate over how carbon behaves under planetary-scale compression.

Now Marius Millot and colleagues have measured the atomic structure, temperature, density and reflectivity of diamond mid-melt. The verdict: molten diamond forms a metallic liquid carbon so dense that solid diamond floats in it — like ice cubes bobbing in a glass of water. The experiments finally line up with quantum-mechanical predictions, resolving two separate discrepancies at once.

It is not just a physics puzzle. A clearer picture of diamond's melt behavior could help researchers triple the energy gain in inertial confinement fusion, where carbon capsules crush fuel to spark fusion. And because ice giants like Neptune and Uranus are thought to harbor diamond-rich interiors, the measurements sharpen models of how planets form and evolve.

Sources: Science Daily · Nature Physics