They Made Diamond Rain in a California Lab

On August 21, 2017, a team led by Dominik Kraus at the Helmholtz-Zentrum Dresden-Rossendorf published a paper in Nature Astronomy describing what looked, briefly, like Neptune. They had taken a sliver of polystyrene — the same plastic that makes packing peanuts — and hit it with the optical laser at SLAC's Matter in Extreme Conditions instrument. The shock front compressed the sample to about 150 gigapascals at roughly 5,000 kelvin, the conditions theorized to exist about 10,000 km below Neptune's cloud tops.

While the sample was at pressure, the team pulsed it with X-rays from the Linac Coherent Light Source. The diffraction pattern that came back was the signature of diamond. Almost every carbon atom in the polystyrene had snapped into a tiny diamond crystal a few nanometers across. The window for the measurement was a fraction of a billionth of a second.

Why this matters for the ice giants: Uranus and Neptune are mostly hydrogen, helium, water, ammonia and methane. Methane is CH4. At the right depth — high pressure, high temperature — the carbon and hydrogen separate. The carbon then crystallizes into diamond and sinks. Models predicted this for thirty years; nobody had seen the carbon-on-carbon step happen in a lab until 2017.

The predicted natural diamonds aren't nano-scale. Inside a planet, with millions of years to grow and a constant feed of methane, individual stones may be millions of carats. They sink toward the rocky core, releasing gravitational heat as they fall — possibly enough to explain why Neptune still radiates more heat than it gets from the Sun.