It Rains Diamonds on ‘Ice Giant’ Planets

A new experiment at SLAC National Accelerator Laboratory reveals how large diamonds may be formed from just hydrogen and carbon in the deep interior of ice giant planets such as Neptune and Uranus.

A cutaway depicts the interior of Neptune (left). In the new experiment, Kraus et al studied a plastic simulating compounds formed from methane — a molecule with just one carbon bound to four hydrogen atoms that causes the distinct blue cast of Neptune; methane forms hydrocarbon (hydrogen and carbon) chains that respond to high pressure and temperature to form ‘diamond rain’ in the interiors of ice giants like Neptune; the authors were able to recreate similar conditions using high-powered optical lasers and watch the small diamonds form in real time with X-rays. Image credit: Greg Stewart, SLAC National Accelerator Laboratory.

Dr. Dominik Kraus, a scientist at Helmholtz Zentrum Dresden-Rossendorf in Germany, and co-authors simulated the environment found around 6,200 miles (10,000?km) below the surfaces of Neptune and Uranus by creating shock waves in a plastic material (polystyrene) with an intense optical laser at the Matter in Extreme Conditions (MEC) instrument and SLAC National Accelerator Laboratory’s X-ray free-electron laser, the Linac Coherent Light Source (LCLS).

Polystyrene is made from a mixture of hydrogen and carbon, key components of ice giants’ overall chemical makeup. In the experiment, the team was able to see that nearly every carbon atom of polystyrene was incorporated into small diamond structures up to a few nanometers wide.

“In the intermediate layers of ice giants, methane forms hydrocarbon chains that were long hypothesized to respond to high pressure and temperature in deeper layers and form diamond rain,” Dr. Kraus explained.

“We used high-powered optical laser to create pairs of shock waves in the plastic with the correct combination of temperature and pressure.”

“The first shock is smaller and slower and overtaken by the stronger second shock. When the shock waves overlap, that’s the moment the pressure peaks and when most of the diamonds form.”

On Uranus and Neptune, the scientists predict that diamonds would become much larger, maybe millions of carats in weight.

They also think it’s possible that over thousands of years, the diamonds slowly sink through the planets’ ice layers and assemble into a thick layer around the core.

“We assume that the diamonds on Neptune and Uranus are much larger structures and likely sink down to the planet core over a period of thousands of years,” Dr. Kraus said.

“Our experiments are also providing us with better insights into the structure of exoplanets.”

“Previously, researchers could only assume that the diamonds had formed. When I saw the results of this latest experiment, it was one of the best moments of my scientific career,” he added.

Earlier experiments that attempted to recreate diamond rain in similar conditions were not able to capture measurements in real time, because we currently can create these extreme conditions under which tiny diamonds form only for very brief time in the laboratory.

The high-energy optical lasers at MEC combined with LCLS’s X-ray pulses — which last just femtoseconds — allowed the authors to directly measure the chemical reaction.

“For this experiment, we had LCLS, the brightest X-ray source in the world,” said team member Professor Siegfried Glenzer, of SLAC.

“You need these intense, fast pulses of X-rays to unambiguously see the structure of these diamonds, because they are only formed in the laboratory for such a very short time.”

The research appears today in the journal Nature Astronomy.

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D. Kraus et al. Formation of diamonds in laser-compressed hydrocarbons at planetary interior conditions. Nature Astronomy, published online August 21, 2017; doi: 10.1038/s41550-017-0219-9