Scientists create 'diamond rain' that forms in the interior of icy giant planets like Neptune

A cutaway shows the interior of Neptune (Download Image)

A cutaway shows the interior of Neptune (left). In an experiment conducted at the Linac Coherent Light Source, the team studied 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 icy giant planets like Neptune. The scientists recreated similar conditions using high-powered optical lasers and watched the small diamonds form in real-time with X-rays. Image by Greg Stewart/SLAC National Accelerator Laboratory.

In an experiment designed to mimic the conditions deep inside the icy giant planets of our solar system, scientists observed "diamond rain" for the first time as it formed in high-pressure conditions. Extremely high pressure squeezes hydrogen and carbon found in the interior of these planets to form solid diamonds that sink slowly down further into the interior.

The glittering precipitation has long been hypothesized to arise more than 5,000 miles below the surface of Uranus and Neptune, created from commonly found mixtures of just hydrogen and carbon. The interiors of these planets are similar — both contain solid cores surrounded by a dense slush of different ices. With the icy planets in our solar system, "ice" refers to hydrogen molecules connected to lighter elements, such as carbon, oxygen and/or nitrogen.

The findings were published Monday in Nature Astronomy by a team of collaborators from Lawrence Livermore National Laboratory (LLNL), Helmholtz-Zentrum Dresden-Rossendorf in Germany, University of California-Berkeley, Lawrence Berkeley National Laboratory, GSI Helmholtz Centre for Heavy Ion Research in Germany, Osaka University in Japan, Technical University of Darmstadt in Germany, European XFEL, University of Michigan, University of Warwick in the United Kingdom and SLAC National Accelerator Laboratory.

Researchers simulated the environment found inside these planets by creating shock waves in plastic with an intense optical laser at the Matter in Extreme Conditions (MEC) instrument at SLAC’s X-ray free-electron laser, the Linac Coherent Light Source.

In the experiment, they could see that nearly every carbon atom of the original plastic was incorporated into "nanodiamonds" — small diamond structures up to a few nanometers wide. On Uranus and Neptune, the research team predicts that diamonds would become much larger, maybe millions of carats in weight. Researchers 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.

"Previously, researchers could only assume that the diamonds had formed," said Dominik Kraus, lead author on the publication, currently at Helmholtz Zentrum Dresden-Rossendorf. "When I saw the results of this latest experiment, it was one of the best moments of my scientific career." The idea for this experiment was born within LLNL’s NIF & Photon Science Directorate, where Kraus was stationed as a University of California-Berkeley postdoc.

Improving our understanding of how and when mixtures of carbon and hydrogen separate under extreme conditions also is relevant for inertial confinement fusion (ICF) experiments.

"It is important to mitigate mass density fluctuations as a consequence of species separation because they could be seeds for hydrodynamic instabilities," said  Tilo Doeppner, coauthor on the paper and LLNL’s experimental lead for ICF implosions using a CH (plastic) ablator. "In current ICF implosions the first shock drives plastic at higher pressures to significantly higher temperatures compared to this recent LCLS work, to conditions where we are confident that species separation does not occur."

Prior static compression experiments also saw hints of carbon forming graphite or diamond at lower pressures than the ones created in this experiment, but with other materials introduced and altering the reactions. The combination of high-energy optical lasers at MEC with LCLS’s bright X-ray pulses allowed the scientists for the first time to directly measure the species separation at ultra-fast time scales and free from the impact of materials that hold the sample.

The results presented in this experiment are the first unambiguous observation of high-pressure diamond formation from mixtures and agree with theoretical predictions about the conditions under which such precipitation can form and will provide scientists with better information to describe and classify other worlds. The researchers also plan to apply the same methods to look at other processes that occur in the interiors of planets.

In addition to the insights they give into planetary science, nanodiamonds made on Earth could potentially be harvested for commercial purposes – uses that span medicine, scientific equipment and electronics. Currently, nanodiamonds are commercially produced from explosives; laser production may offer a cleaner and more easily controlled method.

LLNL scientists Arthur Park, Alison Saunders, Andy MacKinnon and Simon Frydrych also contributed to the research.

The research was supported by the Department of Energy’s Office of Science and the National Nuclear Security Administration.