Lab experiment yields evidence of superionic ice

Scientists at Lawrence Livermore National Laboratory have found experimental evidence of superionic ice -- a first.

Various models and numerical simulations have long suggested that water ice assumes unusual behavior at extremely high pressures. The numbers suggest water under extreme-pressure features liquid-like hydrogen ions confined by a solid lattice of oxygen.

Until now, however, scientists have been able to identify experimental evidence of superionic ice.

Scientists began by using shock compression to melt ice under extremely high pressure. Their efforts proved ice melts at around 5,000 degrees Kelvin at 200 gigapascals of pressure. The experiment showed ice can indeed remain in a uniquely solid state at high temperatures when subjected to extreme pressure.

"Our work provides experimental evidence for superionic ice and shows that these predictions were not due to artifacts in the simulations, but actually captured the extraordinary behavior of water at those conditions," Marius Millot, a physicist at LLNL, said in a news release.

In followup experiments, scientists used diamond anvil cells to pre-compress water ice at room temperature. Then, researchers used laser-driven shock compression to further compress and heat the ice.

"Because we pre-compressed the water, there is less shock-heating than if we shock-compressed ambient liquid water, allowing us to access much colder states at high pressure than in previous shock compression studies, so that we could reach the predicted stability domain of superionic ice," Millot said.

Advanced imaging technology allowed the researchers to measure the optical properties of the highly pressurized ice. The optical properties revealed the water ice's unique thermodynamic properties before the sample was vaporized.

"These are very challenging experiments, so it was really exciting to see that we could learn so much from the data -- especially since we spent about two years making the measurements and two more years developing the methods to analyze the data," Millot said.

Researchers believe their efforts -- detailed this week in the journal Nature Physics -- could help planetary scientists better understand the composition of planets like Neptune and Uranus.

Astronomers have hypothesized Neptune and Uranus feature a mantle made of superionic ice. A superionic core could explain the unique magnetic fields found surrounding the planets.

"Magnetic fields provide crucial information about the interiors and evolution of planets, so it is gratifying that our experiments can test -- and in fact, support -- the thin-dynamo idea that had been proposed for explaining the truly strange magnetic fields of Uranus and Neptune," said Raymond Jeanloz, a professor at the University of California, Berkeley. "It's also mind-boggling that frozen water ice is present at thousands of degrees inside these planets, but that's what the experiments show."

While the recent experiments have moved scientists closer to an understanding of the liquid-like hydrogen ions found in superionic ice, the researchers hope follow-up experiments will provide a more accurate model of the crystalline structure of the oxygen lattice.

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