Researchers Demonstrate First Experimental Evidence for Superionic Ice

Using diamond anvil cells (DAC), the team applied 2.5 GPa of pressure (25 thousand atmospheres) to pre-compress water into the room-temperature ice VII, a cubic crystalline form that is different from “ice-cube” hexagonal ice, in addition to being 60 percent denser than water at ambient pressure and temperature. They then shifted to the University of Rochester’s Laboratory for Laser Energetics (LLE) to perform laser-driven shock compression of the pre-compressed cells. They focused up to six intense beams of LLE’s Omega-60 laser delivering a 1 nanosecond pulse of UV light onto one of the diamonds. This launched strong shock waves of several hundred GPa into the sample, to compress and heat the water ice at the same time.

“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. 

This work also has important implications for planetary science because Uranus and Neptune might contain vast amount of superionic water ice. Planetary scientists believe these giant planets are made primarily of a carbon, hydrogen, oxygen and nitrogen (C-H-O-N) mixture that corresponds to 65 percent water by mass, mixed with ammonia and methane.

Many scientists envision these planets with fully fluid convecting interiors. Now, the experimental discovery of superionic ice should give more strength to a new picture for these objects with a relatively thin layer of fluid and a large “mantle” of superionic ice. In fact, such a structure was proposed a decade ago – based on dynamo simulation – to explain the unusual magnetic fields of these planets. This is particularly relevant as NASA is considering launching a probe to Uranus and/or Neptune, in the footsteps of the successful Cassini and Juno missions to Saturn and Jupiter.

“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, co-author on the paper and professor in Earth & Planetary Physics and Astronomy 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.”

“The next step will be to determine the structure of the oxygen lattice,” said Federica Coppari, LLNL physicist and co-author of the paper. “X-ray diffraction is now routinely performed in laser-shock experiments at Omega and it will allow to determine experimentally the crystalline structure of superionic water. This would be very exciting because theoretical simulations struggle to predict the actual structure of superionic water ice.”

Looking ahead, the team plans to push to higher pre-compression and extend the technique to other materials, such as helium, that would be more representative of planets like Saturn and Jupiter.

Contacts and sources:
Lawrence Livermore National Laboratory