In 2019, scientists confirmed the existence of “superionic ice,” a new almost metal-like phase of water that is black and hot, first theoretically predicted more than 30 years ago. Although it has never been seen until then, scientists think this new state of matter might be among the most common forms of water in the universe. “It’s not quite a new phase of water. It’s really a new state of matter,” said physicist Livia Bove of France’s National Center for Scientific Research and Pierre and Marie Curie University. “Which is rather spectacular.”
The discovery of superionic ice at the Laboratory for Laser Energetics at the University of Rochester, reported Joshua Sokol in Quanta, “potentially solves the puzzle of what giant icy planets like Uranus and Neptune are made of. They’re now thought to have gaseous, mixed-chemical outer shells, a liquid layer of ionized water below that, a solid layer of superionic ice comprising the bulk of their interiors, and rocky centers.”
Physicists have been unlocking the secrets of superionic ice for years. In 1988, chemist Pierfranco Demontis at the University of Sassari predicted water would take on this strange, almost metal-like form if pushed beyond the map of known ice phases.
Great Revelation of the X-rays
The team at the Laboratory for Laser Energetics using one of the world’s most powerful lasers, blasted a droplet of water, creating a shock wave that raised the water’s pressure to millions of atmospheres and its temperature to thousands of degrees. The X-rays beamed through the droplet offered humanity’s first look of water under those extreme conditions.
Time-integrated image of a laser-driven shock compression experiment to recreate planetary interior conditions and study the properties of superionic water. (M. Millot/E. Kowaluk/J.Wickboldt/LLNL/LLE/NIF)
The great revelation of the X-rays was that the water inside the shock wave didn’t become a superheated liquid or gas, instead the atoms froze solid, forming crystalline ice.
“You hear the laser blast,” said Marius Millot of Lawrence Livermore National Laboratory in California, and “right away you see that something interesting was happening.” Millot co-led the experiment with Federica Coppari, also of Lawrence Livermore.
Ubiquitous on Ice Giants?
Across the solar system, writes Sokol, “more water probably exists as superionic ice — filling the interiors of Uranus and Neptune — than in any other phase, including the liquid form sloshing in oceans on Earth, Europa and Enceladus. The discovery of superionic ice potentially solves decades-old puzzles about the composition of these “ice giant” worlds.”
Superionic ice is a new crystal, “but with a twist,” observed Sokol. “All the previously known water ices are made of intact water molecules, each with one oxygen atom linked to two hydrogens. But superionic ice, the new measurements confirm, isn’t like that. It exists in a sort of surrealist limbo, part solid, part liquid. Individual water molecules break apart. The oxygen atoms form a cubic lattice, but the hydrogen atoms spill free, flowing like a liquid through the rigid cage of oxygens”.
Puzzle of Ice-Giant Worlds of the Solar System
When asked about how the discovery of superionic ice potentially solves decades-old puzzles about the composition of “ice giant” worlds” in our solar system, physicist Marius Millot wrote in an email to The Daily Galaxy: “Our experimental discovery of superionic water ice opens the way to imagining new kinds of water-rich planets with mostly frozen interiors. For example, superionic water ice could dominate the interior of Neptune and Uranus which offers a plausible origin to their unique, multi-polar and non-axisymmetrical magnetic fields. This is enabled by the extra-entropy associated with the “liquid-like” behavior of the hydrogen ions which helps superionic water ice sustain higher temperature without melting. In contrast, before our discovery, most planetary scientists were imagining these planets as having water-rich fluid interiors with swirling motions extending very deep.”
Millot is with the Lawrence Livermore National Laboratory, High Energy Density Science (HEDS) Center, as part of a multi-institutional team that recently created superionic ice during a series of dynamic compression experiments (“Experiments Verify ‘Mind-Boggling’ Behavior of Water Ice”). The results offer new insights into the formation and composition of the solar system’s ice giants, Uranus and Neptune, as well as similar exoplanets.
“All of this would not have been possible, say, five years ago,” said Christoph Salzmann at University College London, who discovered ices XIII, XIV and XV. “It will have a huge impact, for sure.”
Read more about the physics of the discovery at Quanta
Image credit top of the page: NASA’s Voyager 2 snapped this picture of a crescent Neptune on 31 August 1989 during the spacecraft’s flyby of the planet. Voyager 2 is the only mission to have visited Uranus or Neptune, the two ice giant planets of our solar system. NASA/JPL-Caltech/Kevin M. Gill