They Squeezed Water to 2 Million Atmospheres and 1,800 Kelvin. A New Kind of Ice Appeared, and It May Run Neptune's Magnetic Field.

Deep inside Uranus and Neptune, water is neither liquid nor anything a person would recognize as ice. Under millions of atmospheres of pressure and temperatures hotter than the surface of some stars, it becomes "superionic": the oxygen atoms freeze into a rigid crystal lattice while the hydrogen nuclei slip through that lattice like a fluid, turning the ice into an electrical conductor. Physicists have known for years that this phase exists. What they did not know was what shape the oxygen lattice actually takes. A team led by Alexis Forestier of the CEA, France's Alternative Energies and Atomic Energy Commission, has now caught it changing shape, and the answer could reshape models of the two ice giants.

The experiment, reported in Physical Review Letters, used a diamond anvil cell, in which a speck of water is crushed between the tips of two gem-quality diamonds. The team squeezed samples to 230 gigapascals, more than two million times atmospheric pressure at sea level, and heated them with lasers to above 1,800 kelvin, roughly 2,800 degrees Fahrenheit. They then fired X-rays through the sample at the European Synchrotron Radiation Facility in Grenoble and read the diffraction pattern, which reveals how the atoms are arranged.

At lower pressures the oxygen atoms sat in a face-centered cubic arrangement, the same packing pattern as the atoms in a block of aluminum. Above about 200 gigapascals and 1,800 kelvin, the pattern changed. The oxygen atoms locked into a hexagonal close-packed lattice, the arrangement found in magnesium and titanium. That hexagonal form became the dominant phase of superionic ice at the highest pressures. "We report the unambiguous observation of a novel H2O ice phase adopting an hcp oxygen sublattice," the authors wrote.

The change is what metallurgists call a martensitic transition, a rapid, diffusionless rearrangement of a crystal in which atoms shift cooperatively rather than migrating one by one. It is the same class of transformation that hardens steel when it is quenched. Seeing one in hot, conducting water under planetary conditions was not something the field had predicted with confidence.

Why it matters comes down to the magnetic fields of Uranus and Neptune, which are among the strangest in the solar system. Both are tilted sharply from the planets' spin axes and offset from their centers, unlike the neat dipoles of Earth, Jupiter and Saturn. Planetary scientists have long suspected that a conducting layer of superionic ice, rather than a molten iron core, generates those fields. If the hexagonal and cubic forms of superionic ice have different electrical and mechanical properties, and there is good reason to think they do, then where the transition sits inside each planet would help determine how thick the conducting layer is and how the dynamo behaves. "The presence of an fcc-hcp martensitic transition in the superionic regime of warm dense ice may have implications for planetary models of Uranus and Neptune," the team wrote.

The work also fills in a decades-old gap in the phase diagram of the most familiar substance on Earth. Water has at least twenty known crystalline forms at various pressures and temperatures, and the superionic regime, first predicted in 1988 and confirmed in shock experiments in 2018, has been the least mapped of all because it exists only under conditions that are hard to hold steady in a laboratory. Forestier's group managed to keep the sample stable long enough for a clean X-ray measurement, which is what separates an "unambiguous observation" from a hint. A preprint of the paper is on arXiv, and the planners of NASA's proposed Uranus orbiter now have one more number to build into their interior models.