Scientists Made Ice Stay Solid Above 2,000°C. The ‘Superionic Ice’ Could Be Found Inside Uranus and Neptune

The researchers squeezed water to millions of atmospheres and discovered a bizarre new form of ice.

09 Sep 2026, 19:56 by Tibi Puiu · ZME Science

Illustration made with the help of AI. Credit: ZME Science.

Between the tips of two diamonds, researchers squeezed a speck of water until the pressure rose to millions of times that at Earth’s surface. Then they fired lasers at it, heating the sample above 2,000 degrees Celsius.

The water did not boil away. Instead, its oxygen atoms settled into a crystal arrangement that scientists had predicted but never clearly observed in water before. The highly pressurized water essentially got locked in an exotic form of solid ice, despite the temperature exceeding the melting point of iron.

The researchers found that this newly observed form of ice, called hexagonal close-packed or hcp ice, increasingly took over as pressure and temperature rose. At 219 gigapascals and 2,630 kelvins — about 2,357°C (4,274°F) — the hcp pattern dominated the X-ray signal. Researchers call this type of ice “superionic.”

The new study, which is set to appear in the journal Physical Review Letters (arXiv link), suggests that this previously overlooked form of superionic ice may exist deep inside Uranus and Neptune, with possible consequences for models of their unusual magnetic fields.

Superionic ice barely resembles the cubes in a freezer. Its oxygen atoms remain locked into a crystal lattice, while hydrogen nuclei move through that framework more like particles in a liquid. That mobile hydrogen can carry electrical charge.

Scientists predicted such a state more than 30 years ago, but it was only in 2018 that they provided the first strong laboratory evidence for superionic water. A year later, researchers used nanosecond X-ray diffraction to examine its atomic structure directly in a 2019 Nature study.

Squeezing water into a planetary interior

Alexis Forestier of France’s Alternative Energies and Atomic Energy Commission and his colleagues used diamond-anvil cells, devices that trap microscopic samples between two diamond tips. They surrounded the water with laser-absorbing boron-doped diamond and probed it with an X-ray beam less than a micrometer across at the European Synchrotron Radiation Facility. At the highest pressures, the water sample itself was only about 12 micrometers wide.

As the researchers pushed the water to ever more extreme conditions, the balance began to change. At lower pressures (but still high by everyday standards), the familiar superionic structure still shared space with the newly observed form. But as the pressure climbed toward more than two million times that at Earth’s surface, the new structure steadily took over. By the hottest, most compressed point in the experiment, it was clearly dominant.

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What changed was not the basic chemistry of the water, but the way its oxygen atoms lined up. The rearrangement is tiny, almost like shifting the way stacked balls sit on top of one another. Yet under these conditions, that small atomic shuffle may determine which form of superionic ice is most stable.

The transition did not appear to happen all at once. The researchers saw signs that one structure gradually slipped into the other as the atomic layers shifted position. Different arrangements of the same atoms can behave very differently — including in how they conduct heat, electricity and moving hydrogen deep inside a planet.

Previously, a different Nature Communications study of shock-compressed superionic water found mixed fcc and hcp structures above roughly 150 gigapascals and around 2,500 kelvins. Those results fit the broad transition seen in the new experiments.

“The unique structure of superionic water likely gives rise to its conductive properties,” said SLAC physicist Arianna Gleason, a co-author of that earlier research.

Why an atomic shuffle could matter to a planet

The new hcp phase also showed unusual thermal expansion near 1,700 kelvins (1,427°C or 2,600°F), which the researchers interpret as a signature that it had entered the superionic state. Intriguingly, most of the expansion happened in just one crystallographic direction, suggesting that hydrogen may flow more easily through the ice along some paths than others.

That is where planetary science enters the picture.

Voyager 2 found that Uranus and Neptune have magnetic fields unlike Earth’s relatively orderly dipole. Their fields are strongly tilted and strikingly non-axisymmetric. Models have long proposed that electrical currents in a relatively thin conducting layer inside the planets could generate those strange fields, with superionic water among the candidate materials.

If hcp superionic ice occupies part of the planets’ interiors, those properties could differ from the commonly assumed fcc form and change models of how matter and electrical charge move through the planets.