Scientists made a new form of ice at more than 2,000 degrees

A team of physicists has produced a previously unconfirmed form of ice by squeezing water to nearly 2.3 million times normal atmospheric pressure while heating it above 2,000 degrees Celsius, temperatures at which water would normally exist only as a gas. The new phase, called hexagonal close-packed, or hcp, ice, had been predicted in theory for years but never clearly observed in an experiment. Researchers say the discovery could reshape ideas about what is happening deep inside ice giant planets such as Uranus and Neptune.

The work, led by physicist Alexis Forestier of the French Alternative Energies and Atomic Energy Commission, recreated conditions closer to a planetary interior than to anything found at Earth’s surface, where extreme pressure keeps water locked in dense, exotic solid states rather than letting it expand into vapor. Ordinary water already behaves oddly compared with most liquids, becoming less dense as it freezes and boiling at a far higher temperature than its molecular weight would predict; researchers have long suspected that pushing it to planetary extremes of pressure and heat would uncover even stranger behavior than anything visible at Earth’s surface.

Squeezing Water Between Diamond Tips

To reach these conditions, the researchers compressed tiny water samples between the tips of two diamonds, a technique known as a diamond anvil cell, while firing lasers at the sample to heat it. The pressures involved reached as high as 230 gigapascals, roughly 2.3 million times sea-level atmospheric pressure and well above the roughly 360 gigapascals estimated at the center of Earth. Temperatures climbed as high as 2,630 kelvins, or about 2,357 degrees Celsius, hot enough that, without the crushing pressure holding it in place, the water would have broken apart into a gas of oxygen and hydrogen.

Superionic Ice: Neither Fully Solid Nor Fully Liquid

Under these conditions, water can enter a state known as superionic ice, in which oxygen atoms stay fixed in a rigid crystal lattice while hydrogen nuclei move freely through that lattice, behaving more like a liquid than a solid. Roughly twenty known phases of water ice exist, and only a handful become superionic under this kind of extreme pressure and heat, a distinction that has made them a target for physicists trying to map water’s full range of behavior. Because it conducts an electric current the way a liquid electrolyte does while still holding a solid crystal’s rigid framework, superionic ice is often described as neither a true solid nor a true liquid, and its properties are difficult to reproduce or measure directly outside a laboratory built around diamond anvils and high-powered lasers.

How hcp Ice Differs From Its fcc Counterpart

The distinction between forms of superionic ice comes down to how their oxygen atoms are stacked. One previously confirmed phase, called face-centered cubic, or fcc, ice, arranges its layers in one repeating sequence; the newly confirmed hcp ice stacks its layers differently, in the same way that identical balls can be packed tightly in more than one repeating pattern. The researchers found evidence that the crystal can flip from one stacking sequence to the other as pressure and temperature shift, with the hcp arrangement becoming more stable as conditions grow more extreme. That kind of stacking transition is a small change on the atomic scale, but it can alter how easily electric charge and heat move through the material, properties that matter far more once the ice sits inside a planet than they do in a laboratory sample a few micrometers across.

Watching the Transition With Synchrotron X-Rays

To track the shift, the team used a narrow beam of synchrotron X-rays to probe the crystal structure of the compressed ice at each set of conditions. At 155 gigapascals and 2,000 kelvins, the X-ray signal showed a mix of fcc and hcp structure. Pushing to 197 gigapascals and 2,250 kelvins made the hcp signal noticeably stronger, and by the study’s most extreme conditions, 219 gigapascals and 2,630 kelvins, the fcc signature had nearly disappeared, leaving hcp ice dominant. As the hcp crystal was heated further, its expansion pattern suggested it entered the superionic state at around 1,700 kelvins.

A Signal the Researchers Had Missed Before

Looking back at data from an earlier experiment, the team realized they may have already produced hcp ice without recognizing it: an unexplained X-ray diffraction peak recorded above 130 gigapascals in that earlier run appears, in hindsight, to have been the same hcp signature. That earlier miss suggests the new phase may be more stable, and more common under these conditions, than the researchers initially assumed once pressures climb above roughly 200 gigapascals.

Why the Finding Matters for Uranus and Neptune

Superionic ice is thought to exist deep inside Uranus and Neptune, where its unusual mix of solid structure and mobile charge carriers may help generate the two planets’ notably lopsided and irregular magnetic fields. If hcp ice conducts electricity differently than fcc ice, as the researchers suspect it might, its presence inside those planets could change scientists’ models of how charge and material move through their interiors. The team is calling for further theoretical and experimental work to pin down hcp ice’s mechanical and electrical properties, as well as the precise pressure-temperature boundary where it takes over from the fcc form. Uranus and Neptune have long puzzled planetary scientists because their magnetic fields are not centered or aligned the way Earth’s, Jupiter’s or Saturn’s are, and one leading explanation is that the field-generating layer inside each planet is not a simple, uniform ocean but a shifting mix of exotic ice phases like these. Confirming exactly which phase dominates at a given depth, and how well each one conducts electricity, is a step toward testing that idea more rigorously rather than relying on models built with incomplete data about the ice itself. The findings were published in Physical Review Letters, and additional detail on the experiment is available in ScienceAlert’s coverage of the discovery.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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