Bizarre form of hot ice seen on Earth

Thought to lurk deep within Neptune and Uranus, the extreme material is actually half as hot as the surface of the sun.

Illustration by Millot, Coppari, Hamel, Krauss (LLNL)
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From left to right in this artistic rendering, high-power lasers focus on the surface of a diamond, generating a sequence of shock waves that propagate through a sample of water, simultaneously compressing and heating the initially liquid sample and forcing it to freeze into superionic ice.

Illustration by Millot, Coppari, Hamel, Krauss (LLNL)

Bizarre form of hot ice seen on Earth

Thought to lurk deep within Neptune and Uranus, the extreme material is actually half as hot as the surface of the sun.

From the seas of Antarctica to the depths of your freezer, most ice on Earth is relatively tame stuff. But throughout the solar system and beyond, extreme temperatures and pressures can crush the frozen substance into increasingly odd varieties.

Now, researchers have snapped x-ray images of what might be the newest entrant to ice’s diversity: a highly electrically conductive material known as superionic ice. As the team reports today in the journal Nature, this ice exists at pressures between one and four million times that at sea level and temperatures half as hot as the surface of the sun.

“Yes, we’re talking about ice,” says study leader Marius Millot, a physicist at Lawrence Livermore National Laboratory in California. “But the sample is at several thousand degrees.”

While normally unachievable on Earth, such conditions should be present deep inside the watery giants Uranus and Neptune, potentially helping to explain how these distant planets work, including the origins of their unusual magnetic fields.

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Beyond Vonnegut

Scientists already know of 17 varieties of crystalline ice (fans of Kurt Vonnegut might be relieved to know that Ice IX is quite innocuous compared to its fictional counterpart). And more than 30 years ago, physicists predicted that crushing pressure should squeeze water into superionic forms.

Superionic materials are dual beasts, part solid and part liquid, that on a microscopic level consist of a crystal lattice permeated by free floating atomic nuclei that can carry electrical charge. In water—aka H2O—the oxygen atoms would crunch into a solidified crystal while the hydrogen’s protons would zip around like a liquid. (Recently, another team of scientists working with potassium confirmed the existence of a state of matter that is solid and liquid at the same time.)

“It’s quite an exotic state of matter,” says coauthor Federica Coppari, also of the Livermore lab.

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In this time-integrated photograph of an x-ray diffraction experiment, giant lasers focus on the water sample, which is sitting on the front plate of the diagnostic tool used to record diffraction patterns. Additional laser beams generate an x-ray flash off an iron foil, which allows the researchers to take a snapshot of the compressed and heated water layer.

Last year, Millot, Coppari, and their colleagues found the first evidence for superionic ice, using diamond anvils and laser-induced shock waves to compress liquid water so much that it turned to solid ice for a few billionths of a second. The team’s measurements showed that the water ice briefly became hundreds of times more electrically conductive than it had previously been, a strong hint that it had gone superionic.

In their latest tests, the researchers used six giant laser beams to generate a sequence of shockwaves that crunched a thin layer of liquid water into solidified ice at millions of times Earth’s surface pressure and between 3,000 and 5,000 degrees Fahrenheit. Precisely timed x-ray flashes probed the configuration, which again only lasted for a few billionths of a second, and revealed that the oxygen atoms had indeed taken on a crystalline form.

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The fact that matter can arrange itself in such a large variety of forms is quite astonishing.
Roberto Carr, Princeton University

The oxygen was seen to be tightly packed into face-centered cubes—little boxes with an atom at each corner and one in the middle of each side. This is the first time that water ice has been seen taking on such an arrangement, Coppari says. The team has proposed calling this new formation Ice XVIII.

While there was some overlap in conditions between the team’s two experiments, more investigations will be necessary to definitively prove that the ice is superionic, says Roberto Car, a Princeton University physicist who was not involved in the work. Nevertheless, he considers the study to be an important illustration of water’s variableness.

“The fact that matter can arrange itself in such a large variety of forms is quite astonishing,” he says.

Magnetic mysteries

The team’s results are already informing models of Uranus and Neptune. Often known as ice giants, these enormous worlds are around 65 percent water, plus some ammonia and methane, which forms layers much like the rocky-metallic surface, mantle, and core of Earth.

The new experiments indicate that Uranus and Neptune should have a superionic ice layer that acts like our planet’s mantle, which is made of solid rock that still flows on extremely long geological timescales. And that could help explain why they have unusual magnetic fields.

The magnetic fields of Earth, Jupiter, and Saturn are all thought to be created by internal dynamos near their cores. These planets’ fields are aligned fairly closely with their axes, as if they are coming from bar magnets sticking through the planets’ centers. (Here’s what happened when scientists had to update our best map of Earth’s magnetic field.)

Neptune’s magnetic field, by contrast, seems to come from an internal bar magnet that has drifted down to one side, with its ends emerging from spots halfway to the equator. Uranus’ is even more outlandish, like a bar magnet that has flipped upside-down, meaning its magnetic south juts out from the planet’s northern hemisphere. Both ice giants’ magnetic fields are suspected of being unstable.

Millot has suggested that there could be a liquid layer at the top edge of Uranus and Neptune’s superionic ice layer, but that it is also a highly electrically conductive phase of water. The planets’ magnetic fields might originate here, far closer to the surface than the magnetic fields of other worlds, potentially explaining their wonky characteristics. And since astronomers have discovered many Neptune and Uranus-sized exoplanets, the findings could be applicable to far-flung parts of the cosmos.