Scientists find strange black “superionic ice” that could exist deep inside other planets

Uranus and Neptune

Superionic water is found in the ice giants Uranus and Neptune. Credit: LLNL

Using the advanced photon source, scientists have recreated the structure of ice formed in the middle of planets such as. Neptune and Uranus.

Everyone knows about ice, liquid and steam – but depending on the conditions, water can actually form more than a dozen different structures. Researchers have now added a new phase to the list: superionic ice cream.

This type of ice is formed at extremely high temperatures and pressures, such as those deep inside planets like Neptune and Uranus. Earlier superionic ice had only been glimpsed in a brief moment when scientists sent a shock wave through a drop of water, but in a new study published in Physics, scientists found a way to create, maintain and study the ice reliably.

Superionic ice experiment

Scientists used diamonds and a beam of radiant X-rays to recreate conditions deep inside the planets and found a new phase of water called “superionic ice.” Credit: Picture of Vitali Prakapenka

“It was a surprise – everyone thought this phase would not emerge until you are under much higher pressure than where we first find it,” said study co-author Vitali Prakapenka, a University of Chicago research professor and beamline scientist at Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory. “But we were able to very accurately map the properties of this new ice cream, which forms a new phase of matter, thanks to several powerful tools.”

Although humans have looked back in time to the beginning of the universe – and down to the smallest particles that make up all matter – we still do not understand exactly what is hidden deep inside the Earth, let alone inside the sibling planets in our sun. system. Scientists have only dug about seven and a half kilometers below the earth’s surface before the equipment began to melt due to the extreme heat and extreme pressure. Under these conditions, rocks behave more like plastics, and the structures of even basic molecules like water begin to shift.

“We were able to very accurately map the properties of this new ice cream, which forms a new phase of matter, thanks to several powerful tools.” – Vitali Prakapenka, University of Chicago.

Since we cannot reach these places physically, scientists must turn to the laboratory to recreate conditions with extreme heat and pressure.

Prakapenka and his colleagues use APS, a massive accelerator that drives electrons at extremely high speeds close to the speed of light to generate radiant rays of X-rays. They squeeze their samples between two pieces of diamond – the hardest substance on Earth – to simulate the intense pressure, and then shoot lasers through the diamonds to heat up the sample. Finally, they send a ray of X-rays through the sample and assemble the arrangement of the atoms inside based on how the X-rays scatter from the sample.

When they first ran the experiments, Prakapenka saw readings of the structure that were very different than he had expected. He thought something had gone wrong and there had been an unwanted chemical reaction that often happens with water in such experiments. “But when I turned off the laser and the sample returned to room temperature, the ice went back to its original state,” he said. “That means it was a reversible, structural change, not a chemical reaction.”

Looking at the structure of the ice, the team realized that it had a new phase on the way. They were able to accurately map its structure and properties.

“Imagine a cube, a lattice with oxygen atoms in the corners connected by hydrogen,” Prakapenka said. “When transformed into this new superionic phase, the lattice expands so that the hydrogen atoms can migrate around while the oxygen atoms remain stable in their positions. It’s a bit like a solid oxygen lattice sitting in a sea of ​​liquid hydrogen atoms.”

It has consequences for how the ice behaves: It becomes less dense, but significantly darker because it interacts differently with light. But the full spectrum of the chemical and physical properties of superionic ice has yet to be explored. “It’s a new state of matter, so it’s basically acting as a new material, and it may be different than we thought,” Prakapenka said.

The results were also a surprise, because while theoretical scientists had predicted this phase, most models thought it would not appear until the water was compressed to more than 50 gigapascals pressure (roughly the same as the conditions inside rocket fuel as it detonates before departure ). But these experiments were only at 20 gigapascals. “Sometimes you get surprises like this,” Prakapenka said.

But mapping the exact conditions in which different phases of the ice occur is important for, among other things, understanding planet formation and even where to look for life on other planets. Scientists believe that similar conditions exist in the interior of Neptune and Uranus and other cold, rocky planets like those elsewhere in the universe.

The properties of these ices play a role in a planet’s magnetic fields, which have a huge impact on its ability to host life: Earth’s powerful magnetic fields protect us from harmful incoming radiation and cosmic rays, whereas the surfaces of barren planets March and mercury is exposed. Knowing the conditions that affect magnetic field formation can guide scientists as they search for stars and planets in other solar systems that may host life.

Prakapenka said there are many more angles to explore, such as conductivity and viscosity, chemical stability, which changes when water is mixed with salts or other minerals, as it often does deep below the earth’s surface. “This should stimulate many more studies,” he said.

Reference: “Structure and properties of two superionic ice phases” by Vitali B. Prakapenka, Nicholas Holtgrewe, Sergey S. Lobanov and Alexander F. Goncharov, 14 October 2021, Natural physics.
DOI: 10.1038 / s41567-021-01351-8

Synchrotron X-ray diffraction was performed by GeoSoilEnviroCARS, a beamline on Advanced Photon Source at the Argonne National Laboratory, and optical spectroscopy was performed by the Carnegie Institution for Science. The other authors on paper were Nicholas Holtgrewe of CARS and the Carnegie Institution of Washington, Sergey Lobanov of the Carnegie Institution and the GFZ German Research Center for Geosciences, and Alexander Goncharov of the Carnegie Institution.

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