Scientists finally understood the internal structure of superionic ice.

Rahul Rao | posted on October 25, 2021 at 11:58 AM

If you put tap water at the same pressure and temperature as the center of the earth (yes, water that crushes bones and burns flesh), it will turn into a strange, black ice that is neither liquid nor hard of. Scientists call this substance superionic ice, and its properties are still a bit mysterious to most scientists. 

Researchers have only recently been able to make super-ionized ice in the laboratory. Now, a group of scientists sorted out its characteristics. Their results can answer the mystery of the magnetic properties of Uranus and Neptune: superionic ice may be the cause of the strange and unstable magnetic fields of these planets, which has puzzled scientists for some time. 

The researchers published their work in the journal Nature Physics on October 14.

When water freezes in the earth’s atmosphere, its molecules naturally arrange into hexagons; this is why, if you live in a sufficiently cold climate, you will see hexagonal snowflakes. But by pushing water under extreme conditions that don't normally exist on Earth, a large number of strange ice phases may be produced. The shape of these ices is strange, some can exist at room temperature-or in fact, the temperature is much higher.

Out of enthusiasm for creative names, scientists have marked the different stages of ice with Roman numerals. For example, the ice in cold drinks is "Ice I". When ice is squeezed at 10,000 times the pressure of the earth's atmospheric pressure, it may become ice VI, and its molecules form rectangular prisms. Increase the pressure, it may become ice VII, and its molecules will become cubes.

You can also find ice XI, whose charge flips in an electric field, and ice XVI, which is imprisoned in other ice "cages". Yes, if you read Kurt Vonnegut's "Cat's Cradle", you will find "Ice IX", even though it is completely harmless.

From a larger perspective, this strange ice may not be that strange. In particular, Ice VII is believed to exist under the outer seas of the ocean world and in the depths of Jupiter's moon Europa. Closer to us, scientists discovered that Ice VII was buried in diamonds formed in the mantle; the pressure there might allow ice like this to exist.

The latest entry into the Ice Temple is Super Ion Ice. Here, the boundary between liquid water and solid ice begins to fall apart. The oxygen atoms of water molecules are arranged in order, just as they are in a solid. But the hydrogen atoms give up their electrons, become charged ions, and start jumping in the ice—just like they are in a fluid.

This is indeed a strange kind of ice. On the one hand, it becomes dark. Moreover, unlike pure water at room temperature, these freely rotating protons make superionic ice a good electrical conductor.

Since the late 1980s, scientists have predicted superionic ice. "Since then, we have been thinking about conducting that kind of experiment," said Alexander Goncharov, a physicist at the Carnegie Institute of Science in Washington, D.C., and one of the authors of the paper. 

But until recently, scientists have been able to use it in the laboratory. Some researchers basically create super-ion ice by spraying a little bit of water with a high-pressure shock wave. In 2018, they measured the electrical conductivity; in 2019, they determined the structure of the oxygen atom labeled superionic ice. They named it "Ice Eighteen".

But the shock wave won’t last long: According to one of the researchers, Sebastien Hamel, a physicist at Lawrence Livermore National Laboratory in California, the entire experiment lasted. A few nanoseconds.

At the same time, however, another group of researchers is making their own superionic ice in a different way. They don't want to find ice in the shock wave, but want to actually create ice in a more static environment so that they can study it.

"We can recognize structures," said Vitali Prakapenka, a physicist at the University of Chicago and another author of the paper. "We can measure optical properties."

But getting there is a tedious and difficult process, especially because it involves incredible temperature and pressure in the center of the earth: hotter than the surface of the sun, and 3.5 million times the pressure of the earth’s atmosphere.

To do this, the scientists squeezed ice in an anvil made of 0.2 carat diamonds. Because diamonds are the hardest material known on earth, they are a great way to push ice into crazy pressure. Then, they can heat the sample to a temperature similar to that of a star by using a laser. 

In order to actually observe the ice, the scientists took their anvils and samples to the Argonne National Laboratory in the suburbs of Chicago, using a synchrotron, a machine that can produce amazingly bright X-rays. When these X-rays pass through the ice, they scatter, and scientists can measure them to reconstruct the characteristics of the ice.

To complicate matters further, when X-rays pass through the diamond, they are refracted. It's a lot like how things look distorted when you see things through water. They need to correct this.

"It's very challenging, but we are doing it," Prakapunka said.

Their experiments lasted exactly microseconds, not just nanoseconds, which gave them more time to make measurements. "They have been able to explore this system in more detail... than we can do," said Hamel, who was not involved in the paper. However, he said that the temperature gradient caused by laser heating introduces a lot of uncertainty.

Nevertheless, in addition to the discovery of ice XVIII, the researchers also discovered a second type of superionic ice, which they called "ice XX". (Ice XIX is a non-superionic phase that happened to be discovered and named during all these processes.) In addition, they were able to measure the structure and conductivity of superionic ice.

Researchers don't just make super-ion ice to play with diamond anvils. Just as Ice VII may be found on Europa, superionic ice may also be located in the outer solar system.

In many ways, Uranus and Neptune are very similar. Their sizes are close to each other. They are all "ice giants" and their atmosphere is full of hydrogen, helium, and methane. 

Their magnetic fields are very, very strange.

The earth's magnetic field mainly coincides with the planet's rotation. The physical poles of our planet are not too far away from our magnetic poles. However, the magnetic fields of these two planets are quite tilted. In addition, the magnetic poles are not aligned, cutting awkwardly into the sides of the planets instead of drawing a line through their centers.

Now, ice scientists are looking for an explanation: super-ion ice is buried deep under the gas cover of huge planets. Because of the conductive properties of superionic ice, scientists believe that it can play with magnetic fields. If the researchers are correct, then the huge superionic ice layer — which dwarfs everything we see on Earth — may shift the magnetic field of each planet away from the center.

Ultimately, any guesses must rely on simulation and modeling. "In the field of planetary science, we have a large detective game playing," Hamel said. "It's not like we can cut through the earth to see how it is made."

But Plaka Penka said his team’s latest experiment adds to the evidence that superionic ice can be found there. "We estimate that there should be a lot of ice," he said, "and under conditions deep in the planet, the temperature and pressure are exactly the same as where we found superionic ice."

Rahul is a freelance science journalist, graduated from NYU SHERP, and a fan of Doctor Who. Contact the author here.

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