Experiments Confirm the Interiors of Uranus and Neptune Are Made of Superionic Ice

Uranus, on the left, Neptune, on the right

A unique form of water ice that is both solid and liquid at the same time might be found inside Uranus and Neptune, according to a recent set of experiments that mimicked the conditions inside the icy giants. 

The results, published in the journal Nature Physics, confirmed a 30-year old theory that a form of water ice called superionic ice likely exists in certain planetary conditions where liquids endure extreme heat and pressure. This includes the ice giants in our own solar system, as well as similar exoplanets discovered in other solar systems throughout our galaxy.  Superionic ice, however, is not found naturally on Earth.

“We wanted to see if we could confirm the prediction for superionic water ice and measure its properties in the laboratory,” lead author Marius Millot, a researcher at Lawrence Livermore National Laboratory, said in an email to Seeker. “It is such an unusual state of matter, we wanted to see if we could create it with shock waves.”

There are perhaps 17 — or more — types of water ice, although some remain theoretical. On Earth’s surface, only one kind of ice occurs naturally — the ice in your drink or that makes up the Antarctic ice sheets — called ice lh (pronounced “ice one h”). As water freezes and turns from a liquid into a solid, the water molecules crystalize into a hexagonal shape.

But depending on the pressure and temperature, the water molecules can line up into different shapes, creating different types of ice. Even water at high temperatures can turn solid when compressed under enough pressure. Ice of this type, called ice VII (pronounced “ice eight”), is known to exist deep within Earth, and it was recently found inside of diamonds. Ice VII has also been created in laboratories.

Superionic ice is thought to form at extreme temperatures and pressures, where oxygen atoms are locked into a crystal structure, but the hydrogen ions move around, making the ice simultaneously solid and liquid, somewhat similar to lava. Over the years, various research groups have explored the properties of water under high pressure using computer simulations of the structure of water.

“These simulations showed that when water is compressed to millions of [Earth] atmospheres and heated to thousands of degrees it forms a crystal of oxygen ions with hydrogen ions moving rapidly through the crystal in a fluid-like manner,” co-author Sebastien Hamel, also from LLNL, said in an email. “However, such simulations have been approximations and so we wanted to verify those predictions by reaching those pressure and temperature conditions for a sample of water in the lab and measuring whether or not it solidified and whether or not the hydrogen ions were fluid-like.”

Millot, Hamel, and their colleagues first created ice VII in their laboratory by putting a small, sub-millimeter-sized water sample inside a diamond anvil cell (DAC), a high-pressure device made up of two opposing diamonds, which places water under extreme pressure. They then hand-carried the sample to a laser facility at the University of Rochester.

“What was novel about our experiment was to combine the compressed ice with a laser-generated shock wave to compress and heat up the water sample to reach the conditions of pressure and temperature that we wanted,” Hamel said.

The water was pre-compressed in the DAC to about 30,000 atmospheres and the shockwave briefly increased the pressure to 2,000,000 atmospheres, while heating the sample to about 4,000 degrees Kelvin.

Over a year, the researchers conducted multiple tests and were able to confirm that the extreme pressure and temperatures created superionic ice. They measured the optical reflectivity and absorption levels, showing the samples were opaque, suggesting that the ions were moving.

“Physicist often measure the optical properties to understand the electronic structure,” Millot explained. “Superionic water ice is a semiconductor, and because there are not enough ‘free electrons’ able to carry electrical current, it is not shiny like a metal. Instead, it absorbs visible light and looks black, opaque if there is a thick enough layer.”

These results were consistent with the computer-simulated predictions and Hamel said the researchers are now working on developing a general capability for performing this type of experiment for various other materials.

Interestingly, the team brought the DAC carrying the ice sample inside a carry-on case on a commercial flight from LLNL in California to the laser facility in New York. Asked if that method of transport was nerve-wracking, Millot and Hamel said “not at all. “

“We often hand-carry our targets for laser experiments, so there is always a chance that one cell will break during the trip, but they are usually okay,” Millot said, adding that they usually bring multiple samples. “The final countdown for the laser shots is more stressful, because each cell is destroyed once we have fired the laser. So if the diagnostic did not record, the whole time preparing the target and setting up the laser shot is lost!”

But the researchers said understanding superionic ice could also solve a mystery about the odd, lopsided magnetic fields of Uranus and Neptune detected by the Voyager 2 mission in the 1980’s. Planetary magnetic fields are produced by the movement of electrically conducting internal fluids at high pressures, and any unusual magnetic fields are thought to be related to the consistency of the fluids that generate them.

“Given how we think planets like Neptune and Uranus form, a large fraction of their mass is water,” Hamel said. “Under the pressures and temperatures achieved in the interior of those giant planets, water will be a fluid for the outer part of the planet and a super-ionic solid for the deeper layers of the planet.”

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