Ice is the familiar solid phase of water, but there's much more to it than meets the eye. The kind of ice we find on the planet's surface, from your winter backyard to Antarctica's giant ice sheets and glaciers, is all the same and has a hexagonal crystal structure. But there are 18 other known types of molecular arrangements for ice, even though it's all water. In fact, one study suggested there should be as many as 300 different forms of ice, most of which still await discovery.
A new one was added to the list this week by researchers at the University of Nevada Las Vegas (UNLV). Like most other types of ice that aren't naturally found on the planet's surface, the new type of ice forms at incredibly high pressure, comparable to the kind experienced by matter deep in Earth's bowels. Scientists believe this new type of ice, known as Ice-VIIt (the regular variety found in your freezer is called Ice I), could be found deep in Earth's mantle or even on distant watery planets.
In 2017, scientists at the Los Alamos National Laboratory observed water turning into Ice-VII (Ice Seven), a cubic phase, for the first time. It was a huge breakthrough in science and involved using an array of lasers to squeeze water to a pressure exceeding 30,000 times that of Earth’s atmosphere at sea level.
At UNLV's Nevada Extreme Conditions Lab, physicists used two diamond anvil cells to squeeze water between their tips and recreate pressures as high as those found at the center of the Earth. The ice crystals were subjected to lasers that temporarily melted them before they quickly froze into a powder-like collection of tiny crystals.
After a series of incremental rises in pressure and periodic blasting with the laser beam, the water ice turned into Ice-VII, then into the newly discovered intermediate Ice-VIIt, before settling into Ice-X.
In the process, the physicists not only discovered a new form of ice but also learned that the transition to Ice-X can occur at pressures much lower than they previously thought. The water molecules turned into Ice-VIIt at around 5.1 gigapascals, or 51,000 atmospheric pressures, whereas the transition to Ice-X occurred at around 30.9 gigapascals. More than 300,000 atmospheres are ungodly high, but that's almost three times less than the one million atmospheres previously thought to be required to make Ice-X, the most extreme form of ice. Given that Ice-X is also thought to be stable to very high temperatures (up to 2500 K so far) then it could be an important part of the interiors of the icy gas giant planets, like our very own Uranus and Neptune.
"This transformation to an ionic state occurs at much, much lower pressures than ever thought before,” UNLV physicist Ashkan Salamat said. “It’s the missing piece, and the most precise measurements ever on water at these conditions.”
Salamat added that the Ice-VIIt phase of ice could exist in abundance in the crust and upper mantle of expected water-rich planets outside of our solar system, meaning they could have conditions habitable for life.
The findings appeared in the journal Physical Review B.