Scientists Discover New Form of Ice – May Be Common on Distant, Water-Rich Planets

Findings could have implications for our understanding of distant, water-rich planets.

Researchers from the University of Nevada, Las Vegas have discovered a new form of ice, redefining the properties of water at high pressures. This finding, according to physicist Ashkan Salamat, could recalibrate our understanding of the makeup of exoplanets. Furthermore, the newly discovered form of ice, Ice-VIIt, might be abundant on planets outside our solar system.

Solid water, or ice, is similar to many other materials in that it may create a variety of solid materials depending on temperature and pressure, much like how carbon can form diamond or graphite under certain conditions. Water, on the other hand, is remarkable in this regard, as we are aware of at least 20 solid forms of ice.

A group of researchers from UNLV’s Nevada Extreme Conditions Lab developed a new method for assessing the characteristics of water under high pressure. The water is initially squeezed between the points of two diamonds pointing in opposing directions, resulting in a jumble of ice crystals. The ice then is momentarily melted with a laser before immediately reforming into a powdery collection of small crystals.

Ice-X Transition Occurs at Lower Pressures

By incrementally raising the pressure, and periodically blasting it with the laser beam, the team observed the water ice make the transition from a known cubic phase, Ice-VII, to the newly discovered intermediate, and tetragonal phase, Ice-VIIt, before settling into another known phase, Ice-X. 

Zach Grande, a UNLV Ph.D. student, led the work which also demonstrated that the transition to Ice-X, when water stiffens aggressively, occurs at much lower pressures than previously thought.

While it’s unlikely we’ll find this new phase of ice anywhere on the surface of Earth, it is likely a common ingredient within the mantle of Earth as well as in large moons and water-rich planets outside of our solar system.

The team’s findings were reported in the March 17, 2022 issue of the journal Physical Review B.

Takeaways

The research team had been working to understand the behavior of high-pressure water that may be present in the interior of distant planets.

To do so, Grande and UNLV physicist Ashkan Salamat placed a sample of water between the tips of two round-cut diamonds known as diamond anvil cells, a standard feature in the field of high pressure physics. Applying a little bit of force to the diamonds enabled the researchers to recreate pressures as high as those found at the center of the Earth.

By squeezing the water sample between these diamonds, scientists drove the oxygen and hydrogen atoms into a variety of different arrangements, including the newly discovered arrangement, Ice-VIIt.

Not only did the first-of-its-kind laser-heating technique allow scientists to observe a new phase of water ice, but the team also found that the transition to Ice-X occurred at pressures nearly three times lower than previously thought — at 300,000 atmospheres instead of 1 million. This transition has been a highly debated topic in the community for several decades.

“Zach’s work has demonstrated that this transformation to an ionic state occurs at much, much lower pressures than ever thought before,” Salamat said. “It’s the missing piece, and the most precise measurements ever on water at these conditions.”

The work also recalibrates our understanding of the composition of exoplanets, Salamat added. Researchers hypothesize 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.

Reference: “Pressure driven symmetry transitions in dense H₂O ice” by Zachary M. Grande, C. Huy Pham, Dean Smith, John H. Boisvert, Chenliang Huang, Jesse S. Smith, Nir Goldman, Jonathan L. Belof, Oliver Tschauner, Jason H. Steffen, and Ashkan Salamat, 17 March 2022, Physical Review B. 
DOI: 10.1103/PhysRevB.105.104109

Collaborators at the Lawrence Livermore National Laboratory used a large supercomputer to simulate the bond rearrangement—predicting that the phase transitions should happen precisely where they were measured by the experiments.

Additional collaborators include UNLV physicists Jason Steffen and John Boisvert, UNLV mineralogist Oliver Tschauner, and scientists from the Argonne National Laboratory and the University of Arizona.

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