A team of physicists recently discovered a new phase of water ice, after they put ordinary ice under extraordinarily high pressure and melted it before letting it refreeze. The previously unknown phase — called Ice-VIIt — is organised differently from typical water ice. It doesn’t occur naturally on Earth’s surface, but it may exist in the mantle or on distant moons and planets.
Ice-VIIt has a tetragonal symmetry, as opposed to the cubic structure of the ice phase from which it forms, Ice-VII. That tetragonal structure also sets Ice-VIIt (the ‘t’ is for ‘tetragonal’) apart from the hexagonal symmetry of natural water ice (known as Ice-I). That means its crystal structure looks like a rectangular prism instead of a cube. The findings were published last week in Physical Review B.
“The main significance is that the community that has been studying ice has been very adamant that cubic ice-VII is the dominant high-pressure phase,” said Zachary Grande, a physicist at the University of Nevada, Las Vegas and the study’s lead author, in an email to Gizmodo. “But we were able to use our new technique to obtain much more accurate measurements than anyone before, allowing us to observe this subtle quantum transition.”
To synthesise the unique ice phase, the researchers froze a water sample under the quash of a diamond anvil cell, which pressed the water molecules between two diamonds. They used a laser to briefly melt the sample, before allowing it to freeze again in a new configuration. By squeezing the sample under pressures similar to those at Earth’s centre, they forced Ice-VII into Ice-VIIt. Grande said that the newly discovered phase was similar enough to Ice-VII to be named similarly.
“Zach’s work has demonstrated that this transformation to an ionic state occurs at much, much lower pressures than ever thought before,” said Ashkan Salamat, also a physicist at UNLV and a co-author of the research, in a university release. “It’s the missing piece, and the most precise measurements ever on water at these conditions.”
Ice-VIIt may occur naturally in the Earth’s mantle; though our planet’s interior is hot, high-pressure ices like Ice-VIIt have higher melting points. Instead of thawing out at 32° Fahrenheit, it takes 1,340° F heat to make the rare ice phase liquify.
Ice-VIIt transitions to Ice-X, a phase that the team found will occur at just one-third the pressure physicists previously thought was necessary to induce the state. Grande said that the Ice X finding had even more extraterrestrial implications than the existence of Ice-VIIt.
“If there are planets with a significant amount of water within their mantle, the planets would actually be bigger than we previously thought, since the water won’t compress,” Grande said, “and because of this, astronomers will need to re-evaluate the water supply in many of the large exoplanets that have been discovered in recent years.”
The Webb Space Telescope may help that re-evaluation; among its many tasks is to study exoplanets in unprecedented detail. The telescope is expected to be operational by the summer.