New form of ice could help planetary scientists – 02/08/2023 – Science

Shaken and cooled—but not stirred—ordinary frozen water turns into something else: a newfound form of ice made from a mixture of molecules with unique properties.

“This is completely unexpected and very surprising,” said Christoph Salzmann, professor of chemistry at University College London, UK, and author of a paper published last week in the journal Science describing the ice.

Water is a simple molecule that has been intensely studied by scientists for centuries: two hydrogen atoms projecting at a 104.5 degree V-shaped angle from a central oxygen atom.

The new discovery shows, once again, that water, a molecule without which it is known that life does not exist, hides scientific surprises that have not yet been revealed. This experiment used relatively simple and inexpensive equipment to reveal a form of ice that could exist in other parts of the solar system and throughout the universe.

In everyday life, we encounter three forms of water: a vapor-like gas, flowing liquid water, and hard, slippery ice. The ice of our everyday lives consists of water molecules lined up in a hexagonal pattern, and these hexagonal lattices stack neatly on top of each other. The hexagonal structure is not tightly packed, so ice is less dense than liquid water and floats.

With temperature and pressure permutations outside of what generally occurs on Earth, water molecules can be forced to assume other crystalline structures. Scientists are now aware of 20 crystalline forms of water. The twentieth form of ice was discovered last year.

In addition, the researchers also documented two types of ice with mixed molecules, what they call amorphous materials. Since one of the amorphous ices is denser than water, it is known as high-density amorphous ice; the other, with a density less than that of water, is low-density amorphous ice. Amorphous ices are not found on Earth, but can be prevalent in outer space, on comets, interstellar clouds and icy worlds like Europa, a moon of Jupiter.

There is even a type of water that is liquid and solid. In 2018, scientists announced the creation of “superionic water”, which was both solid and liquid.

Salzmann and his colleagues did not intend to add to the catalog of water ices. Instead, they wanted to study very small ice crystals, because tiny pieces of something sometimes have very different properties than larger pieces of the same material.

So Alexander Rosu-Finsen, a postdoctoral scientist in Salzmann’s research group and lead author of the Science paper, set about crushing ice. The water ice was first cooled in liquid nitrogen to -320 degrees Fahrenheit (-195.5°C) and then placed in a container along with steel balls. A machine then shook the ice and steel balls, still chilled to ultra-cold temperatures, back and forth 20 times a second, pulverizing the ice into tiny pieces, a process known as ball milling.

Think of it as a high-tech cocktail shaker.

Then Rosu-Finsen opened the container.

“Something completely unexpected happened,” said Rosu-Finsen, who is now associate editor of Nature Reviews Chemistry.

The white material inside looked like what you’d expect from crushed ice, but it had been transformed.

The material was now denser and much of the crystalline structure had been destroyed, producing an amorphous material. The density, however, did not correspond to the already known amorphous ices of high and low density. Interestingly, it stayed in the middle; in fact, it was almost exactly the same density as liquid water. Until now, all solid forms of ice, crystalline or amorphous, have been significantly denser or less dense than liquid water.

The researchers called it medium-density amorphous ice, or MDA.

The hitting of the steel balls applied a shear force to the ice crystals, enough to knock the water molecules out of their crystalline positions, allowing them to pack more firmly together.

“It’s really cool,” said Marius Millot, a physicist at Lawrence Livermore National Laboratory in California, who led the experiment that created superionic water. “What this tells us is that there are still a lot of things we don’t understand.”

The fact that medium-density amorphous ice has about the same density as liquid water raises the possibility that it really is glass, a liquid cacophony of molecules flowing until it cools, slows down, and freezes in time without crystallizing, still disorderly.

“That’s the key question,” Salzmann said. “The MDA is the glass of liquid water?”

Follow-up experiments can add impurities to the ice. “We did the experiments with pure ice,” said Salzmann. “The next question is, what happens if we start mixing other things together?”

The findings could be useful to planetary scientists. Temperatures drop within what is found on Europa, and Jupiter exerts enormous tidal forces on the icy oceanic moon, which will be visited and studied closely by NASA and European orbiters.

“You get exactly the same kind of shear motion,” Salzmann said. “The speculation now is that there could be some MDA in the outer solar system.”

The researchers also discovered a property of MDA that is unique among water ices. For most materials, if you compress it and then release the pressure, it just goes back to how it was before. But compressing the MDA and then releasing the pressure and heating it released a huge burst of energy.

That energy, released as amorphous ice recrystallizes, can trigger ice earthquakes, for example.

This means that perhaps the physics of new ice could play a role in the formation of Europa’s icy crust and the dynamics of ice further down in the Moon’s ocean, with implications for whether conditions there could be conducive to life.

^{Translated by Luiz Roberto M. Gonçalves}