by Valerie Wong (’24) | March 10, 2023
On February 2, chemistry professor Christoph Salzmann and researcher Alexander Rosu-Finsen published a study about their accidental discovery of a new form of amorphous ice with a density similar to that of water. Their discovery could have huge implications for the study of water’s mysterious properties and the use of water in outer space.
Most substances become denser as their temperature decreases. However, frozen water is an exception to this rule. Ice forms a hexagonal crystalline structure as it cools, becoming less dense than its liquid counterpart and explaining why ice floats in cold drinks.
There are twenty known crystalline structures of water, created by varying temperatures and pressures. Scientists have also previously discovered two types of non-crystalline, or amorphous, ice. One type is a high-density amorphous ice, formed by compressing ordinary ice at low temperatures under high pressure. The other has low density and forms when water vapor freezes on a cold surface like a comet. Though uncommon on Earth, these forms of ice are often found in outer space.
Salzmann and his team originally sought to study tiny ice crystals for their unique properties. They conducted an experiment at University College London, shaking normal ice with stainless steel balls at extremely cold temperatures (-200℃) in a process called “ball milling,” which is usually used to blend materials in mineral processing. This procedure would theoretically break up the larger ice crystals into smaller ones.
However, Salzmann found something new when he inspected the white, granular powder created by the process. Using X-ray diffraction, he discovered that the material was actually a form of amorphous ice with a density similar to that of liquid water. The stainless steel balls applied a “shearing force” to the crystals, causing them to lose their crystalline structure.
This discovery is especially relevant to the study of water, a unique substance because it does not follow the rules that other liquids do—one example being its aforementioned decrease in density as it freezes. Another is its behavior as a supercooled liquid, or a liquid at temperatures below freezing. When water is supercooled, it is thought to be both a high-density and low-density liquid at the same time, which corresponds to the high-density and low-density amorphous ices that have been discovered. The new medium-density amorphous ice, however, raises new questions.
One theory is that the amorphous ice might be a “glass,” or a flowing liquid that is cooled until its molecules become extremely slow but remain un-crystallized. If this is true, it would provide another piece of information that could lead to a better understanding of the strange properties of water.
Another point of interest is the unique properties of the medium-density amorphous ice. When compressed and then decompressed, the ice released a large amount of energy as it recrystallized. This behavior could be present on icy moons like Europa, a moon of Jupiter.
A similar effect to the shearing force of the steel balls occurs on Europa’s icy surface as a result of Jupiter’s gravitational pull. The energy released as amorphous ice recrystallizes could affect the shaping of Europa’s crust and liquid-water oceans underneath. By changing the dynamics of water, the amorphous ice could offer insight into potential life under Europa’s surface.
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