Scientists Discover Form of Ice Unexpectedly Similar to Liquid Water

(Credit: Christoph Salzmann/University College London)

(Credit: Christoph Salzmann/University College London) Water is the basis of life as we know it, so you’d think we would have a pretty good handle on how it works. However, researchers from Cambridge and University College London have discovered a previously unknown type of water ice, dubbed medium-density amorphous ice (MDA). It’s frozen but has a density similar to that of liquid water, presenting a number of fascinating possibilities about where we might find this material in nature.

Producing MDA might be one of the simplest techniques known to humanity. All you have to do is shake the ice really hard. But not by hand — that’s not going to cut it. The team used a process known as ball-milling. Essentially, they placed regular crystalline ice in a small metal container with steel balls. When mechanically shaken at high speed, the balls break down the crystal structure of the ice, but it remains frozen due to the low experimental temperature of -376 degrees Fahrenheit (-200 degrees Celsius).

This is the third form of amorphous ice discovered. The others are low-density and high-density amorphous ice. As the name implies, medium-density amorphous ice slots right in the middle, but scientists previously doubted it existed. The high-density variety is denser than liquid water, and low-density amorphous ice is less dense. With water in between, no one expected to find a form of amorphous ice in the same range, but that’s MDA. The team says MDA looks like a white powder, but its molecular structure has been completely shredded by the milling process to leave it with a water-like density.

A new form of ice very similar in molecular structure to liquid water (left), compared to ordinary crystalline ice (right). Credit: University of Cambridge

By simulating random shearing of ice molecules with a computer, the team created a model of MDA (see above) that shows remarkable similarities with liquid water. MDA certainly has some fascinating properties, but it’s not likely to exist in nature on Earth. However, the team points to several notable moons in our solar system as possible reservoirs of the material. Tidal forces from gas giants like Jupiter deform moons, causing tidal heating. That’s how worlds like Europa can possess liquid interiors despite being so far from the sun. The researchers say tidal flexing could produce similar shearing forces as the ball-milling process. Thus, we could find that amorphous ice is a common material in these satellites.

MDA has one more interesting property that could make it important as we explore other worlds. When it recrystallizes, MDA releases a large amount of heat. This process could play a role in activating tectonic processes and other geophysical events. Future missions like Europa Clipper might give us a better idea of how much amorphous ice is hiding in our own backyard and how it affects the possibility of life.

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