New form of ice could exist on ocean moons

CHEMISTS HAVE discovered a new form of ice, and their work may have major consequences for our understanding of the outer solar system.

Thanks to Earth’s temperate conditions, the solid ice we’re familiar with comes in one variety, where water molecules arrange themselves into an orderly crystalline structure. But there is another class of ice that does not contain repeated structures. This is called amorphous ice, and it is ubiquitous in the frigid realms of deep space, from molecular clouds to comets and icy moons.

For decades, scientists have known of two kinds of amorphous ice: low-density and high-density. But recently, a team of chemists created a third kind, which they called medium-density amorphous (MDA) ice.

The team’s process was simple: They put ice chilled to –340 degrees Fahrenheit (–255 degrees Celsius) into a container with small steel balls and then used a machine to vigorously shake the container back and forth 20 times per second.

“We shook the ice like crazy for a long time and destroyed the crystal structure,” said Alexander Rosu-Finsen of University College London in a press release. “Rather

than ending up with smaller pieces of ice, we realized that we had come up with an entirely new kind of thing, with some remarkable properties.” Rosu-Finsen was the first author of a paper on the new form of ice, published Feb. 2 in the journal

Nature.

While low-density amorphous ice is less dense than liquid water and high-density amorphous ice is more dense, MDA ice has nearly the same density as liquid water. It’s as if water has simply become frozen in time rather than crystalizing to conventional ice.

The researchers also found that when they warmed up MDA ice, it turned back into a regular crystal lattice, releasing a surprising amount of energy. This could have huge implications for the nature of frozen moons like Jupiter’s Europa and Ganymede.

Those worlds have thick ice shells surrounding globe-spanning oceans of liquid water — prime places to search for signs of past or present life. If MDA ice is present there, shifts in temperature could cause it to suddenly crystalize, triggering tectonic motions in the moons’ giant ice sheets and completely altering the boundary of ice and liquid in their interiors.

NASA/JPL-CALTECH CHRISTOPH SALZMANN SETTING SAIL. NASA’s Europa Clipper mission, scheduled to arrive at Jupiter in 2030, will scrutinize Jupiter’s icy moon Europa, which is known to be covered in amorphous ice. BEARING IN MIND. The key to producing mediumdensity amorphous ice was to pulverize ultra-cold crystalline ice with small steel balls.