Scientists have created a new type of ice that matches the density and structure of water, possibly opening the door to studying water’s mysterious properties.
“It could be liquid water frozen in time,” says Martin Chaplin, a water structure specialist at London South Bank University who was not involved in the work. “It could be very important.
The ice is called amorphous ice of intermediate density. The team that created it, led by Alexander Rosu-Finsen of University College London (UCL), shook ordinary ice in a small container with centimeter-wide stainless steel beads at temperatures of -200ËšC to create a variation that had never been seen before. The ice looked like a white granular powder that stuck to the metal balls. The results were published today in Science1.
Random molecules
Normally, when water freezes, it crystallizes and its molecules arrange themselves into the familiar hexagonal solid structure we call ice. Ice is less dense than its liquid form – an unusual property for a crystal. Depending on conditions such as pressure and rate of freezing, water can also solidify in any of two dozen other regular arrangements. Amorphous ice is different: it has no such order. “You have a lot of molecules that randomly come together,” Chaplin says.
Two types of amorphous ice had previously been discovered, both in the twentieth century. Amorphous “low-density” ice is the result of water vapor freezing on a very cold surface at a temperature below -150 ËšC; “high-density” amorphous ice is formed by compressing ordinary ice at similar temperatures under high pressure.
Although neither type is common on Earth, both are abundant in space. “Comets are large chunks of low-density amorphous ice,” says Christoph Salzmann, a UCL chemist and co-author of the latest paper.
The team used a ball mill, a tool commonly used to grind or mix materials in mineral processing, to grind the crystalline ice. Using a container with metal balls inside, they shook a small amount of ice about 20 times per second. The metal balls created a ‘shear force’ on the ice, says Salzmann, breaking it down into a white powder.
Firing X-rays at the powder and measuring them as they bounce back – a process known as X-ray diffraction – allowed the team to work out its structure. The ice had a molecular density similar to liquid water, with no apparent ordered structure of molecules — meaning the crystallinity was “destroyed,” Salzmann says. “You’re looking at very disordered material.
The results are “quite convincing,” says Marius Millot, a physicist at Lawrence Livermore National Laboratory in California. “This is a great example of how we still have to get along with water.”
The results matched models created by scientists from a team at the University of Cambridge in the UK that predicted what would happen if regular ice was broken in this way. However, it is unclear whether the resulting powder actually matches the properties of liquid water, given that it was previously frozen as crystalline ice. An investigation that will require further work.
Big consequences
If confirmed, the new form of ice could allow the study of water in a way not previously possible. “Liquid water is a strange material,” says Chaplin. “We still don’t know as much about it as we’d like to. For example, water is commonly assumed to consist of two forms, low-density and high-density water, which correspond to previously known variants of amorphous ice. The discovery of intermediate-density amorphous ice could challenge this idea.
If the intermediate-density amorphous ice is indeed associated with liquid water, it would mean that this model is incorrect,” says Salzmann. “It could open a new chapter in ice research.”
There are also implications for understanding other worlds. Some moons in our solar system, such as Jupiter’s moon Europa and Saturn’s moon Enceladus, have icy surfaces. If two ice regions on such a moon were to rub against each other due to tidal forces, they could produce medium-density amorphous ice between them by the same shearing process used by the scientists.
The increase in density could create gaps in the surface, causing disruption of the moons as the ice cracked together. “There would be a massive collapse of the ice,” says Salzmann. “This could have serious implications for the geophysics of icy moons.”
This in turn could have implications for the potential habitability of the liquid water oceans that lie beneath the icy surfaces on these moons. “One of the key things about these moons is whether you can have an interface between liquid water and rocks – this is where life could emerge,” says Millot. “Amorphous ice could have a role that we need to understand.”
Reference : https://www.nature.com/articles/d41586-023-00293-w
