Imma Perfetto
Cosmos science journalist
We’ve got a new kind of ice on the block – medium-density amorphous ice (MDA).
It’s amorphous, which means that the water molecules are in a disorganised form instead of being neatly ordered like they are in the ordinary, crystalline ice you find floating in your Scotch on the rocks..
Amorphous ice is super rare on Earth, but scientists think that it might be the main type found in the frigid environment of outer space – because ice wouldn’t have enough thermal energy there to form crystals.
In a new study published in the journal Science, researchers used a process called ball milling, involving vigorously shaking normal ice together with steel balls inside a jar cooled to -200°C, to make it for the first time ever.
“We shook the ice like crazy for a long time and destroyed the crystal structure,” explains lead author Dr Alexander Rosu-Finsen, who carried out the experimental work while at the University College London in the UK.
“Rather than ending up with smaller pieces of ice, we realised that we had come up with an entirely new kind of thing, with some remarkable properties.”
The process resulted in a form of ice that looks like a fine white powder, whose disorganised state more closely resembles liquid water in solid form than a crystalline lattice.
“Water is the foundation of all life. Our existence depends on it, we launch space missions searching for it, yet from a scientific point of view it is poorly understood,” says Christoph Salzmann, professor of Chemistry at UCL and senior author of the study.
“We know of 20 crystalline forms of ice, but only two main types of amorphous ice have previously been discovered, known as high-density and low-density amorphous ices.”
Read more: Ice, ice, maybe?
Low-density amorphous ice was first discovered in the 1930s when scientists condensed water vapour on a metal surface cooled to -110°C, whereas the high-density state was discovered in the 1980s when ordinary ice was compressed at nearly -200°C.
“There is a huge density gap between them, and the accepted wisdom has been that no ice exists within that density gap,” Salzmann says.
This large density gap, and the fact that the density of liquid water lies right there in the middle, has been a cornerstone of scientists’ understanding of liquid water until now. It even led them to suggest that water exists as two different liquids at very cold temperatures – one high- and one low-density – with one floating above the other like oil and water do.
But the researchers say that their new study may raise questions about the validity of this idea.
“Our study shows that the density of MDA is precisely within this density gap and this finding may have far-reaching consequences for our understanding of liquid water and its many anomalies,” says Saltzmann.
“Existing models of water should be re-tested. They need to be able to explain the existence of medium-density amorphous ice. This could be the starting point for finally explaining liquid water.”
Amorphous ice in general is thought to only occur in the cold upper reaches of the atmosphere on Earth, but it might be much more common in space. The team says that similar to their experiment, ordinary ice may undergo similar shear forces in ice moons due to the tidal forces exerted by gas giants (like Jupiter) to form MDA.