Water ice has many crystalline phases, along with a few amorphous structures. Amorphous ice, although rare on Earth, is the primary type found in space. They govern several cosmological processes and are potentially key materials for explaining the anomalies of liquid water. Considering the widespread importance of ice, it is important to understand the complex structural diagram.
Now, scientists at UCL and the University of Cambridge have discovered a new type of ice that resembles liquid water more closely than any other known ice, which may rewrite our understanding of water and its many anomalies. The newly discovered ice is amorphous: Its molecules are disorganized. They need to be properly ordered as ordinary, crystalline ice.
In a jar frozen to -200 degrees Celsius, scientists employed a technique known as ball milling, aggressively shaking common ice and steel balls. Ball milling is used in several industries to grind or blend materials, but it has yet to be applied to ice.
In the study, liquid nitrogen was used to cool a grinding jar to -200 degrees Centigrade, and the density of the ball-milled ice was determined from its buoyancy in liquid nitrogen. Scientists used several other techniques, including X-ray diffraction and Raman spectroscopy, to analyze the structure and properties of ice. They also used small-angle diffraction to explore its long-range structure.
They discovered that the procedure produced a novel amorphous type of ice, unlike all other known ices, which had the same density as liquid water and whose condition resembled water in solid form rather than microscopic chunks of ordinary ice. Their novel ice was given the term medium-density amorphous ice (MDA).
They also looked into the heat released when the medium-density ice recrystallized at warmer temperatures using calorimetry. They discovered that compressing and heating the MDA caused it to recrystallize with a significant energy release, demonstrating that water can be a high-energy geophysical material that may be responsible for the tectonic processes of the solar system‘s ice moons.
As tidal pressures from gas giants like Jupiter and Saturn may impose similar shear stresses on common ice as those caused by ball milling, the scientists proposed that MDA, which resembles a fine white powder, may exist inside ice moons of the outer solar system.
The scientists also discovered that MDA produced an unusual amount of heat when it warmed up and recrystallized, which meant it might cause tectonic vibrations and “icequakes” in the kilometers-thick layer of ice covering moons like Jupiter’s Ganymede.
Senior author Professor Christoph Salzmann (UCL Chemistry) said: “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. There is a huge density gap between them, and the accepted wisdom has been that no ice exists within that density gap. 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.”
Scientists believe that water actually exists as two liquids at shallow temperatures and that, theoretically, at a specific temperature, both of these liquids could coexist, with one type floating above the other, as when mixing oil and water. The density gap between the known amorphous ices supports this theory. This theory has been proved in a computer simulation rather than by experimentation.
According to scientists, their new study may raise questions about the validity of this idea.
Scientists proposed that the newly discovered ice may be the true glassy state of liquid water. It is a replica of liquid water in solid form. Another scenario is MDA is not glassy at all but is in a heavily sheared crystalline state.
Co-author Professor Andrea Sella (UCL Chemistry) said: “We have shown it is possible to create what looks like a stop-motion kind of water. This is an unexpected and quite amazing finding.”
Lead author Dr. Alexander Rosu-Finsen, who carried out the experimental work while at UCL Chemistry, said: “We shook the ice like crazy for a long time and destroyed the crystal structure. 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.”
By mimicking the ball-milling procedure via repeated random shearing of crystalline ice, the team also created a computational model of MDA. Dr. Michael Davies, who carried out the computational modeling while a Ph.D. student in the ICE (interfaces, catalytic & environmental) lab at UCL and the University of Cambridge, said: “Our discovery of MDA raises many questions on the nature of liquid water and so understanding MDA’s precise atomic structure is very important.”