Water ice has many crystalline phases, along with some amorphous structures. Amorphous ice, although rare on Earth, is the primary type found in space. They control several cosmological processes and are potentially important materials for explaining the anomalies of liquid water. Given the widespread importance of ice, it is important to understand its complex structural diagram.

Now scientists at UCL and the University of Cambridge have discovered a new type of ice that is more like liquid water than any other known ice, which could rewrite our understanding of water and its many anomalies. The newly discovered ice is amorphous: the molecules are disorganized. They must be properly arranged like regular, crystalline ice.

In a jar frozen to -200 degrees Celsius, scientists applied a technique known as ball milling, aggressively shaking ice and steel balls. Ball milling is used in various industries to grind or mix 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 Celsius, 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 narrow-angle diffraction to explore the long-range structure.

They found that the procedure produced a new amorphous type of ice, unlike any other known ice, which had the same density as liquid water and whose state resembled solid water rather than microscopic pieces of regular ice. Their new ice was given the term medium density amorphous ice (MDA).

They also looked at the heat released when the medium-density ice recrystallized at warmer temperatures using calorimetry. They found that compressing and heating the MDA caused it to recrystallize with a significant release of energy, showing that water may be a high-energy geophysical material that may be responsible for the tectonic processes of the solar system’s icy moons.

Since the tidal pressure of gas giants like Jupiter and Saturn can create similar shear stresses on ordinary ice to those caused by ball milling, the scientists suggested that MDA, which resembles a fine white powder, may exist in icy moons of the outer solar system.

The scientists also found that MDA produced an unusual amount of heat as it warmed and recrystallized, meaning it could cause tectonic tremors and “icequakes” in the miles-thick layer of ice that covers moons like Jupiter’s Ganymede.

Just crystalline ice
Ordinary crystalline ice (left) and MDA (right) at the atomic scale. Credit: Michael Davies

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 been previously discovered, known as high-density and low-density amorphous ice. There is a huge density gap between them, and the generally accepted wisdom is that within that density gap there are no ice exists. Our study shows that the density of MDA is right within this density gap, and this finding could have far-reaching implications 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 liquids can coexist, with one type floating above the other, as in the mixing of oil and water. The density gap between the known amorphous ices supports this theory. This theory has been proven in a computer simulation rather than by experiments.

According to scientists, their new study may raise questions about the validity of this idea.

Scientists suggested that the newly discovered ice could be the true glassy state of liquid water. It is a replica of liquid water in solid form. Another scenario is that MDA is not glassy at all, but is in a highly cracked crystalline state.

Co-author Professor Andrea Sella (UCL Chemistry) said: “We have shown that it is possible to create water that resembles a kind of stop-motion. This is an unexpected and quite astonishing 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. Instead of ending up with smaller pieces of ice, we realized we had come up with a whole 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 computer model of MDA. Dr. Michael Davies, who performed the computational modeling while pursuing a Ph.D. student in the ICE (Interfaces, Catalytic and Environmental) Laboratory at UCL and the University of Cambridge, said: “Our discovery of MDA raises many questions about the nature of liquid water and so it is very important to understand the precise atomic structure of MDA.”

Magazine reference:

  1. Alexander Rosu-Finsen, Michael B. Davies et al. Medium density amorphous ice. Science. DOI: 10.1126/science.abq2105