Water, the main component of our body and essential for life on Earth, can be in a solid, liquid or gaseous state. When frozen, it can assume a multitude of forms: 20 crystalline phases are known, where H2O molecules are regularly arranged in a hexagonal network, and at least two families of amorphous ice, without such an ordered crystalline structure. On our planet, almost all the frozen water known to us is in crystalline form, but on the scale of the Universe, amorphous is considered the most common.
Amorphous ice has been discovered with an average density close to that of liquid water, which has implications for understanding water anomalies and how it works in the universe.
In general, amorphous ice is divided into groups: low density (0.94 grams/cm3) and high density (1.13 g/cm3 or more). However, neither these nor the crystalline lenses present a form with an average density close to that of liquid water (1 g/cm3), at least until now. This density gap forms a cornerstone of our current understanding of water.
But researchers at University College London (UCL) have just shown that ordinary ice at nearly -200°C can be crushed to produce medium-density amorphous ice (MDA) with a density of 1.06 g/cm3, i.e. much close to that of liquid water. The study was published in the journal Science.
According to the authors, this discovery suggests that water is more complex at low temperatures than previously thought, which has implications not only for a better understanding of this substance and its curious and unexplained anomalies (it does not sink in the solid state, high heat specificity, good conductivity…), but also how water exists and works in the universe.
A cold ball mill
To create this previously unknown amorphous form of ice, the team used the low-temperature ball milling method, a process that involves vigorously shaking a cryogenically refrigerated container filled with crushed frozen water and steel balls. It is commonly used to create other amorphous materials such as metallic alloys and inorganic compounds, but it has not been applied to ice before.
“As soon as you start shaking the ice with the ball bearing, the MDA starts to form, but to get a good conversion, we shake it for about a day”, explains one of the authors, Christoph Salzmann, to SINC. “Then – he adds – the sample is placed back in a container with liquid nitrogen at –200 °C. At that temperature, it would last forever. Only when you warm up do you see the transition back to normal ice.”
Using various experimental techniques and computer simulations (including random ‘shearing’ such as shear shears), the researchers were able to assess and characterize the nature of this new form of ice, revealing its distinctive structure and unique mechanical properties.
According to the authors, the discovery of this medium-density amorphous ice now raises interesting questions. The key is whether it is just a very ground or ‘cut’ crystalline state or whether it can represent the true glassy state of liquid water.
Now the question arises as to whether or not this new amorphous ice represents the true glassy state of liquid water.
“The density of MDA and liquid water are very similar. The big question is whether they are the same thing”, says Salzmann, who anticipates the next steps to find out: “computer simulations will be very important in the future, and I think we will need high-resolution electron microscopy for the experimental part”.
Whatever the exact structure of this new amorphous ice, the authors think it plays a role in the geology of ice at low temperatures, for example, on the many icy moons of the solar system and beyond.
Similar process on icy moons
“The amorphous ice that exists in the universe is of low density, has been known for 90 years and is the most abundant version in the cosmos. It forms when water condenses onto dust grains in space,” explains Salzmann, “but this medium-density MDA may exist inside icy moons, where the ‘shear’ forces act in a similar way to the grinding of balls.
This medium-density amorphous ice can exist in the interior of icy moons, where ‘shear’ forces similar to ball milling at low temperatures act.
Christoph Salzman (UCL)
The tidal forces that, by gravitational attraction, induce the gas giant planets on these icy moons can cause this process. As a result, under certain conditions, medium density ice can be generated from another type.
The reverse process could even occur and, when heated to certain pressures, MDA could be recrystallized, releasing energy. This could play a role in triggering tectonic movements.
“We think this might be relevant to the geophysics of icy moons,” says Salzmann, “however, to produce energy on Earth, temperatures would simply be too low, as the vast amount of heat is released at minus 120 degrees.”
Alexander Rosu-Finsen, Christoph G. Salzmann et al. “Medium density amorphous ice”. Science, 2023