Scientists at University College London and the University of Cambridge have discovered a new type of ice that more closely resembles liquid water than any other known ices and that may rewrite our understanding of water and its many anomalies. The ice is called medium-density amorphous ice (MDA).
Key points
- For the study, published in the journal Science, the research team used a process called ball milling, vigorously shaking ordinary ice together with steel balls in a jar cooled to -200 degrees Centigrade.
- The ice appeared as a white granular powder that stuck to the metal balls. Normally, when water freezes, it crystallizes and its molecules are arranged into the familiar hexagonal, solid structure that we call ice. Ice is less dense than its liquid form — an unusual property for a crystal.
- Depending on conditions such as pressure and the speed of freezing, water can also solidify in any of two dozen other regular arrangements.
- Amorphous ice is different: it has no such order. If confirmed, the new form of ice could enable studies of water in a manner that was not possible before. It had the same density as liquid water and whose state resembled water in solid form.
- The team suggested that MDA (which looks like a fine white powder) may exist inside ice moons of the outer solar system, as tidal forces from gas giants such as Jupiter and Saturn may exert similar shear forces on ordinary ice as those created by ball milling.
- Amorphous ice, although rare on Earth, is the main type of ice found in space. That is because, in the colder environment of space, ice does not have enough thermal energy to form crystals.
- There are 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.
- The new 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.
