Water has many unique properties: it exists in three phases, it reaches its highest density at four degrees Celsius, and it exists in different forms of ice. A team led by Christoph Salzmann from University College London is now adding another type of ice to these, as described in “Science”: medium-density amorphous ice (MDA, medium-density amoprhous ice) in contrast to amorphous ice with high or low density .
“We know of 20 crystalline forms of ice, but only two main types of amorphous ice have been discovered so far: high-density and low-density amorphous ice. In between there is a huge density gap, and the conventional wisdom was that there is no ice within this density gap,” says Salzmann. For their experiment, the working group placed crystalline ice with steel balls cooled to minus 200 degrees Celsius in a kind of shaking machine that not only crushed the ice with its original hexagonal crystal structure, but even reshaped it.
Instead of the expected white powder in the usual form, the process produced the MDA, which had the same density as liquid water. So it could be water that behaves like a glass that continues to behave like a liquid even at extremely low temperatures. For short periods it appears solid like a glass, but for very long scales it flows like a highly viscous liquid.
The difference in densities between the previously known amorphous bodies of ice has led scientists to believe that water does indeed exist as two liquids at very cold temperatures: theoretically, at a given temperature, both liquids could coexist, with one type floating above the other – similar to a mixture of oil and water. This hypothesis was demonstrated in a computer simulation but not confirmed by experiments. According to the researchers, their new study now raises questions as to whether this hypothesis is viable at all.
On Earth, amorphous ice is extremely rare or naturally non-existent. However, Salzmann and Co. speculate that MDA could exist on the icy moons of Saturn and Jupiter. The tidal forces of the two gas giants can exert similar shearing forces on ordinary ice as the steel balls in the experiment. If you heat MDA and let it recrystallize again, it also releases a lot of thermal energy. Transferred to an ice moon like Ganymede, this might trigger tectonic movements and “icequakes” in its kilometer-thick layers of ice, which could be detected with satellites.
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The original of this article “Ice-cold steel balls create a new type of ice” comes from Spektrum.de.
