Shaken
and chilled — but not stirred — ordinary frozen water turns into something
different: a newly discovered form of ice made of a jumble of molecules with unique
properties.
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“This is
completely unexpected and very surprising,” said Christoph Salzmann, a
chemistry professor at University College London in England and an author of a
paper published recently in the journal Science that described the ice.
Water is a simple
molecule that has been intently studied by scientists for centuries: two
hydrogen atoms jutting off at a 104.5-degree angle in a V-shape from a central
oxygen atom.
The new discovery
shows, once again, that water, a molecule without which life is not known to be
able to exist, is still hiding scientific surprises yet to be revealed. This
experiment employed relatively simple, inexpensive equipment to reveal a form
of ice that could exist elsewhere in the solar system and throughout the universe.
Water in its
many formsIn day-to-day
life, we encounter three forms of water: a vaporous gas like steam, flowing
liquid water and hard, slippery ice. The ice of our everyday lives consists of
water molecules lined up in a hexagonal pattern, and those hexagonal lattices
neatly stack on top of each other. The hexagonal structure is not tightly
packed, which is why ice is less dense than liquid water and floats.
There is even a type of water that is both liquid and solid. In 2018, scientists announced the creation of “superionic water”, which was simultaneously solid and liquid.
With permutations
of temperature and pressure outside what generally occurs on Earth, water molecules
can be pushed into other crystal structures. Scientists now know of 20
crystalline forms of water. The 20th form of ice was discovered last year.
In addition,
researchers also have documented two types of ice with jumbled molecules, what
they call amorphous materials. Because one of the amorphous ices is denser than
water, it is known as high-density amorphous ice; the other, with a density
less than that of water, is low-density amorphous ice. Amorphous ices are not
found on Earth, but they could be prevalent in outer space, in comets,
interstellar clouds and icy worlds like Europa, a moon of Jupiter.
There is even a
type of water that is both liquid and solid. In 2018, scientists announced the
creation of “superionic water”, which was simultaneously solid and liquid.
Salzmann and his
colleagues were not looking to add to the catalog of water ices. They instead
wanted to study very tiny ice crystals, because minuscule bits of something
sometimes possess properties very different from larger bits of the same stuff.
Medium-density
amorphous iceSo Alexander
Rosu-Finsen, a postdoctoral scientist in Salzmann’s research group and the lead
author of the Science paper, started smashing up ice. The water ice was first
chilled in liquid nitrogen to minus 195 degrees Celsius and then placed in a
container along with steel balls. A machine then shook the ice and steel balls,
still chilled at ultracold temperatures, back and forth at 20 times per second,
pulverizing the ice into tiny bits, a process known as ball milling.
Think of it as a
high-tech cocktail shaker.
Rosu-Finsen then
opened the container.
“Lo and behold,
something completely unexpected happened,” said Rosu-Finsen, who is now an
associate editor at the journal Nature Reviews Chemistry.
That medium-density amorphous ice has almost the same density as liquid water raises the possibility it is actually a glass, a liquid cacophony of molecules flowing until it cooled and slowed and froze in time without crystallizing
The white
material inside looked like what one would expect smashed-up ice to look like,
but it had been transformed.
The material was
now denser, and much of the crystalline structure had been destroyed, producing
an amorphous material. The density, however, did not match the already known
high- and low-density amorphous ices. Intriguingly, it fell in between; indeed,
it was almost exactly the same density as liquid water. Until now, all of the
solid forms of ice, crystalline or amorphous, were either significantly denser
or less dense than liquid water.
The researchers
named it medium-density amorphous ice, or MDA.
The banging of
the steel balls applied a shearing force on the ice crystals, enough to knock
the water molecules out of their crystal positions, allowing them to be packed
more tightly.
Water in glass
form?“It’s really
cool,” said Marius Millot, a physicist at Lawrence Livermore National
Laboratory in California who led the experiment that created superionic water.
“What it tells us is that there’s still a lot of things that we don’t
understand.”
That
medium-density amorphous ice has almost the same density as liquid water raises
the possibility it is actually a glass, a liquid cacophony of molecules flowing
until it cooled and slowed and froze in time without crystallizing, still
disordered.
“This is the key
question,” Salzmann said. “Is MDA the glass of liquid water?”
Follow-up
experiments could add impurities to the ice. “We’ve done the experiments with
pure ice,” Salzmann said. “The next question is, what will happen if we start
mixing in other things?”
For most materials, if you compress it and then release the pressure, it simply returns to how it was before. But compressing MDA and then releasing the pressure and heating it released a large burst of energy.
The findings
could be of use to planetary scientists. The temperatures fall within what is
found on Europa, and Jupiter exerts huge tidal forces on the icy ocean moon,
which will be visited and studied closely by NASA and European orbiters.
“You get exactly
the same kind of shearing motion,” Salzmann said. “The speculation is now that
there could be some MDA in the outer solar system.”
Making
icequakesThe researchers
also found a property of MDA that is unique among water ices. For most
materials, if you compress it and then release the pressure, it simply returns
to how it was before. But compressing MDA and then releasing the pressure and
heating it released a large burst of energy.
That energy,
released as the amorphous ice recrystallizes, could set off icequakes, for
example.
That means
perhaps the physics of the new ice could play a role in the shaping of the icy
crust of Europa and the dynamics of ice farther down in the moon’s ocean, with
implications for whether conditions there could be hospitable for life.
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