We might need to rethink our understanding of water, both on Earth and other planets. New experiments with water ice at high pressures has revealed unexpected behaviour, which could upend our assumptions about the makeup of聽icy exoplanets.
There are two ways to make ice: by lowering the temperature, as in a freezer, or by raising the pressure, essentially compressing the water into a solid. Ashkan Salamat at the University of Nevada Las Vegas and his colleagues used the second method, crushing water between a pair of diamonds to examine how the H2O molecules bond together at high pressures.
They found that at a pressure of around 31 gigapascals 鈥� almost 300 times the pressure at the bottom of the Mariana Trench 鈥� the crystal structure of the ice changed to a form called ice-X and the internal bonds got stronger. That made the ice itself harder.
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鈥淭hat transition in the way that the two hydrogens and the oxygen are bound together makes it less compressible, you can鈥檛 squish the water as much,鈥� says Michele Bannister at Queen鈥檚 University Belfast. That means our models of planets that contain water will be very different.
Inside Earth, for example, water could exist deep in the mantle, far deeper than we thought, says Salamat. There may actually be a reservoir of ancient water deep under our feet.
For exoplanets, the effect of this dense ice will vary. For some for which we know their size, it may mean that they have less water because if ice can鈥檛 compress, less of it can fit in the same space. For others, it may mean that they have more, because this ice can have characteristics that we previously mostly attributed to rocks.
鈥淲hat looks like a very simple system actually shows this very complex behaviour,鈥� says Salamat. 鈥淏ecause of that, we don鈥檛 know how much water is really in these exoplanets.鈥�
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