Two bizarre new forms of ice discovered that could exist deep inside planets

Two new types of ice may help us understand what’s going on inside Neptune and Uranus. These ice giant planets have extremely unusual magnetic fields, which may be explained by strange forms of ice deep under their surfaces.

The water ice that we are familiar with in everyday life is just one of more than 20 different phases, each of which has a different crystal structure to its molecules. Under increased temperature and pressure, the hydrogen and oxygen that make up the water rearrange, giving each phase unique electric and mechanical properties.

To search for new phases of ice, researchers use a device called a diamond anvil to crush water molecules to pressures of millions of atmospheres. Gunnar Weck at the French Alternative Energies and Atomic Energy Commission and his colleagues also heated the anvil with a laser until the samples reached temperatures in the thousands of degrees.

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The new phase of ice that they found at such high temperature and pressure is called hexagonal close-packed (hcp) ice. Its main difference from “normal” ice is the way that the hydrogen atoms move through it: “The oxygen stays in a solid lattice, while the hydrogen can move like in a liquid,” says Weck. This makes hcp ice what’s called a superionic phase, which is far more electrically conductive than other forms of ice.

That conductivity is important, because it could help explain why the ice giant planets have magnetic fields that are tilted and off-centre – if they are caused by charge flowing through superionic ices closer to the planets’ surfaces, rather than some sort of dynamo at their centres like Earth has, that could easily match what spacecraft have observed.

“The next step is to try to define, very accurately, the transition line between the different phases,” says Weck. “You need to know the domain of stability of these superionic phases to see at which depth they can be present within the giant planets.” Researchers are already working on this: Israel Osmond at the University of Edinburgh in the UK and his team have also just found a new phase of ice called ice XXII that is stable at pressures nearly double those required for hcp ice.

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Once we understand where each of these ices could exist within the ice giants, we will have to figure out how, exactly, each of them behaves, both electrically and mechanically. The type of magnetic field they can create will depend not only on how charge flows through them, but also on how they themselves move and deform at the high pressures and temperatures within the ice giants. Each new phase is one piece of this extraordinarily complex puzzle.

Journal Reference:

Physical Review Letters DOI: 10.1103/sdrk-3m4t