Nancy Huang

Feb 9th 2022

Superionic Ice May Be the Most Common Form of Water in the Solar System


It can be hard to imagine what happens to common molecules under the extreme conditions on other planets. For example, Earthlings know water as an essential ingredient for life that exists as a solid, liquid and gas. However, scientists examining how water behaves under different conditions have identified about 20 different types of ice. The most recent addition to this list is “superionic ice,” which is thought to be the main form of ice on water-rich planets like Uranus and Neptune.

A Solid and a Liquid

Superionic ice is unique because the oxygen atoms lock into place like a solid, but the hydrogen atoms give up their electrons to become positively charged ions that flow through the oxygen lattice like a fluid. Because the water molecules break apart — unlike other known types of ice — some scientists consider it a new state of matter rather than a new phase of water.

The existence of superionic ice was first proposed in 1988 based on theoretical calculations, as reported in Physical Review Letters. The highly unusual structure suggested that it could conduct electricity like a metal, with the swimming hydrogen ions playing the role usually assumed by electrons. The fluidity of the hydrogen ions would also allow this ice to absorb a lot of heat before melting. Therefore, it would remain a solid at temperatures hotter than the surface of the sun, which exist at the core of large planets.

Superionic ice

Experimental Confirmation

In 2019, scientists were finally able to create the extreme temperatures and pressures required to produce this type of ice in a laboratory and published the results in Nature. The technique involved blasting a water droplet with a high-pressure shock wave, raising the pressure to 100-400 gigapascals and the temperature to 2,000-3,000 Kelvin. For comparison, the center of the Earth has a pressure of 360 gigapascals, and the surface of the Sun has a temperature of 5,778 Kelvin.

Superionic ice formed under these experimental conditions but only lasted for about 20 nanoseconds (20 billionths of a second). That was enough time to use X-ray diffraction to confirm the predicted structure of the oxygen lattice, and a few additional features.

In November 2021, a different research group developed a more reliable method for producing this new form of ice, as Nature Physics reports. They squeezed a water droplet between two diamonds and then blasted it with powerful lasers to achieve temperatures and pressures similar to what exists at the center of the Earth. This method produced ice that lasted for microseconds (millionths of a second) instead of just nanoseconds (billionths of a second), allowing it to be studied in more detail.

The most surprising finding was that superionic ice formed at much lower pressures than originally expected (20 gigapascals versus 50 gigapascals). Using these methods, scientists have also confirmed that superionic ice is stable up to 5,000 Kelvin, conducts electricity, is extremely dense, is dark because the swimming hydrogen ions block light from passing through, and returns to being liquid water when conditions are reversed. Additional experiments will test viscosity, chemical stability and how the ice behaves when it’s mixed with salt and other molecules like methane and ammonia, which are known to exist on water-rich planets.

Explaining the Mysteries of Uranus and Neptune

This newly discovered ice forms under conditions similar to those found at the center of Neptune and Uranus, which are chemically very similar planets. These ice giants are now thought to have gaseous, mixed-chemical outer shells, a liquid layer of ionized water below and a solid layer of superionic ice comprising the bulk of their interiors and rocky centers. If this is true, superionic ice might be the most common form of water in our solar system.

This planetary structure could explain the unusual features of the magnetic fields that surround Uranus and Neptune. The other planets in our solar system with strong magnetic fields have clearly defined north and south poles, like Earth. According to Quanta magazine, these strongly defined north and south poles are due to interior regions of conductive fluids that rise and swirl as the planet rotates.

The magnetic fields of Uranus and Neptune have more than two poles, and they don’t align well with the planets’ rotation. One possible explanation is that these planets might have large solid cores surrounded by a thin liquid layer responsible for generating these odd magnetic fields. However, the idea of a large solid core didn’t seem realistic because of the huge amounts of water on these planets. Now, the characterization of superionic ice makes this a real possibility, as it can get extremely hot but remain a solid.

Discovering this new material sheds light on just how much there is still to discover in space and here at home.

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