Hot Ice

Science Alert checks out “superionic ice,” an exotic form of ice that stays solid at extremely hot temperatures and is probably floating around inside the cores of Uranus and Neptune – where it’s responsible for giving them their very strange planetary magnetic fields: Superionic ice is strangely different, and yet it may be among the most abundant forms of water in the Universe – presumed to fill not only the interiors of Uranus, Neptune, but also similar exoplanets.

These planets have extreme pressures of 2 million times the Earth’s atmosphere, and interiors as hot as the surface of the Sun – which is where water gets weird.

Scientists confirmed in 2019 what physicists had predicted back in 1988: a structure where the oxygen atoms in superionic ice are locked in a solid cubic lattice, while the ionized hydrogen atoms are let loose, flowing through that lattice like electrons through metals.

This gives superionic ice its conductive properties. It also raises its melting point such that the frozen water remains solid at blistering temperatures.

In this latest study, physicist Arianna Gleason of Stanford University and colleagues bombarded thin slivers of water, sandwiched between two diamond layers, with some ridiculously powerful lasers.

Successive shockwaves raised the pressure to 200 GPa (2 million atmospheres) and temperatures up to about 5,000 K (8,500 °F) – hotter than the temperatures of the 2019 experiments, but at lower pressures.

The resulting diffraction patterns confirmed the ice crystals were in fact a new phase distinct from superionic ice observed in 2019. The newly discovered superionic ice, Ice XIX, has a body-centered cubic structure and increased conductivity compared to its predecessor from 2019, Ice XVIII.

Conductivity is important here because moving charged particles generate magnetic fields. This is the basis of dynamo theory, which describes how churning conductive fluids, such as Earth’s mantle or inside another celestial body, give rise to magnetic fields.

If more of a Neptune-like ice giant’s insides were taken up by a mushy solid, and less of by a swirling liquid, then it would change the kind of magnetic field produced.

And if towards its core that planet had two superionic layers of differing conductivity, as Gleason and colleagues suggest Neptune might contain, then the magnetic field generated by the outer liquid layer would interact with each of them differently, making things stranger still.

You can read Gleason’s research here, in Scientific Reports.