If you think of very low temperatures, there's a good chance you are picturing ice. Ice is a quintessential "cold" thing for us. But at extreme pressures, like in the core of large planets, something peculiar can happen. Ice can remain solid but have a temperature hotter than the surface of the Sun.

This type of water ice is called "superionic ice" and has been added to the list of around 20 phases water can structurally form, including ice, liquid, and vapor. Now, researchers report in Nature Physics the discovery and characterization of two superionic ice phases, having found a way of reliably and stably recreating the ice for longer than has previously been achieved to be able to study it. 

One superionic phase extends between 200,000 and 600,00 times the atmospheric pressure at sea level and at a temperature of several hundred to over 1,000°C. The other phase extends to half the pressure experienced at the center of the Earth and with temperatures of thousands of degrees.

“It was a surprise — everyone thought this phase wouldn’t appear until you are at much higher pressures than where we first find it,” co-author Vitali Prakapenka, a University of Chicago research professor and beamline scientist at the Advanced Photon Source at the Argonne National Laboratory, said in a statement. “But we were able to very accurately map the properties of this new ice, which constitutes a new phase of matter, thanks to several powerful tools.”

At higher temperatures and incredible pressures, the ice remains solid but the atomic structure is dramatically shifted. Once the pressure and temperature are removed though, the ice return to its regular state.

“Imagine a cube, a lattice with oxygen atoms at the corners connected by hydrogen,” Prakapenka said. “When it transforms into this new superionic phase, the lattice expands, allowing the hydrogen atoms to migrate around while the oxygen atoms remain steady in their positions. It’s kind of like a solid oxygen lattice sitting in an ocean of floating hydrogen atoms.”

Superionic ice is less dense than regular ice, which we know already to be less dense than liquid water. It also changes color. While water ice can be transparent to cloudy white, depending on how it freezes, superionic ice is darker as it interacts with light differently.

“It’s a new state of matter, so it basically acts as a new material, and it may be different from what we thought,” Prakapenka said.

Planetary scientists believe similar extreme conditions in pressure and temperatures might exist inside Neptune and Uranus, as well as other ice giant planets beyond the Solar System. Understanding the properties of superionic ice might help us understand the properties of these planets.