When you think of very low temperatures, one of the first things that usually comes to mind is ice floes. Ice is the perfect “cold” entity for us. But at extreme pressures and temperatures, such as in the cores of major planets, something strange can happen. In such an environment, even though there is a higher temperature than the surface of the Sun, the ice can remain in a solid state.
This type of water ice is called “superionic ice” and is on a list of about 20 phases that water can form structurally, including ice, liquid and steam. The researchers report the discovery and characterization of two superionic ice phases in Nature Physics, noting that they have found a way to reliably and consistently recreate ice over longer periods of time than has been achieved before to be able to work on it.
A superionic phase occurs at a temperature range of 200,000 to 600,000 times the atmospheric pressure at sea level and a few hundred to over 1,000°C. The other extends to half the pressure found in the center of the Earth and temperatures rise to thousands of degrees.
“It was a surprise – everyone thought this phase wouldn’t occur until much higher pressure than where we first found it,” said co-author Vitali Prakapenka, a University of Chicago research professor and Argonne National Laboratory Advanced Photon Source beamline scientist, in a statement. Thanks to the powerful tool, we were able to very accurately map the properties of this new ice forming a new phase of matter.”
At higher temperatures and incredible pressures, ice remains solid, but its atomic structure changes dramatically. When the pressure and temperature are removed, the ice returns to its normal state.
“Imagine a cube, a lattice with oxygen atoms at its corners,” Prakapenka says, and continues: “When this transforms into the new superionic phase, the lattice expands and allows the hydrogen atoms to move while the oxygen atoms remain stationary in their positions. Solid standing in an ocean of floating hydrogen atoms. like an oxygen cage.”
Superionic ice is less dense than regular ice, which we already know is less dense than liquid water. It also changes color. Water ice can be transparent to cloudy white depending on how it is frozen, while superionic ice appears darker because it interacts differently with light.
“It’s a new state of matter, so it behaves like a fundamentally new material, and it may be different from what we thought,” Prakapenka says.
Planetary scientists believe that similar extreme conditions in pressure and temperature may exist on Neptune and Uranus, as well as on other icy giant planets beyond the Solar System. Understanding the properties of superionic ice can help us understand the properties of these planets.
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