Water can take on far more forms than many people give it credit for, and now scientists have recreated a particularly bizarre one in the lab – a “hot black ice” that may exist deep inside planets like Uranus and Neptune.
This phase of water is known as superionic ice, and it forms under extremely high temperatures and pressures, causing the water molecules to separate into their constituent hydrogen and oxygen ions. The oxygen ions then arrange themselves into a cube-shaped lattice, around which the hydrogen ions move freely. Altogether, this gives superionic ice a relatively high conductivity and low density, and a darker color.
Actually studying the stuff has been tricky though. The phase has been theorized for decades, and experimental evidence began emerging in the 1990s, but it wasn’t until 2019 that scientists managed to produce superionic ice in the lab. However, in that experiment it only lasted a split second.
Now, scientists at Argonne National Laboratory have managed to make stable superionic ice, which lasts long enough to study in more detail. First, pressure is applied by squeezing a sample of water in a diamond anvil cell, then the water is heated using lasers. Finally, the team uses a high-energy X-ray light source facility called the Advanced Photon Source (APS) to image the arrangement of the atoms in the sample, to figure out what kind of phase the water is in.
And sure enough, the studies showed that the experiment was producing superionic ice. It began appearing at temperatures between 627 °C and 1,627 °C (1,160 °F and 2,960 °F), and at pressures of 20 GigaPascals. Intriguingly, that’s a much lower pressure than models had predicted for this phase to form.
“It was a surprise – everyone thought this phase wouldn’t appear until you are at much higher pressures than where we first find it,” says Vitali Prakapenka, co-author of the study. “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.”
Producing superionic ice in the lab is more than a curiosity – studying it could help us understand how planets form and even inform where we look for alien life. It’s thought that this kind of ice would be found on ice giant planets like Uranus and Neptune, and if so slushy mantles of superionic ice could generate the magnetic fields of these worlds.
The team says that far more work still needs to be done to explore what makes superionic ice tick. Properties like its conductivity, viscosity, and stability remain murky, and things could change dramatically when it’s mixed with salts or other minerals.
The research was published in the journal Nature Physics.
Sources: Argonne National Laboratory, Nature
