The enigmatic outer solar system planets may harbor a strange form of water ice that is simultaneously a liquid and a solid. Now, researchers have created this odd "superionic water" here on Earth.
The experiment adds weight to a theory that superionic ice, which can conduct electricity like metal does, may explain the lopsided magnetic fields of Uranus and Neptune. The researchers describe their findings in a study published this week in the journal Nature Physics.
As Kenneth Chang reports for The New York Times, water is a deceptively simple molecule made of just three atoms: two hydrogens and an oxygen linked together in a V-shape. When many water molecules get together, their collective structure can change depending on what state they are in. In liquid, the Vs slosh around. During freezing, they link up in a crystal lattice that is more spacious, hence ice's tendency to expand.
Researchers probing water's structural properties have found almost 20 different forms of ice crystals, with water molecules arranged in configurations from disordered to hexagonal to cubic. (Fortunately, none have the lethal and ocean-freezing properties of the fictional Ice-nine dreamt up by Kurt Vonnegut for Cat's Cradle. )
Superionic ice's water molecules are packed together so tightly that their oxygen atoms form a rigid lattice, Change writes. The hydrogen atoms, carrying a positive charge, are then free to flow through it. The flowing hydrogen ions can also carry electrical current.
“It’s as though the water ice is partially molten,” Raymond Jeanloz, one of the paper's authors and a professor of earth and planetary science at the University of California, Berkeley, tells The New York Times.
To create superionic water, Jeanloz and his colleagues compressed water ice between two diamonds at Lawrence Livermore National Laboratory to 25,000 times the pressures we experience on Earth, according to a press release from the lab. The ice under pressure then took a cross-country plane ride in carry-on luggage to reach the University of Rochester's Laboratory for Laser Energetics in New York. Six intense laser beams blasted a pulse of ultraviolet light into the sample, sending shock waves that compressed the substance further as well as heated it.
At about 1.9 million times atmospheric pressure, the researchers observed a spike in density and temperature, Emily Conover writes for Science News. That change is evidence that the sample melted, just as theoretical calculations would predict if it was superionic ice. The sample also conducted electricity.
Ice giants Uranus and Neptune may carry superionic ice in their mantles, a phenomenon that would generate magnetic fields that look different from one generated at the core of a planet, as is the situation on Earth, writes Chang for The New York Times.
The experiment may also have implications closer to home. Jeanloz tells The Times that superionic ice could lead to new materials. "As one starts validating those kinds of predictions, it gives a hope that one could start thinking about engineering new materials," he says, “where you tell me what properties you want, and someone can use a computer now to figure out what kind of material, what kind of elements you have to put together, and how they’d have to be packed together to come up with those properties."