Focus: In Still Waters, Protons Run Deep
At high temperatures and under extreme pressures, water’s hydrogen nuclei can roam and conduct electricity, as electrons do in a metal, a team of physicists and chemists reports in the 1 April PRL. Through computer simulations and experiments, the team has found clear hints of a long-sought “superionic” solid phase of water. The discovery could help explain what powers the magnetic fields of Neptune and Uranus, planets that harbor large amounts of “hot ice” in their depths.
Scientists have observed superionic states in other materials. In certain crystals, some charged atoms can move freely, while other atoms remain fixed in a lattice. Computer simulations in the 1980s and the 1990s suggested that a similar phase could exist in water: Oxygen atoms would freeze in an irregular lattice, while hydrogen nuclei–each of which consists of a single proton–could jump from one oxygen to the next. Wandering protons could conduct electricity, something pure water or ice won’t do under normal conditions. Experiments involving explosively-created shock waves have shown electrical conductivity in water but were unable to confirm the superionic state.
Alexander Goncharov of the Lawrence Livermore National Laboratory in California and his colleagues have performed new supercomputer simulations that predict a superionic state in water, but in less extreme conditions than previously thought. To check their models, the team squeezed a droplet of water between two diamond tips to pressures of hundreds of thousands of atmospheres, where water forms ice even at high temperatures. The researchers then melted the mini ice cube by heating it with a laser beam to 1000 degrees Kelvin and more. To measure the melting point, they monitored the molecules’ vibrations by observing their scattered light from another laser–a set-up no one had tried before. Above a critical pressure of about half a million atmospheres, the molecular vibrations showed two abrupt changes at distinct temperatures during heating, instead of a single change at the melting point. An intermediate state between normal ice and liquid water appeared precisely where the simulations predicted a superionic state should be.
The team does not have direct evidence that this intermediate phase is superionic, but if the simulations are correct, protons can move around at high speeds and conduct electricity. They might even provide the electric currents inside the planets Neptune and Uranus that generate the intense magnetic fields discovered by NASA’s Voyager 2 probe. “Previously it was thought that these currents were related to the presence of liquid phases,” Goncharov says, but the new work suggests that the superionic state can exist at the pressures inside these planets.
The results are “a major step forward in our understanding of water under extreme conditions,” says Anders Nielsson of the Stanford Synchrotron Radiation Laboratory in California. Russell Hemley of the Carnegie Institution of Washington, DC, agrees. “These are beautiful measurements and calculations, but more needs to be done to confirm the superionic phase.” He says a conclusive proof will only come from a direct measurement of conductivity. He also points out that Earth’s mantle may contain significant amounts of water, some of which could be in the superionic state. Hemley and his collaborators recently recorded experimental data similar to Goncharov’s, but his team hasn’t yet published its results.