How to make hot ice

Using an enormously powerful laser to vaporise diamond, a team of American researchers have blasted water into “hot ice”, an exotic crystalline state in which water may exist on Saturn’s moon Titan and on planets beyond our solar system.

To understand why hot ice might exist elsewhere on other worlds, let’s go back a step.

When you pour a glass of water from the tap, it’s easy to forget that simple H₂O is a molecule with rich and complex chemistry. Many of the properties of water that we accept as normal are actually strange and exotic in the chemical world.

It is unusual, for example, for a liquid to expand when it freezes. Most shrink. So the fact that icebergs float in water is actually quite odd. Water’s remarkably high surface tension, which allows insects to walk across it like rubber, is also unprecedented among liquids that are not ionic or metallic.

Another extraordinary property of water is that it can freeze in myriad different ways, leading to 17 known phases (or different crystalline forms) of ice. At Earth’s temperature and pressure, almost all ice is in a hexagonal structure known as ‘ice I_h’.

But if we venture to planets where more extreme pressures are found, we might encounter a variety of different phases of ice. One of those is ‘ice VII’, which is thought to make up the ocean floors of Titan. Ice VII is formed at pressures of greater than 30,000 Earth atmospheres. It is stable across a wide range of temperatures, even exceeding 350°C.

Scientists have long studied the phase changes of ice, but have been restricted by the incredibly fast speeds at which they occur. Now, Arianna Gleason, with the Shock and Detonation Physics Group at Los Alamos National Laboratory in New Mexico, and colleagues have developed a way to follow the formation of ice VII over the course of six nanoseconds. The research is published in Physical Review Letters.

The technique uses the world’s most powerful laser, at the SLAC National Accelerator Laboratory, to vapourise a layer of diamond next to a sample of water. This laser blast generates more than 50,000 atmospheres of pressure, enough to convert the water into ice VII.

As ice VII is forming under this extreme pressure, a second laser delivers pulses of X-ray radiation at femtosecond intervals (a femtosecond is 10^-15 seconds, or one-quadrillionth of a second).  By analysing the diffraction of these X-ray pulses, a series of snapshots could be taken of the changes to the water’s structure.

The researchers were able to observe the formation of an initial “slush” of water and ice VII, and then mathematically define the transition process. This will help in understanding the nature of water-rich moons and exoplanets, and will also help scientists model the collision of large water-containing objects such as comets.