How ice forms is a mystery — now scientists are cracking the case

Most ice crystals have a hexagonal symmetry, which can be seen using a light microscope. Credit: Frank Fox/Science Photo Library

Beneath a suburb of Hamburg in Germany, an underground particle accelerator propels electrons at close to the speed of light through a slalom course of magnets. Racing through the twists and turns, the electrons emit bursts of radiation that produce one of the world’s most powerful X-ray laser beams.

This prized machine, the European X-ray Free Electron Laser Facility (XFEL), has helped researchers to make ultrafast movies of chemical reactions and to map the atomic structures of viruses. Now, they are using it to crack the secrets of a seemingly simple process that has bedevilled scientists for decades: how water and other liquids freeze.

For 150 years, theorists have been trying to explain the process that turns pure liquids into solids. But their models of how quickly this happens are often wildly inaccurate when compared with experiments — results can be off by as much as 20 orders of magnitude.

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