I Didn’t Know That. . .

Q: Why is ice slippery?

A: Scientists have long pondered that problem, but it is only recently that they have obtained an answer. Using a new imaging technique called low-energy helium atom scattering, a team from the Max Planck Institute for Fluid Dynamics in Gottingen, Germany, reported in March that a very thin layer of water molecules on the surface of ice move around as if they were not frozen, even at temperatures well below freezing. This mobile layer of molecules allows most objects placed on the surface of ice to slide easily. It also explains why pollutants in the atmosphere get trapped on the surface of ice crystals--they dissolve in the water--and why two pieces of ice placed in contact freeze together. The slippery layer persisted until the ice was cooled to below -400 degrees Fahrenheit.