Once an ice skater gets going, friction between her skate and the ice creates a microscopic layer of water that allows the skate to hydroplane.

But before she can get up to speed, and friction can melt the ice, it’s still slippery enough for her to start her glide. Why is ice so slippery?

In the 1800s, scientist Michael Faraday conducted experiments to show that ice, even well below freezing, has a very thin layer of water on its surface.

But the technology to see this layer did not exist. Nor did the scientific understanding to prove that it was there.

It would be more than 100 years before scientists could finally see Faraday’s water layer using X-ray imaging. And still later that they could measure it.

Turns out this thin layer is very thin indeed—thousands of times thinner than a sheet of paper. In fact, it’s just a couple of molecules thick.

When water freezes, its molecules interlock tightly to create the crystalline structure of ice, held together by four hydrogen bonds.

But the molecules on the surface of ice can only bond to the molecules just beneath them, with just three hydrogen bonds. This won’t allow a stable crystalline surface.

This strange, disordered molecular state of water on the surface of ice will persist down to -36o Fahrenheit.

But if the temperature goes below that, ice will no longer be slippery—sometimes with disastrous results, which we’ll talk about on another EarthDate.