Does ice melt due to weight and/or friction of a boot, skate or tire creating a thin layer of water? Experiements are unable to prove this leading some to suggest special qualities of H2O in it’s “solid” form. You can find more in The New York Times article, Explaining Ice: The Answers Are Slippery. Free registration may be required.

The pressure-melting explanation also fails to explain why someone wearing flat-bottom shoes, with a much greater surface area that exerts even less pressure on the ice, can also slip on ice.

Two alternative explanations have arisen to take the pressure argument’s place. One, now more widely accepted, invokes friction: the rubbing of a skate blade or a shoe bottom over ice, according to this view, heats the ice and melts it, creating a slippery layer.

The other, which emerged a decade ago, rests on the idea that perhaps the surface of ice is simply slippery. This argument holds that water molecules at the ice surface vibrate more, because there are no molecules above them to help hold them in place, and they thus remain an unfrozen liquid even at temperatures far below freezing.

In 2002, Dr. Salmeron and colleagues performed an experiment. They dragged the tip of an atomic force microscope, resembling a tiny phonograph needle, across the surface of ice.

“We found the friction of ice to be very high,” Dr. Salmeron said. That is, ice is not really that slippery, after all.