Ice, The Sequels

Mary Katharine Ham
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Posted: Feb 22, 2006 11:48 AM

Did y'all know that we don't know exactly why ice is slippery? I didn't. I mean, obviously I don't know why it's slippery, but I figured someone knew. Somewhere out there in the great, smart humanity, someone must know, I figured. I was wrong. There are actually several theories. The traditional theory--the one in many textbooks--has been disproved, and scientists are working on a couple others.

First, the explanation you may have heard before.

According to the frequently cited — if incorrect — explanation of why ice is slippery under an ice skate, the pressure exerted along the blade lowers the melting temperature of the top layer of ice, the ice melts and the blade glides on a thin layer of water that refreezes to ice as soon as the blade passes...

But the explanation fails, he said, because the pressure-melting effect is small. A 150-pound person standing on ice wearing a pair of ice skates exerts a pressure of only 50 pounds per square inch on the ice...That amount of pressure lowers the melting temperature only a small amount, from 32 degrees to 31.97 degrees. Yet ice skaters can easily slip and fall at temperatures much colder.

Here are the other theories:

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.

Fascinating. Also, did you know there are as many as a dozen different kinds of ice, numbered Ice II, Ice III, Ice IV, and so on? Ice VII may be found deep inside the Earth, at temperatures of 3 and 400 degrees, and is rumored to be much more entertaining than Rocky VI:

The scientists started considering what happens to tectonic plates after they are pushed back down into Earth's interior. At about 100 miles down, the temperature of these descending plates is 300 to 400 degrees — well above the boiling point of water at the surface — but cool compared with that of surrounding rocks. The pressure of 700,000 pounds per square inch at this depth, Dr. Bina and Dr. Navrotsky calculated, could be great enough to transform any water that was there into a solid phase known as Ice VII.

Score one for the New York Times. What a delightful article. I learned much, and the writer managed to make no inappropriate puns about the ice being "more slippery than the White House communications shop, har-dee-har-har." Well played, Times. Well played.