Fire And Ice

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THE PLANET MERCURY IS TRULY A HELLISH PLACE: IT is one-third as far from the sun as Earth, and its daytime temperatures can reach 430 degreesC (800 degreesF). The last thing scientists expected to find there was ice. But that is just what a new radar study of Mercury, reported in Science, has detected. Planetary scientists at NASA's Jet Propulsion Lab and at Caltech aimed powerful radar beams at both of the planet's poles; the return signals bore the telltale signs of having bounced off a frozen surface. Like Earth and Mars, Mercury appears to have polar ice caps.

How is it possible? An analysis by UCLA scientists points out that while the poles are bathed in scorching sunlight, the light hits at such a shallow angle that the floors of some craters are permanently in shadow. With no atmosphere to move heat around, the temperature in these spots is far below zero. Any ice that condensed as frost in the craters billions of years ago when water boiled on the planet's surface as it formed would still be around today -- and evidently is.