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The Sciences

The Snow on Pluto

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Anyone with a reasonably good telescope can take a picture of Pluto, but seeing anything more than a blurry dot is all but impossible. Even with the Hubble telescope, Pluto is only five picture elements wide. But this past year a team led by Alan Stern, an astronomer at the Southwest Research Institute in Boulder, Colorado, teased some surface details out of that low-resolution view. Stern and his colleagues used the Hubble to take images of Pluto at various times during its 6.4-day rotation; by superimposing the overlapping images and plotting them on a globe, they could distinguish light and dark patches on Pluto’s icy surface. They found between seven and nine such patches, each of them hundreds of miles across, including a polar ice cap bisected by a dark strip.

Although some of the regions may be permanent physical features like basins or craters, Stern and his colleagues believe that most are transitory patterns in the frost: the frost tends to be light, they think, where it has freshly precipitated out of the atmosphere, and dark where it has been on the surface long enough to have been heavily bombarded by dusty micrometeorites and radiation. Pluto’s surface, which is thought to consist of frozen methane, nitrogen, and carbon monoxide, is on the whole salmon- colored--the result, says Stern, of ultraviolet sunlight converting some of the ice into complex hydrocarbons. Pluto had always been just a pinpoint, Stern says. It became a real place this year.

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