Hubble space telescope reveals a patchy Pluto.

Viewed through any telescope on Earth, Pluto looks like an undistinguished fuzzball. But images taken with the keen eye of the Hubble Space Telescope, which were unveiled a year ago but now have been enhanced, directly reveal the first details of the remote planet.

Released last week at a press briefing in Washington, D.C., the pictures show that Pluto has a mottled appearance, with nearly a dozen large bright and dark patches on its icy surface. These include a bright northern polar ice cap bisected by a dark strip, a group of bright spots rotating with the planet, a cluster of dark spots, and a bright linear marking. Pluto's patchiness, probably due to deposits of frost, endow the planet with higher contrast than any other denizen of the outer solar system.

"The things we couldn't see before . . . are turning out to be at least as interesting as we thought they would be," says S. Alan Stern of the Southwest Research Institute, in Boulder, Colo.

Last year, Stern and his collaborators, including Marc W. Buie of Lowell Observatory in Flagstaff, Ariz., and Laurence M. Trafton of the University of Texas at Austin, presented "raw" ultraviolet and visible-light images taken by Hubble's Faint Object Camera (SN: 4/1/95, p. 204). Because even Hubble can barely discern detail on tiny Pluto, the astronomers used computer techniques to enhance the visible-light images.

The darker regions in the enhancement may consist of old methane frost, broken down by the sun's ultraviolet light into dark hydrocarbons. The brighter regions may be fresh nitrogen ice, Stern says.

Scientists say that the distribution of frosts on Pluto may change dramatically as the planet's elliptical, 248-year orbit takes it further into the chillier reaches of the outer solar system. As Pluto, now just past its closest approach to the sun, continues to recede, more of the planet's atmosphere is expected to condense as snow. Fresh, bright frost covering old, dark layers may give Pluto a considerably more uniform appearance in just 20 years.

Five years ago, researchers including Buie and Richard P. Binzel of the Massachusetts Institute of Technology modeled Pluto's surface brightness after viewing partial eclipses of the planet by its moon, Charon (SN: 6/6/92, p. 379). The Hubble images match that model well, notes Binzel, and "set the stage for the next step-exploration of Pluto by spacecraft." NASA is now considering launching a spacecraft, Pluto Express, in the next decade, that would take 12 years to reach the planet.