National Geographic : 1999 Sep
CAI ITC T SOMETHING IS EATING rCLL I U AWAY AT THE SURFACE OF JUPITER'S MOST DISTANT MAJOR MOON, BUT EVEN MORE MYSTERIOUS IS WHAT MAY LIE BENEATH ITS CRATERED FACE: AN OCEAN OF SALTY WATER. Judging from Voyager images, scientists expected to find craters of every size on shell shocked Callisto, from giant bull's-eyes to tiny pockmarks. "But what we saw," says Greeley, "was an eroded surface with very few craters smaller than a half mile. The surface is being eaten away and blanketed by soft, fluffy stuff." At the upper rim of an impact crater (right) a section has given way in a landslide. The distance traveled by the debris-nearly two miles-suggests a loose, fine-grained surface. An older, eroded crater called Har (left) has a large, rounded mound on its floor that was probably caused by upwelling ice. Ice may play a key role in smoothing Callis to's surface. A highly detailed view taken in the moon's largest impact structure, Valhalla (below)-marked by a shadow from a fault scarp-shows a blanket of fine debris. "As the ice sublimates and is lost, all that is left is the dirt," says Greeley. "It's like after your snowman melts, and all that's left is that little pile of soil." But why here on Callisto and not on heavily cratered Ganymede? "Who knows?" Greeley says. Another question for another mission.