Callisto conundrum.

When the Voyager spacecraft flew past Jupiter's icy moon Callisto 18 years ago, it discovered a battered world, riddled with overlapping craters and huge basins. Although those images didn't show much detail, they suggested strongly that Callisto was as pockmarked as our own moon.

Last December, when the Galileo spacecraft began sending close-up pictures of selected areas on Callisto, the outermost of Jupiter's four large satellites, some planetary scientists had a rude awakening. The high-resolution Galileo images show that although the moon indeed has plenty of large craters, it possesses surprisingly few smaller than 100 meters in diameter.

That's a conundrum, says Clark R. Chapman of the Southwest Research Institute in Boulder, Colo., because bombardment by asteroids and comets would have produced craters of all sizes. Moreover, Callisto, like Earth's moon, is thought to have been geologically dead for more than 3 billion years. Both moons lack current volcanic activity, for example, which might have erased craters.

"Apparently, giant craters last throughout Callisto's history, but building-size things are continuously destroyed in just tens of millions of years," Chapman says.

On other planetary bodies, "it's fairly clear what causes a crater to disappear. Another crater might lie on top of it, or a volcanic event or [eruption of ice] on the surface will flood it," he notes. On Callisto, in contrast, "craters . . . are kind of falling apart in place. It's as though material is coming unglued, disintegrating like a dirty snowbank in Boston in the spring."

Not only does it seem that small craters are falling apart, but some mysterious process has apparently moved the debris over distances of several kilometers, blanketing and smoothing over other pockmarked features on the surface, Chapman adds. None of the other icy Jovian moons shows such a pattern.

The debris isn't simply rolling downhill, because the smoothed regions do not lie at the bottom of slopes. In addition, Callisto has neither an atmosphere to blow the debris about nor liquid water on its surface to foster migration.

Jeffrey M. Moore of NASA's Ames Research Center in Mountain View, Calif., and his colleagues propose that debris from some of the craters disintegrates as ices evaporate and then develops an electric charge. The charged dust particles repel one another and rise above the surface, traveling a significant distance before settling back down, they suggest. Planetary scientists have found evidence of electrostatic levitation on the moon and on asteroids, Moore notes.

One caveat, notes Chapman, is that Galileo has so far revealed only a few small patches of Callisto at high resolution. It's possible that other parts of Callisto show a more heavily cratered facade, he notes.

"It's embarrassing that we don't have better answers, but it's only been less than a year that we've had this [high-resolution] data to look at," says Galileo scientist Torrence V. Johnson of NASA's Jet Propulsion Laboratory in Pasadena, Calif. The craft's Jovian tour, which began in December 1995, had been scheduled to end this December, but NASA recently extended the mission for 2 years.