CALLISTO is spiked with towering spires in some regions, new images show, and these giant icicles appear to be melting.

Just like the Energizer Bunny of television fame, NASA's Galileo spacecraft keeps going and going ... and going. Scientists have extended the mission three times since its start, and now, in its sixth year of orbit around Jupiter, the long-lasting satellite continues to send home valuable data about the gas-giant planet and its many moons. This summer, the probe made several passes at Jove's moons, providing new information about Callisto's surface and Io's core.

In May, Galileo snapped the sharpest pictures ever taken of Callistoor any of Jupiter's moons, for that matter. The satellite whizzed a mere 138 kilometers above Callisto's surface, photographing a strange and unexpected landscape below. Indeed, the images show that parts of the moon are spiked with icy spires, measuring 80 to 100 meters (260 to 330 feet) tall (see top inset in image below). Moreover, these giant icicles look as if they are melting.

"We haven't seen terrain like this before," says James Klemaszewski of Academic Research Lab, who is analyzing the recent pictures with Ronald Greely of Arizona State University. "It looks like erosion is going on, which is pretty surprising."

In fact, researchers long assumed that Callistoone of Jupiter's largest moons and the farthest from the planetwas fairly dull and unchanging. They knew its face was heavily scarred with impact craters; this moon bears the most pockmarked surface in the solar system. On Earth, the action of wind, water and volcanoes helps erase away such features fairly quickly. But Callisto lacks any similar environmental influences. Thus scientists believed that, apart from acquiring new craters, Callisto's surface had remained more or less constant for billions of years.

EROSION continues to melt icy spikes on some parts of Callisto's surface (top inset). Elsewhere the process has stopped, leaving behind little but gaping impact craters (bottom inset).

The close-up shots that Galileo took of Callisto's spiky spires, however, indicate that sections of the moon's exterior are constantly in the remaking. In addition to ice, the spires contain darker, dusty material, presumably thrown away from the planet during a major impact billions of years ago. And this darker material is slowly sliding down the sides of the spires and settling in low-lying areas.

The scientists think that this erosion may take place as some of the ice in the spires turns into vapor. The dust left behind may then absorb heat from the sun and turn more ice into vapor, perpetuating the process. "[The spires] are continuing to erode and will eventually disappear," Klemaszewski says. Elsewhere on the moon, erosion appears to have ceased, leaving behind gaping impact craters (see bottom inset in image below).

What these new images don't offer is any further information about Callisto's interior. Based on data Galileo collected in 1996 and 1997, researchers suspect the moon harbors a vast, subterranean ocean of salty ice and slush. The same is likely untrue for Io, another of Jupiter's moons visited by Galileo, most recently on August 6, 2001. Although the data from this flyby have yet to be fully analyzed, they indicate so far that Io doesn't house any hidden fluids.

The movement of liquids deep inside Earth, Jupiter and Jupiter's moons Ganymede and Callisto creates strong intrinsic magnetic fields. Galileo's journey over Io's north pole, however, found that any internally generated magnetic field there is either very weak or entirely absent. Based on the moon's density and heat output, researchers believe that, like Earth, it has a molten iron core. But given Io's apparent lack of an intrinsic magnetic field, that core may not churn convectively as does Earth's own center.

As scientists interpret these latest measurements, more data continue to stream in. Galileo is scheduled to return to Io, swinging past its south pole on October 16. Hopefully the hearty craft will keep the date. In early August the satellite missed several planned pictures of Io because of a recurring electronic problem with its camera. But Galileo has, at this point in its mission, withstood three times the radiation it was designed to handle. In all likelihood it will keep on going and going and going, for at least a little while longer.