Galileo finds spires on Callisto.

The sharpest images ever taken of Jupiter's icy moon Callisto show features never seen before on the remote body--icy, knoblike spires that show slow but steady signs of erosion. That's a puzzle because other evidence indicates that Callis to, the most distant of Jupiter's four largest moons, has been inactive for billions of years.

The Galileo spacecraft recorded the spires, which are 80 to 100 meters high, when it passed just 138 kilometers above the moon last May. The images, unveiled Aug. 22, provide the highest-resolution view of any of Jupiter's moons, showing features as small as 3 m across.

Scientists speculate that the spires were created by material thrown upward when a projectile struck Callisto several billion years ago. The spires contain dark dust, which appears to be sliding down from their tops and collecting in low-lying regions.

This slow erosion occurs although that Callisto has long been dormant. The moon's pockmarked appearance strongly suggests Callisto hasn't undergone significant volcanic activity or fracturing, which would have erased the craters and other features that have accumulated on the surface.

James Klemaszewski of the Academic Research Lab in Phoenix and his colleagues theorize that as some of the ice in the spires vaporizes, it leaves behind the dark dust. As the material accumulates, it absorbs heat from the sun, warming the surrounding ice and continuing the erosion process. Supporting that theory, the Galileo images show areas where the spires have apparently vaporized entirely, leaving a flat region blanketed with dark material.