Posted: Thu, Aug 23, 2001, 12:35 PM ET (1635 GMT)
New images from NASA's Galileo spacecraft released Wednesday show evidence of hills on the moon Callisto that appear to be slowly eroding away. The images, taken by Galileo in May when it passed less than 140 km from the surface the moon, show a series of jagged hills with regions of dark dust slumping down their sides. Planetary scientists believe the icy hills may have been formed billions of years ago as either the debris of an impact or the remnants of an impact structure. The dust appears to play the key role in eroding the hills: sunlight warms the dust, which then sublimes the surrounding ice. The images are surprising since Callisto, the outermost of Jupiter's four large Galilean moons, has shown no evidence of geological activity that would reshape its surface, and as a result is the most heavily cratered moon in the solar system.

Related Links:

•  NASA press release
•  Galileo web site