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The concentric rings surrounding Valhalla are perhaps the most distinctive geological feature on Callisto. This Voyager 1 close-up shows a segment of the ridged terrain. The presence of superimpact craters shows that the rings formed early in Callisto's history; however, the density of craters is less here than on other parts of the satellite, where the surface is older.
The Voyager Project is managed for NASA by the Jet Propulsion Laboratory, Pasadena, Calif.