Callisto, the outermost of Jupiter's four Galilean satellites, as viewed by NASA's Voyager 2 spacecraft in 1979. Callisto possesses the greatest concentration of impact craters of any object in the solar system. Most of the craters have remained unchanged for the past 4 billion years. The two largest impact features are Valhalla (shown here near Callisto's left limb) and Asgard. Valhalla has a bright patch at its center, surrounded by a series of rings extending outward about 1,800 miles.
Image Credit: Jet Propulsion Laboratory and NASA.