English: All of Callisto, the fourth Galilean satellite, looks similar to the dark regions on the surface of Ganymede. The surface of Callisto is covered with numerous impact craters, indicating little geologic activity has taken place here since the intense bombardment associated with the early history of the solar system. The Valhalla impact structure consists of a bright central region roughly 600 kilometers (400 miles) across (near the top of the photograph) with a series of concentric bright rings surrounding the central region. Voyager 1 obtained this view of Callisto in March 1979.