Jupiter's icy moon Callisto is shown here in approximate natural color. At the lower left is the impact structure, Asgard surrounded by concentric rings up to 1,700 kilometers (about 1,050 miles) in diameter. A second ringed structure with a diameter of about 500 kilometers (310 miles) is just below Asgard, partially obscured by the more recent, bright-rayed crater, Burr. The icy materials excavated by the younger craters contrast sharply with the darker and redder coatings on older surfaces of Callisto. These bright areas are believed to contain more clean ice than the rest of Callisto's generally 'dirty-ice' surface.