Callisto's surface. Galileo spacecraft image of the surface of Callisto, one of the Galilean moons of Jupiter. At centre is the Asgard impact struct- ure; its centre is light grey and is surrounded by discontinuous rings. The outermost rings of this ancient structure are about 850 kilometres from its centre. The rings of Asgard are formed of escarpments (scarps) near the centre, and troughs near the edge. Several large impact craters (white) have been formed since Asgard, including the young Burr crater (at upper centre). There are also many smaller craters. Image taken on 4 November 1996 by Galileo's Solid State Imaging camera at a range of 111,891 kilometres.