than those in the inner trough zone, they are severely degraded, and are made of a series of small knobs, much like their inner counterparts. There are no indications of volcanic flows or other signs of endogenic activity associated with the graben on the high resolution Galileo images as was suggested based on the low resolution Voyager images. So, all structures within Valhalla basin have impact or tectonic origin. Craters Several prominent impact craters and catenae are located within Valhalla structure. At the northern margin of it Gomul Catena can be found as well as Egdir and Mimir craters. The catena consists of a linear sequence of impact craters and probably resulted from a break-up of a comet (like comet Shoemaker-Levy 9). To the south of Valhalla there are Sarakka and Nar impact craters; to the east of it (at the boundary between the inner and outer trough zones) are Sculd crater and Svol Catena. To the west of Valhalla another large multi-ring basin—Asgard—can be found. The central parts of Valhalla are less cratered than the old plain outside the structure. This indicates that Valhalla is significantly younger than Callisto itself. Origin The Valhalla multi-ring structure (like other Callistan multi-ring basins) probably resulted from a giant impact, which punctured the outer ridged lithosphere of the moon and entered the underlying layer consisting of a much softer material. The latter can be warm ice or even a liquid ocean, the existence of which can be inferred from magnetometric data. The formation of a ringed structure is caused by the concentric failure of the brittle outer shell (lithosphere) following the impact, after the soft material underlying the lithosphere flows towards the center to fill the cavity excavated by the impact. The absolute age of the Valhalla structure is not known; however, it is the youngest such feature among the five known multi-ring structures on Callisto. Estimates of its age vary from 2 to 4 billion years. Consistent with this picture, imaging by the Galileo spacecraft found no evidence of disruption of the area of Callisto antipodal to Valhalla. Such disrupted terrain normally forms as a result of focusing of the seismic energy at the opposite point after the impact. The absence of such disruption supports the presence of a subsurface ocean, which would have absorbed much of the seismic energy, at the time of Valhalla's formation. In fiction Kim Stanley Robinson's Galileo's