General Description
Valhalla is the largest multi-ring basin on Callisto and in the Solar System (with diameter up to 3,800 km). It was discovered by the Voyager probes in 1979–80 and is located on the leading hemisphere of Callisto, in its Jupiter facing quadrant slightly to the north of the equator (at about 18°N latitude and 57°W longitude). From the geological point of view Valhalla consists of three zones: the central zone, the inner ridge-and-trough zone, and the outer trough zone.
The inner zone (diameter of about 360 km) is an example of a palimpsest: a high albedo circular feature of impact origin. The surface in the central zone is relatively smooth and has a mottled appearance. Many impact craters inside it have dark halos. At the high resolution achieved in some Galileo images the central part of Valhalla looks like a knobby terrain, where bright knobs are surrounded by dark smooth plains; there is noticeable deficit of small impact craters.
The inner ridge and trough zone surrounds the central palimpsest. The ridges that immediately surround the central zone have steep flanks facing outward. These scarps, when studied at a high resolution, turned out to be discontinuous consisting of a series of small bright knobs surrounded by the smooth dark material. They are obviously very degraded structures. The troughs that are situated further from the center than ridges are sinuous and appear to be graben (about 20 km wide). The inner trough zone extends up to 950 km from the center of Valhalla.
The outer trough zone has the radius of 1500 to 1900 km; its outer boundary is not well defined. It consists of wide double walled sinuous lineaments (troughs), which like inner troughs appear to be graben. Although these grabens are wider (up to 30 km) 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 lower resolution Voyager images. So, all structures within Valhalla basin have impact or tectonic origin.
Read more about this topic: Valhalla (crater)
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