Valhalla ( ) is a vast multi-ring basin on Callisto, the outermost of the Galilean moons. It is the largest multi-ring impact basin in the Solar System. It is named after Valhalla, the hall where warriors are taken after death in Norse mythology.
Valhalla consists of a bright central region 360 km across, an inner ridge and trough zone, and striking concentric rings extending up to around 1,900 km from the center. It was discovered by Voyager 1 in 1979 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.
thumb|left|200px|The central part of Valhalla structure on Callisto. Bright knobs surrounded by smooth dark plains are visible.
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 central parts of Valhalla are less cratered than the old plain outside the structure. This indicates that Valhalla is significantly younger than Callisto itself. The latter can be warm ice or even a liquid ocean, the existence of which can be inferred<!--shouldn't this be "inferred" rather than "implied"?--> 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.
In fiction
Kim Robinson's Galileo's Dream (2009) contains a detailed description of a large city built around the concentric rings of Valhalla in the 29th century. The same city is briefly mentioned in Robinson's 2312.