Galileo views of the geology of Callisto

Abstract

Callisto was imaged by the Galileo spacecraft on 10 orbits, half of which involved close flybys, producing images with resolution as high as 15 m/pixel. The Calhalla, Asgard, and Alinda multi-ring structures were imaged, along with two features which might also be large impacts, Utgard and Heimdall (both provisionally named). The high albedo interior zones of these structures are relatively rugged and include small knobs. The interior zone of Valhalla includes outward facing scarps, as discovered on Voyager images, while the inner zones of the other, smaller multi-ring structures include grabens and inward facing scarps. The outer ring zones consist of grabens which are sinuous and include multiple parallel faults and fractures. High albedo areas associated with the fractures had been suggested previously as resurfacing zones; they are now seen to consist if numerous, closely spaced fractures. Crater size frequency distributions for the units associated with multi-ring structures suggest that all were formed essentially contemoraneously with their respective impact events.