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 resolutions as high as 15 m/pixel. The Valhalla, Asgard, and Adlinda 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 ring 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 of numerous, closely spaced fractures. Crater size frequency distributions for the units associated with multi-ring structures suggest that all were formed essentially contemporaneously with their respective impact events. Galileo images show that surface degradation occurs on a variety of scales, including large landslides from the rims of craters and apparent sub-decameter surface modifications, leading to the removal of small carters in some areas. Similar styles of degradation are not observed on Ganymede, suggesting fundamental differences in the properties of the surface materials. Catenae (crater chains) were also observed; some appear to be secondaries in which their primary source crater can be identified, while others are probably the result of the breakup of primary objects, as previously proposed. Although most of Callisto’s surface consists of ancient cratered plains, some small ‘smooth’ areas had been suggested to result from cryovolcanic resurfacing. Representative bright and dark areas were viewed in high resolution; bright areas were found to be rugged at the 10s of meters scale, show no indications of flow features, and are not considered to be cryovolcanic. Some smooth dark areas appear to mantle and embay the surrounding cratered plains. Although this is consistent with possible cryovolcanic processes, it is not definitive evidence of endogenic processes. The extension of imaging data, both in coverage and resolution, enable about 90% of the surface to be mapped photogeologically and gives new insight into the nature of surface features and the timing of their development. Cratered plains form the oldest recognizable unit on Callisto, but subtle differences in color and crater frequencies suggest regional differences in its development. The formation of Adlinda, Asgard, Valhalla, Heimdall, and Lofn crater appear to follow in that order. The morphology of associated features is consistent with a differentiated interior for Callisto and the presence of a thin icy lithosphere at the time of the impacts.