Galileo views of the geology of Callisto

Callisto was imaged by the Galileo spacecraft on 10 orbits, half of which i nvolved close flybys, producing images with resolutions as high as 15 m/pix el. The Valhalla, Asgard, and Adlinda multi-ring structures were imaged, al ong with two features which might also be large impacts, Utgard and Heimdal l (both provisionally named). The high albedo interior zones of these struc tures are relatively rugged and include small knobs. The interior ring zone of Valhalla includes outward facing scarps, as discovered on Voyager image s, while the inner zones of the other, smaller multi-ring structures includ e 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 a s resurfacing zones; they are now seen to consist of numerous, closely spac ed fractures. Crater size frequency distributions for the units associated with multi-ring structures suggest that all were formed essentially contemp oraneously 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-decam eter surface modifications, leading to the removal of small carters in some areas. Similar styles of degradation ale not observed on Ganymede, suggest ing fundamental differences in the properties of the surface materials. Cat enae (crater chains) were also observed; some appear to be secondaries in w hich their primary source crater can be identified, while others are probab ly the result of the breakup of primary objects, as previously proposed. Although most of Callisto's surface consists of ancient cratered plains, so me small 'smooth' areas had been suggested to result from cryovolcanic resu rfacing. Representative bright and dark areas were viewed in high resolutio n; bright areas were found to be rugged at the 10s of meters scale, show no indications of how features, and are not considered to be cryovolcanic. So me smooth dark areas appear to mantle and embay the surrounding cratered pl ains. 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 abou t 90% of the surface to be mapped photogeologically and gives new insight i nto the nature of surface features and the timing of their development. Cra tered plains form the oldest recognizable unit on Callisto, but subtle diff erences in color and crater frequencies suggest regional differences in its development. The formation of Adlinda, Asgard, Valhalla, Heimdall, and Lof n crater appear to follow in that order. The morphology of associated featu res is consistent with a differentiated interior for Callisto and the prese nce of a thin icy lithosphere at the time of the impacts. (C) 2000 Elsevier Science Ltd. All rights reserved.