Large mass movements on Callisto

Galileo images reveal the presence of mass movement deposits within impact craters on Callisto. Eleven such deposits were found in 830 candidate craters imaged at sufficient resolutions (86-280 m/pixel) for their identification. All of the deposits are located within impact craters, and their sources appear to be crater wall material. The morphologies of the Callistan deposits are similar to terrestrial mass movements and rock glaciers. Rock glaciers involve ductile now of ice and rock, but in the Callistan environment, water ice is likely to undergo brittle deformation. Consequently, rock glaciers are unlikely analogs for the features on Callisto. Three morphologies are observed: one blocky, one slump-like, and nine lobate deposits. Blocky deposits are characterized by massive blocks on the crater floor, while slump-like deposits appear as debris piles alone, the base of the crater wall. Lobate deposits are 1.7-9.9 km long, average similar to 90 m in thickness, and have tapered or semicircular terminations. Callistan lobate deposits are morphologically similar to rapidly emplaced dry-rock avalanches on Earth. Their significant thickness and steep frontal margins suggest that they may have behaved similarly to Bingham plastic material with a critical yield strength. The Callistan deposits have yield strengths similar to those estimated fur terrestrial dry-rock avalanches. The blocky deposit appears similar to terrestrial block glide deposits, in which large blocks detach and slide downslope. Individual blocks within the blocky deposits may be up to 1 km across and stand 30-60 m high. Callistan slump-like deposits are similar to terrestrial slumps, which fail along rotational planes. Mass movements on Callisto are not confined to craters; they have also been observed along the base of scarps, knobs, and ridges. Three of the Callistan deposits may have been impact triggered because of the proximity of an impact crater near the source. First-order analyses of the ground force generated by impacts show that two of the candidate deposits are capable of being impact triggered. The Callistan mass movements are probably set into motion by a combination of sublimation of ice, which undermines a near-surface layer of "lag" material, and seismic "triggering" associated with nearby impact events.