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R. Wagner, U. Wolf, G. Neukum (DLR Berlin), R. Greeley, J.E. Klemaszewski (ASU Tempe/AZ), Galileo SSI Team
Callisto was imaged by the SSI camera onboard the Galileo spacecraft in three close flybys during the Galileo Nominal Mission, and recently in two close flybys (C20, C21) during the Galileo Europa Mission. These images were geologically mapped, and crater size-frequency distributions were measured to define the sequence of geologic events. Absolute ages were obtained from two cratering chronology models: one based on a lunar-like cratering history, preferentially from asteroids (Neukum and others) (Model I), another one based on a constant cratering rate from Jupiter-family comets (Zahnle and others) (Model II). An area characterized by a dark ''flow'' in low-resolution Voyager images was targeted in C20. Despite this feature was conjectured to have been created by cryovolcanism, the higher-resolution SSI frames (100 and 430 m/pxl) do not reveal past endogenic activity. Instead, there is no distinct boundary between the flow-like patch and adjacent dark material. Dark, blanketing material which has been shown to be globally abundant on Callisto embays bright material of a heavily degraded impact structure (~150 km) not revealed on Voyager images. Measuring the size-frequency of craters superimposed on the degraded feature shows that it may have formed between the events which created the Asgard and Valhalla multi-ring basins, between about 4.2 billion years (b.y.) and 4 b.y. ago (Model I), or between about 4.3 b.y. and 2.3 b.y. ago (Model II). Also in C20, Bran, a large crater featuring bright rays was targeted. The rim seen in Voyager data appears to be an outer ring, the actual crater rim is smaller in diameter (~70 km). Based on crater size-frequency measurements on the continuous ejecta, this crater was formed about 4 b.y. ago (Model I), or 2.45 b.y. ago (Model II).