DPS Meeting, Madison, October 1998
Session 54P. Io, Callisto, and Ganymede II
Contributed Poster Session, Thursday, October 15, 1998, 5:00-6:30pm, Hall of Ideas

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[54P.09] Cratering on Callisto from the Galileo Prime Mission

W. J. Merline, C. R. Chapman, B. Bierhaus, S. Brooks (SwRI), J. Moore (NASA Ames), J. E. Klemaszewski, R. Greeley (ASU), Galileo Imaging Team

To understand the impactor populations in the Jupiter system and crater-degradation processes on Callisto, we have analyzed images returned from the Galileo prime mission. Callisto was observed on 8 of 11 orbits; it was the prime target on 3 orbits, with resolutions as good as about 40 m/pixel. These data have led to several surprises, relative to the impression from Voyager data that Callisto is an inactive world with a surface saturated with craters.

Our analysis of the first Galileo images of Callisto, revealed a distinct lack of small craters. While we confirmed the Voyager conclusion that Callisto's large crater (over 10 km) density approaches that of the lunar highlands, we also found the density rapidly decreases, relative to saturation, at smaller diameters. There is clear evidence for a pervasive blanketing of material over Callisto's surface. However, the depletion of craters with size is so steep that it cannot be explained solely by this degradation process. Images from the latter part of the mission and at various locations about the surface, show the same paucity of craters, indicating that this is a global phenomenon.

By comparison with the small crater population on Ganymede, which has a similar impactor population but perhaps different degradation processes, we have estimated the relative effects of impactor flux versus degradation. We find crater densities about 5 times higher on Ganymede (at diameters of 500 m) than on Callisto, indicating an augmented, ongoing, small-crater degradation process on Callisto, beyond that on Ganymede.

Secondary craters and other, possibly endogenic, features affect our interpretation of the primary small-crater population. For example, we detected a population of ``pits", irregular-shaped depressions having a monomodal size distribution (at about 1 km) in images taken near the Valhalla ring structure on orbit C9. These depressions are oblong, with the long axis directed neither radial nor tangential to the ring structure or other major features.

The author(s) of this abstract have provided an email address for comments about the abstract: merline@boulder.swri.edu

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