DPS 2001 meeting, November 2001
Session 35. Icy Galilean Satellites I: Geology and Geophysics
Oral, Chairs: W. Moore, J. Moore, Thursday, November 29, 2001, 4:40-6:10pm, Regency GH

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[35.05] Callisto's Surface "Activity" from Highest-Resolution Galileo Imaging

J.E. Klemaszewski, D.A. Williams, R. Greeley (Arizona State University)

Very high-resolution images (up to about 3 meters/pixel) of Jupiter's moon Callisto are interpreted to indicate active erosion of its surface. Callisto's surface can be divided into bright and dark subunits, based on their relative albedo. Bright materials are ice-rich and are often associated with impact craters or high-standing features (e.g., knobs). In contrast, dark materials are ice-poor and occur in low-lying areas or plains. Galileo images of Callisto obtained during May 2001 show bright, icy regions contain knobs and spires with elevations of tens to hundreds of meters. In some images knobs populate the surface densely, while other regions contain few knobs or none. Although moderate-resolution context is not available, we speculate that densely populated bright knobs are related to the ejecta blanket of an impact event. In some locations, the bright terrain with numerous icy spires contains very few impact craters, if any. These bright spires are surrounded by smooth, dark material that contains few to no craters. The lack of craters indicates that these surfaces are young, and are likely undergoing active erosion.

We propose the following surface evolution due to erosion and impact cratering: The gradual loss of ice by sublimation liberates non-ice material which slides down-slope, piling up at the base of the spires. These spires continue to shrink, and will eventually disappear, leaving a smooth, dark, surface. This dark surface is less active because the layer of dark material inhibits sublimation of the icy material below. Very small impact craters (which do not penetrate the dark material) will be preserved and accumulate on the surface with time. Larger impacts (greater than a few kilometers) will penetrate the dark material, excavate icy material from below, and sublimation will occur until the ice is removed. Large impacts (10 to 60 km in diameter) have enough surface relief, and the sublimation process is slow enough, that they are still visible at global resolutions.

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