Morphology and origin of palimpsests on Ganymede from Galileo observations

K. B. Jones, J. W. Head, R. T. Pappalardo (Brown University), C. R. Chapman (Southwest Research Institute), R. Greeley (Arizona State University), J. M. Moore (NASA-Ames), G. Neukum (DLR), Galileo SSI Team

Palimpsests are large, circular, high albedo, and very low relief impact scars on Ganymede and Callisto first imaged by Voyager. Even after analysis of Voyager imagery, these structures remain poorly understood. For example, there has not been a consensus on just what the high albedo deposit represents, or on where the original crater rim or transient cavity is located with respect to this deposit. High resolution Galileo images help shed some light on the origin of these poorly understood structures.

Four palimpsests on Ganymede were targeted by Galileo. Memphis Facula was imaged on orbit G1, unnamed palimpsests on G2 and G7, and Buto Facula on G8. Images of the Europan maculae Callanish and Tyre from orbits E4 and G7 and the ganymedian pedestal crater Achelous from G7, based on certain similarities to palimpsests, were used as tools to help understand the palimpsests.

Based on analysis of the Galileo images, the palimpsests consist of four surface units, two located within and two outside of the transient cavity. Secondary craters are present beyond the outermost unit. The palimpsest edge appears to be the continuous ejecta deposit edge, based on the distribution of secondary craters and burial of underlying structure by the outermost units. The transient cavity boundary appears to coincide with the boundary between two of the mapped palimpsest units, based on radius ratio comparisons with pedestal craters and the disappearance of buried structure within this boundary. Further, it appears that the europan maculae and ganymedian palimpsests formed similarly, by impact into thin icy lithospheres.