Ring geometry on Ganymede and Callisto

Arguments have been made, based on geometry, for both an impact and an internal origin for the ancient, partially preserved, major furrow system of Ganymede. Zuber and Parmentier concluded that furrows are not concentric but could be impact related if multiringed structures on icy satellites are initially noncircular. We examine the geometry of the Valhalla ring structure on Callisto in order to assess the circularity of an unmodified ring system. Despite prominent local meandering, the only gross deviations from concentricity in the Valhalla system are found in the outer northeast quadrant of the system. Here, a number of ring segments intersect small circles about the center at angles up to 30°. The Ganymede furrow system was remapped to make use of improvements in coordinate control. The least-squares center of curvature (determined using natural weighting) for all furrows in Marius and Galileo Regio is 20.7 ± 1.1°S, 179.2 ± 1.1°W (2σ uncertainty). Furrows in Marius and Galileo Regio are reasonably concentric, are much more circular than previously estimated, and probably once covered at least an entire hemisphere of Ganymede. In addition, at least three other multiring systems of varying size are identified on Ganymede, indicative of a projectile population. Thus we find furrow geometry and occurrence are consistent with an impact origin. Deviations of some furrows from concentricity about the center of curvature, on the scale of those found at Valhalla, do exist. As in the case of Valhalla these variations are principally confined to outer regions of the structure and are interpreted as inherent properties of multiringed structures on icy satellites. The cause(s) of this may be in the ring formation mechanism itself, but are more likely due to variations in preexisting lithospheric mechanical properties. The perceived present nonalignment of the assumed originally concentric furrows has been used to argue for large-scale lateral motion of dark terrain blocks in Ganymede's crust, presumably in association with bright terrain formation. The overall alignment of furrows as well as the inherent scatter in centers of curvature for subregions of Galileo and Marius Regio do not support this hypothesis.