VOLCANIC CONSTRUCTS ON GANYMEDE AND ENCELADUS - TOPOGRAPHIC EVIDENCE FROM STEREO IMAGES AND PHOTOCLINOMETRY

The morphology of volcanic features on Ganymede differs significantly from that on the terrestrial planets. Few if any major volcanic landfo rms, such as thick flows of shield volcanoes, have been identified to date. Using new stereo Voyager images, we have searched Ganymede for r elief-generating volcanic constructs. We observed seven major types of volcanic structures, including several not previously recognized. The oldest are broad flat-topped domes partially filling many older crate rs in dark terrain. Similar domes occur on Enceladus. Together with sm ooth dark deposits, these domes indicate that the volcanic history of the dark terrain is complex. Bright terrain covers vast areas, althoug h the style of emplacement remains unclear. Smooth bright materials em bay and flood older terrains, and may have been emplaced as low-viscos ity fluids. Associated with smooth bright material are a number of sca lloped-shaped, semi-enclosed scarps that cut into preexisting terrain. In planform these structures resemble terrestrial calderas, The young est volcanic materials identified are a series of small flows that may have flooded the floor of the multiring impact structure Gilgamesh, f orming a broad dome. The identification of volcanic constructs up to 1 km thick is the first evidence for extrusion of moderate-to-high visc osity material on Ganymede. Viscosity and yield strength estimates for these materials span several orders of magnitude, indicating that vol canic materials on Ganymede have a range of compositions and/or were e xtruded under a wide range of conditions and/or eruptive styles.