Formation of Ganymede Grooved Terrain: Results from Galileo Imaging

G. C. Collins, J. W. Head, R. T. Pappalardo (Brown University), Galileo Imaging Team

Bright terrain on Ganymede is pervasively deformed by crosscutting sets of subparallel grooves. High resolution images of bright terrain returned by the Galileo spacecraft, including recent G7 and G8 observations, have greatly improved our understanding of the morphology of grooves and their relationships to each other and their surroundings. The grooves in the Galileo images have been interpreted as predominantly extensional tectonic structures. By examining the crosscutting relationships of these structures in high resolution Galileo images, a sequence of events is derived for several small target areas. The principles and observations gained from these small areas are then applied to the surrounding regions of grooved terrain imaged by Voyager at lower resolution. Application of this analysis to a region in the area of Uruk Sulcus and Nippur Sulcus on the antijovian hemisphere leads us to two preliminary conclusions about the nature of grooved terrain on Ganymede: (1) The style of extensional deformation which formed the grooves is distinct in each stratigraphic interval. In the area mapped so far, early structures are interpreted as horsts and graben, and later structures are interpreted as indicative of tilt block normal faulting and necking of the brittle lithosphere. (2) The orientation of grooves in a given stratigraphic interval is consistent over large distances and changes between different intervals, indicating that the direction of extensional stress has changed through time. In the mapped region, the earliest extensional stresses are oriented in a W-E or NW-SE direction, and the most recent stresses rotated to a NE-SW direction. These observations do not favor low-strain or highly localized mechanisms for the formation of grooved terrain such as the cooling of cryovolcanic flows or the surface expression of small convection cells, and instead favor large-scale mechanisms such as tidal forces and/or global volume change from differentiation.