The role of extensional instability in creating Ganymede grooved terrain: Insights from Galileo high-resolution stereo imaging

Galileo stereo images covering about 1500 km(2) of Uruk Sulcus on Ganymede have revealed two scales of ridges; (1) large-scale ridges and troughs spaced similar to 6 km apart, corresponding to the "grooves" seen in Voyager images, and (2) small-scale ridges spaced hundreds of meters apart superimposed on the large-scale ridges. We interpret the small-scale ridges to be the result of tilt-block normal faulting of the surface brittle layer, while the large-scale ridges may be due to necking of the brittle layer over a ductile substrate. The geometry of the tilt blocks revealed in Galileo images leads to a minimum estimation of 51% to 58% extensional strain in the area. The strain estimate, when incorporated into a model for the formation of grooved terrain by necking of a brittle layer undergoing extension, leads us to estimate a thermal gradient of similar to 20 K/km and a strain rate of similar to 10(-14) s(-1) during groove formation.