Galileo stereo images covering about 1500 km² of Uruk Sulcus on Ganymede have revealed two scales of ridges; (1) large‐scale ridges and troughs spaced ∼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 ∼20 K/km and a strain rate of ∼10−14 s−1 during groove formation.
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Collins et al. (1998) studied this question.