Dark Terrain on Ganymede: New Observations from Galileo

L.M. Prockter, J.W. Head, R.T. Pappalardo (Brown University), D. Senske (JPL), G. Neukum (DLR), R. Greeley (ASU), J. Moore (NASA Ames), Galileo SSI Team

At Voyager resolution, dark terrain on Ganymede (which comprises approximately half of the satellite; the remainder is bright terrain) is a nearly primordial, impact-generated surface of dark plains material with morphological features which include impact structures and huge systems of concentric furrows. One of the outstanding questions from Voyager was the origin of the dark contaminant present in the dark terrain, and one of the primary goals of Galileo is to address this. New Galileo SSI imaging results have shown that the dark terrain is extremely heterogeneous: up to three different plains units are observed; furrow rims and floors can now be distinguished; and brighter material is seen in the form of isolated knobs and massifs, plateaus, and crater and furrow rims. Several processes are observed to have modified the dark terrain including mass wasting, impact resurfacing, tectonic activity, sublimation, and possibly volcanism. Geological mapping enables us to propose and evaluate models for the origin and evolution of the dark terrain. We also examine the progression of dark terrain from its apparently relatively pristine condition (e.g. Galileo Regio), to a highly modified state (e.g. fractured dark terrain in the Uruk Sulcus region) and show examples from the recent G7 and G8 encounters. The dark terrain appears to undergo progressive tectonic modification as bright terrain is approached.