Dark ray craters on Ganymede: Impactor or endogenous origin

Through the spectral analysis of Galileo Near Infrared Mapping Spectrometer (NIMS) observations we now have a better understanding of the origin of dark ray craters (DRCs) on Ganymede. The analysis of NIMS observations show that DRCs are not a single compositional group, and at least some DRCs are the result of the excavation of endogenous material. DRCs overlying bright terrain may be entirely due to the excavation of the darker Ganymede hydrate material; the dark ejecta associated with the Tammuz impact is an example with its rays composed entirely of Ganymede hydrate. In contrast, three DRCs on the trailing hemisphere imaged by NIMS (Kittu, Amon, and Mir craters) show that a third component is present, with the ejecta being a mixture of water-ice, the hydrated non-ice material endogenous to Ganymede, and a less-hydrated, red, non-ice component. The spectrum of this second non-ice component is consistent with hydrated carbonaceous material, implying either a primitive impactor origin or excavation of primordial Ganymede non-ice material. This carbonaceous component is required at an abundance of about 40 to 50% in the ejecta of the three DRCs, assuming discrete mixing. It is absent in icy terrain but often present in regiones unassociated with DRCs, ranging in abundance in that terrain from 0 to again, about 50%. Its abundance in older, more primordial terrain suggests the carbonaceous material is endogenous to Ganymede. Thus, excavation of subsurface material appears capable of accounting for the diversity of DRCs. Additional spectral imaging observations of these and other DRCs are needed to verify this conclusion.