Dark-ray and dark-floor craters on Ganymede, and the provenance of large impactors in the Jovian system

Dark-ray and dark-floor craters are apparently unusual classes of craters on the icy satellite Ganymede that may permit identification of impactor type. Their wide size range and lack of preferential occurrence among bright and dark terrains support the concept that dark rays and dark floors are a result of impactor contamination, rather than excavation of subsurface layering. Dark-ray craters, in particular, are strongly concentrated toward the antapex of orbital motion and occur within ∼25° of the equator. Near the antapex, however, dark rays are at least as common as bright rays, if not more common, indicating that dark material is present in most rays, mixed with pure ice, and that dark rays are one potential stage in ray evolution on Ganymede. We hypothesize that rays darken as a result of near-surface concentration of impactor material, and then once a critical albedo is reached, to thermal sublimation into discrete icy and nonicy patches. This occurs in competition with meteoritic gardening and regolith formation, which dominate on the leading hemisphere. Minimum Voyager clear-filter albedos estimated for each dark-ray crater range from 0.16 to 0.3, about as dark as cratered terrain on Callisto, but not as dark sa C-, P-, and D-type asteroids or (known) comet nuclei, the most likely impactors. The actual albedo of the contaminant may be lower than the minimum values for the dark rays, however, because of possible unresolved albedo segregation. Voyager three-color visible spectra of dark rays indicate that most large ray systems are "redder" than grooved or cratered terrains and are among the "reddest" units on Ganymede. One prominent dark-ray system is relatively "blue," however. We infer that, overall, more than half the recent impactors on Ganymede may have been reddish D-type asteroids or comets, whereas less than ∼25% may have been more spectrally neutral C- and P-type asteroids or comets like Halleyor Chiron. Cumulative impacts of such objects, dominated by D-type asteroids, comets, and materials, may be largely responsible for the albedos and colors of dark terrains on Ganymede and Callisto. Dark-floor craters have a range of spectral character similar to that dark rays, but are randomly distributed and truly rare. They require impactor-specific but unknown processes for formation. The pronounced absorption in the violet for dark rays, dark floors, and Ganymede and Callisto overall may be due to magnetospherically implanted sulfur or to Fe³⁺ in phyllosilicates; the latter may imply aqueous and spectral alteration of anhydrous iron-bearing impactor silicate grains.