Crater densities and crater ages of different terrain types on Ganymede

Abstract

Crater densities and size-frequency distributions on Ganymede were measured for separate areas of dark furrowed and dark smooth terrains, reticulate terrain, complex grooved terrain, and groups of groved polygons and groove lanes having a variety of relative ages. These measurements were used first, together with results of previous studies, to test alternate models of heliocentric and planetocentric crater-forming bombardments of Ganymede and Callisto. The spatial distribution of crater production functions on Ganymede is consistent with a single impactor population having dominated cratering of that satellite's observed surface. In addition, relative crater ages of different Ganymedean material deposits are better reconciled with the deposits' stratigraphic relations if a heliocentric impactor population is assumed. These results are consistent with results of previous studies, interpreted here to indicate that the spatial distributions of most classes of craters on Ganymede and younger craters on Callisto are more consistent with heliocentric than with planetocentric bombardment. The measurements were then used to infer the sequence and style of surface modification of Ganymede; bounds on absolute ages depend on the assumption of heliocentric bombardment. Dark terrain was emplaced during a prolonged period of volcanic resurfacing of older, heavily cratered terrain, probably at least 10 8 yrs in duration and at least 3.8–4.0 Gyr ago. Furrows generally formed on blanket-like deposits of dark material, typically hundreds of meters or more in thickness, soon after their emplacement; furrow formation continued globally throughout the period of dark-material deposition. Next, dark terrain was deformed locally by reticulate terrain formation. Light terrain emplacement subsequently began, at least regionally within “complex grooved terrain.” It continued for an extended period (perhaps several times 10 8 yrs) in grooved lanes, grooved polygons, andsmooth terrain, where a light-material layer averaging several hundred meters or more in thickness was emplaced.