Chapter 19 Icy Satellites of Saturn: Impact Cratering and Age Determination

Abstract Saturn is the first giant planet to be visited by an orbiting spacecraft that can transmit large amounts of data to Earth. Crater counts on satellites from Phoebe in-ward to the regular satellites and ring moons are providing unprecedented insights into the origin and time histories of the impacting populations. Many Voyager-era scientists con-cluded that the satellites had been struck by at least two populations of impactors. In this view, the Population I im-pactors, which were generally judged to be “comets ” orbiting the Sun, formed most of the larger and older craters, while Population II impactors, interpreted as Saturn-orbiting ejecta from impacts on satellites, produced most of the smaller and younger craters. Voyager data also implied that all of the “ring moons, ” and probably some of the mid-sized clas-sical moons, had been catastrophically disrupted and reac-creted since they formed. We examine models of the primary impactor populations in the Saturn system. At the present time, “ecliptic comets, ” which likely originate in the Kuiper Belt/Scattered Disk, are predicted to dominate impacts on the regular satellites and ring moons, but the models require extrapolations in size (from the observed Kuiper Belt Ob-jects to the much smaller bodies that produce the craters) or in distance (from the known active Jupiter family comets to 9.5 AU). Phoebe, Iapetus, and perhaps even moons closer to Saturn have been struck by irregular satellites as well. We describe the Nice model, which provides a plausibl