A unique Saturnian impactor population from elliptical craters

Abstract

The crater populations of Saturn's midsized icy moons are not well matched by the size-frequency distributions of impacting material inferred from other outer planet satellite systems, frustrating attempts to date their surfaces and constrain models of their formation. Elliptical craters record the trajectories of impacting materials and can thus be used to characterize the impactors' dynamics and facilitate crater interpretations. Here, we report evidence of a unique Saturn-orbiting impactor population, distinct from any previously described source population. This population is identified through global mapping and analysis of the elliptical crater populations on Saturn's moons Tethys and Dione. We report the presence of a strong signal of east/west oriented elliptical craters located in the equatorial latitudes of both satellites. Poleward of 30° the orientations of these craters become more widely distributed. On Tethys, a survey of a high latitude area (30°-60° N) reveals that the non-elliptical craters are better matched by the production function constructed from craters on Triton than from the Jovian moons, although the high likelihood of planetocentric material in both the Saturnian and Neptunian systems precludes us from determining an absolute age using the Triton derived production function. Rather, we can conclude that the mid-sized Saturnian satellites have been strongly affected by a potentially similar collisional environment to Triton, giving rise to a Triton-like crater population along with a unique population that is not observed at Triton. Origin scenarios for the elliptical craters on Tethys and Dione would have to explain the orientation and latitudinal clustering of east/west oriented craters, as well as the characteristics of the azimuthally isotropic population, which requires evenly distributed cratering across each satellite without a preferred impact direction. These characteristics can serve as constraints on the formation and evolution of the Saturnian satellites.