THE UNIQUE IMPACTOR POPULATIONS OF THE SATURNIAN SYSTEM; A CASE STUDY ON TETHYS AND DIONE (Invited Presentation)

An examination of the formation ages of the Saturnian satellites by Ćuk et al. (2016) placed a lower limit on the ages of Mimas, Enceladus, Tethys, and Dione of 100 Ma; the upper limit is 4.4 Ga. To help narrow this wide range, we mapped craters across Tethys and Dione, and examined their size-frequency distributions (SFD), to characterize their impactor sources. Better characterization of sources assists in refining the formation and surface age estimates for the satellites. Additionally, we mapped elliptical craters and their orientations across Tethys and Dione between 60° N and 60° S. Elliptical craters can provide an independent constraint on the bombardment source as they record the direction of impact onto the surface. From our regional mapping on Tethys and Dione we find that the SFDs don’t agree with previously established production functions for the outer planets. This deviation is thought to be representative of an unique planetocentric population occurring in the Saturn system. This planetocentric component of the cratering record would be in addition to a background heliocentric flux that affected all of the outer planets, as preserved on Charon and the Jovian satellites. Dione’s surface shows a lower density of smaller impacts (1 km < D < 4 km) and has a different SFD slope than Tethys, which suggests that Dione has experienced eras of regional resurfacing while Tethys has an ancient surface. For the elliptical craters, we find an East/West trend in the crater orientations and a strong concentration of this morphology in the mid-latitudes (30° N to 30° S). These two observations combined lead us to interpret the source of the elliptical craters as being planetocentric in origin. With this result, we mapped an additional region on Tethys between 30° N and 60° N. This mapping allowed us to build an additional SFD that may be less contaminated by a planetocentric signal in an effort to compute the age. Through multiple avenues of crater analysis, we conclude that the oldest regions on Tethys and Dione are likely extremely old. However, models of the longevity and frequency of planetocentric impactors is needed for a robust age determination.