LAST OF THE INNER MOONS: ANALYSIS OF IMPACT CRATER DISTRIBUTIONS OF SATURN’S MOON RHEA

Saturn’s moon Rhea (D = 1527 km) is the farthest from Saturn and the largest out of the inner mid-sized satellites. Rhea has been hypothesized to have formed from Saturn’s protoplanetary disk in part due to prior studies of Rhea’s very dense impact crater population. Conversely, recent dynamical studies of the orbital evolution of Saturn’s mid-sized satellites have suggested that some of the inner moons may be quite young (on the order of 100 Myr) and have accreted recently in Solar System history from a disk of material formed by collisions between prior generations of satellites. This presents a geologic conundrum because Saturn’s satellites show varying amounts of surface geology and modification, all of which represent different timescales needed to record the observed geologic record. Our previous work examining the impact crater distributions on Saturnian satellites suggest that Mimas, Tethys, and Dione don’t show evidence of being extremely young (on the order of 100 Myr), but they also don’t appear to be primordial in age as Rhea may be (i.e., 4.5 Gyr). Mapping craters on Rhea’s surface presents an opportunity to not only study the craters on Rhea, but also complete the examination of system wide trends in age and evolution of the inner mid-sized Saturnian satellites. This work will provide additional insights into the impactor sources for these craters, which can be used to refine model age estimates for the moons.

We will present our two-pronged mapping approach characterizing the history and relative ages of Rhea and the neighboring satellites. We utilize Cassini NAC imagery with pixel scales on the order of ~ 200 m/pix to create regional maps of the crater distributions, including craters’ interior morphologies. We will also present our near-global map of elliptical craters from the global mosaic, which constrains the impactor population(s) responsible for these craters. Elliptical craters are unique in that the orientation of their major axis is indicative of the direction of the impactor when it hit the satellite. By conducting a survey of the elliptical craters and their orientations, we will be able to examine whether these craters come from a heliocentric or planetocentric source. Combining the information we obtain from the regional survey and the elliptical crater study will enable a robust look at the history of Rhea and where it fits in with the other mid-sized satellites.