Dione’s resurfacing history as determined from a global impact crater database

Abstract

Saturn’s moon Dione has an interesting and unique resurfacing history recorded by the impact craters on its surface. In order to further resolve this history, we compile a crater database that is nearly global for diameters (D) equal to and larger than 4km using standard techniques and Cassini Imaging Science Subsystem images. From this database, spatial crater density maps for different diameter ranges are generated. These maps, along with the observed surface morphology, have been used to define seven terrain units for Dione, including refinement of the smooth and “wispy” (or faulted) units from Voyager observations. Analysis of the terrains’ crater size–frequency distributions (SFDs) indicates that: (1) removal of D≈4–50km craters in the “wispy” terrain was most likely by the formation of D≳50km craters, not faulting, and likely occurred over a couple billion of years; (2) resurfacing of the smooth plains was most likely by cryovolcanism at ∼2Ga; (3) most of Dione’s largest craters (D⩾100km), including Evander (D=350km), may have formed quite recently (<2Ga), but are still relaxed, indicating Dione has been thermally active for at least half its history; and (4) the variation in crater SFDs at D≈4–15km is plausibly due to different levels of minor resurfacing (mostly subsequent large impacts) within each terrain.