Polygonal impact craters on Dione: Evidence for tectonic structures outside the wispy terrain

Plan-view impact crater geometries can be indicative of pre-impact structures within the target material. Impact events that occur on a pre-fractured surface generate craters exhibiting one or more straight rim segments, termed polygonal impact craters (PICs). Impact craters that appear to be PICs are abundant on the surface of Saturn's icy satellite, Dione, both within the wispy terrain, a region with large visible fractures, and also outside the wispy terrain (the 'non-wispy terrain'), where less evidence for fracturing has been observed. In the non-wispy terrain, subtle lineaments are hypothesized to be NE-SW, NW-SE, and E-W trending fractures, suggesting that tectonism may have been an important process in this terrain. Results of previous studies have shown that PIC straight rim segment azimuths ('PIC azimuths') commonly parallel pre-impact fracture azimuths, although disagreements about this relationship exist in the literature. We investigated the hypothesis that fractures, either subtle or nonvisible with available spacecraft images, are present within Dione's non-wispy terrain. Our first step was to assess the relationship between PICs and pre-existing fracture azimuths in the wispy terrain. Our results from this initial assessment show a parallel relationship between PIC azimuths and fracture azimuths. Based on this correlation in the wispy terrain, we find it likely that this relationship would hold true in the non-wispy terrain if PICs are present. We tested for PICs using crater rim azimuth data collected from randomly distributed study locations throughout the non-wispy terrain. From these data, we identified widespread PICs in this terrain, which supports the hypothesis that subtle fractures are also present. Analysis of the PIC azimuth data yield a pattern for these inferred fractures across Dione's surface that is consistent with the hypothesized global deformation that would result from a combination of satellite despinning and volume expansion. Our results provide evidence for these previously hypothesized events in Dione's history and demonstrate that mapping PICs and their azimuths is a useful tool for investigating subtle fractures on Solar System bodies.