Tectonic patterns on Vesta and Ceres revealed by polygonal impact craters

Abstract

Polygonal impact craters (PICs), impact craters with straight rims that form polygonal shapes in plan-view, form where pre-existing structures exist in the impact target rock. The straight rim segments of PICs and their preferred orientations are known to reveal hidden tectonic patterns that otherwise are invisible in spacecraft data. Vesta and Ceres are heavily cratered bodies that also experienced tectonics. We systematically mapped crater rims on Vesta and Ceres with crater diameters ≥ 10 and 20 km, respectively. We identified and extracted straight rim segments that maintain consistent orientations for at least 5 km to assess their segment lengths and orientations. The majority of mapped craters on Vesta and all craters on Ceres have at least one straight rim segment and thus are considered PICs. Analyses of straight rim segment orientations reveal multiple fracture sets that form complex but systematic regional and global patterns on both bodies. In particular, we detected an E–W trending fracture pattern in Vesta's north-polar region. Ceres predominantly shows NE–SW oriented fractures in the northern hemisphere and NW–SE oriented fractures in the southern hemisphere that are most pronounced at the poles. None of these patterns correlate with the orientations of large-scale troughs on Vesta and pit chains on Ceres or with large impact basins on Vesta, indicating that their origins are not directly linked. Their differing tectonic patterns indicate that different planetary processes controlled the formation of fractures responsible for the straight rim segments of the PICs. We compare our identified fracture patterns with predicted tectonic patterns caused by changes in spin rate, volume change, and true polar wander. However, predictions of tectonic patterns in the existing literature are not specific to, and thus not applicable to Vesta and Ceres, requiring more detailed modeling efforts and rock-mechanical reassessments to investigate the underlying processes that produced the fracture sets.