Age relationships of large-scale troughs and impact basins on Vesta

Abstract The Dawn mission at Asteroid 4 Vesta revealed two sets of enormous troughs with Divalia Fossae spanning around two-thirds of the equator and Saturnalia Fossae located in the northern hemisphere. Previous studies showed that the Divalia and Saturnalia Fossae are concentric around the Rheasilvia and Veneneia impact basins, respectively, and hence the troughs are widely considered to be genetically linked to the impact events forming the basins. In the existing literature, the Fossae are assumed to be formed directly by, and simultaneously with the basin-forming impacts; however, the temporal relationships of the troughs and how they compare to the impact basins have never been documented. We count crater populations superposed on the Divalia and Saturnalia Fossae to establish their crater frequency distributions and compare that with the previously published data of the Rheasilvia and Veneneia basins, respectively. The comparisons allow us to determine the relative temporal relationships of the troughs and impact basins. Our results show that the Divalia Fossae and the Rheasilvia basin overlap in their crater frequencies, indicating they can be considered coeval within the bounds of the errors. A restrictive age relationship of the Saturnalia Fossae and Veneneia basin emplacement cannot be determined from the crater counts on Vesta since the preserved surface geology of the eastern Veneneia basin may not provide any meaningful information on the emplacement age of the impact structure. However, the Rheasilvia basin superposes the Veneneia basin, and thus the Veneneia basin-forming impact must predate that of Rheasilvia. The crater frequencies of Saturnalia Fossae and the Rheasilvia basin indicate that Saturnalia Fossae are older than or coeval with the Rheasilvia basin emplacement. Taken together, our derived relative age relationships do not contradict but also do not yield tight constraints on the hypothesis that the basin-forming impacts on Vesta directly triggered the formation of troughs.