Asymmetric craters on Vesta: Impact on sloping surfaces

摘要

Cratering processes on planetary bodies happen continuously and cause the formation of a large variety of impact crater morphologies. On Vesta whose surface has been imaged at high resolution during a 14 months orbital mission by the Dawn spacecraft we identified a substantial number of craters with an asymmetrical shape. These craters, in total a number of 2892 ranging in diameter from 0.3 km to 43 km, are characterized by a sharp crater rim on the uphill side and a smooth one on the downhill side. The formation of these unusual asymmetric impact craters is controlled by Vesta's remarkable topographic relief. In order to understand the processes creating such unusual crater forms on a planetary body with a topography like Vesta we carried out the following work packages: (1) the asymmetric craters show various morphologies and therefore can be subdivided into distinct classes by their specific morphologic details; (2) using a digital terrain model (DTM), the craters are grouped into bins of slope angles for further statistical analysis; (3) for a subset of these asymmetric craters, the size-frequency distributions of smaller craters superimposed on their crater floors and continuous ejecta are measured in order to derive cratering model ages for the selected craters and to constrain possible post-impact processes; (4) three-dimensional hydrocode simulations using the iSALE-3D code are applied to the data set in order to quantify the effects of topography on crater shape and ejecta distribution. We identified five different classes (A-E) of asymmetric craters. Primarily, we focus on class A in this work. The global occurrence of these crater classes compared with a slope map clearly shows that these asymmetric crater types exclusively form on slopes. We found that slopes, especially slopes ＞20°, prevent the deposition of ejected material in the uphill direction, and slumping material superimposed the deposit of ejecta on the downhill side. The combination of these two processes explains the local accumulation of material in this direction. In the subset of asymmetric craters which we used for crater counts, our results show that no post-impact processes have taken place since floors and continuous ejecta in each crater show comparable cratering model ages within the uncertainties of the cratering chronology model. Therefore the formation, or modification, of the asymmetric crater forms by processes other than impact can be excluded with some certainty.