IMPACT CRATER MORPHOLOGY AND CHRONOLOGY OF VESTA

The Dawn spacecraft now in orbit has revealed the asteroid 4 Vesta to be a heavily cratered object, with craters in a variety of morphologies and preservation states. Craters with dark rays, bright rays, and dark rim streaks have been observed, suggesting possible buried stratigraphy. The largest fresh craters retain a simple bowl-shaped morphology, with depth/diameter ratios average ~0.18, roughly comparable to lunar values. The largest candidate crater, a ~460 km depression at the south pole, has been shown to have an incomplete inward facing cuspate scarp, and a large central mound surrounded by unusual complex arcuate ridge and groove patterns. Although asymmetric in general form, these characteristics are consistent with an impact origin but may also allow for an internally driven mechanism. Rapid rotation of Vesta during impact may explain some anomalous features, through distortion of excavation, collapse and ejecta processes (Jutzi and Asphaug, 2010). A set of equatorial grooves may be related to impact-related reorientation. The most likely alternative interpretations are internal convective downwelling, or a hybrid modified-impact structure. Vesta’s crater size-frequency distribution has been modeled for the unique impact conditions on Vesta, according to Ivanov (2001). The chronology function is a derivative of the lunar chronology, scaled to impact frequencies modeled for Vesta according to Bottke et al. (1994) and O’Brien et al. (2011) on Vesta. The northern hemisphere is heavily cratered by a large variety of ancient degraded and fresh sharp craters. Curiously, first crater counts from lower resolution images of the south polar depression suggest surface model ages similar to ages for northern cratered regions, despite the highly deformed state of the south polar surface.