Delivering a projectile component to the vestan regolith

Dark material on Vesta may consist of carbonaceous chondrite-like material delivered by impact events. This study uses hypervelocity impact experiments to assess the viability of the impact delivery hypothesis. Experiments reveal that impact events deliver significant fractions of the projectile to the target during impacts at average vestan speeds. Hence, dark material can plausibly be delivered to Vesta by impacts, with the projectile component accumulating in the regolith with time. Projectile retention is sensitive to impact angle, ranging from 7% for 30° impacts (measured from horizontal) to 72% for vertical impacts. Averaged over the probability distribution of impact angles, 17% of the projectile's mass remains in or near the crater. Projectile-contaminated breccias largely remain inside the crater for vertical impacts. In oblique impacts, projectile-contaminated pieces concentrate downrange beyond the crater rim. Based on experiments, projectile delivery is expected for typical vestan impact conditions, not only for extraordinary events such as low-probability and very low-speed (e.g., <2 km s À1) impacts. These experiments indicate that other (non-dark) impactors contaminate the vestan regolith. Regolith-laden bodies in environments with similar impact speeds also may accrete significant amounts of foreign debris. Impact processes sculpt the surfaces of the moons, planets, and small bodies in the Solar System. Some of these impacts leave lingering compositional traces of the projectile on the target object. The delivery of a projectile component is sensitive to many factors, including impact angle, impact speed, projectile composition, the presence of an atmosphere, and the material behavior of the projectile and target. Asteroid 4 Vesta may be a particularly striking case of impactor contamination. Instruments on the Dawn spacecraft observed hydrated, low-albedo material, which many authors have interpreted to be remnants of carbonaceous chondrite-like impactors that collided with Vesta (Prettyman et al. Although impact delivery is these authors' favored explanation for dark material, the efficiency of the delivery process and physical state of the retained projectile component are poorly known. If projectile retention efficiency can be accurately determined, then the amount of dark material on Vesta may constrain the populations of dark and non-dark impactors and the degree of dynamical depletion in the main belt. The experiments reported here directly constrain the projectile retention efficiency of stony impactors for Vesta-like conditions. have explored projectile delivery using numerical impact simulations. These studies, however, did not assess impactor delivery to Vesta specifically. In many cases, the types of impactors or impact speeds were not …