Impact Histories of Vesta and Vestoids Inferred

Introduction: The parent body of the howardites, eucrites and diogenites (HEDs) is thought to be asteroid (4) Vesta [1]. However, several eucrites have now been recognized, like NWA 011 and Ibitira, with major element compositions and mineralogy like normal eucrites but with different oxygen isotope compositions and minor element concentrations suggesting they are not from the same body [2, 3]. The discoveries of abnormal eucrites and V-type asteroids that are probably not from Vesta [see 4] raise the question whether the HEDs with normal oxygen isotopes are coming from Vesta [3]. To address this issue and understand more about the evolution of Vesta in preparation for the arrival of the Dawn spacecraft, we integrate fresh insights from Ar-Ar dating and oxygen isotope analyses of HEDs, radiometric dating of differentiated meteorites, as well as dynamical and astronomical studies of Vesta, the Vesta asteroid family (i.e., the Vestoids), and other V-type asteroids. have been found with abnormal oxygen isotopic compositions indicating they probably come from at least three and probably five separate bodies, not from the HED body [2, 5]. (Since howardites and diogenites with abnormal oxygen isotopes are not known, we use " HED group " and " HED body " for samples with normal oxygen isotopes and their source.) Does the plausible connection between the large group of normal eucrites, howardites and dio-genites and Vesta, the only large, intact basaltic asteroid , and its very prominent family of sub-10 km asteroids exclude the possibility that a eucrite with abnormal oxygen isotopes comes from Vesta and not HEDs? Bogard's study of meteorite radiometric ages [6] suggested that the proportion of meteorites from individual parent bodies that have radiometric ages reflecting impact heating during the Late Heavy Bombardment (LHB) increases as one moves to larger parent bodies: H-type ordinary chondrites (parent body diameter D PB ~ 200 km), HEDs (D PB ~ 500 km), and lunar rocks (D PB ~ 3400 km). Bogard [6] argued that smaller bodies lose impact ejecta more readily and are destroyed by the same impacts that cause most impact heating on larger bodies. For example, the maximum impact energy per kg of target that can be deposited on Vesta during a near-catastrophic collision is ~100× higher than for a near-catastrophic impact on a 10 km