IMPLICATIONS FOR A COMPLEX GEOLOGIC HISTORY OF ASTEROID 4 VESTA: PRIMARY VESTAN LITHOLOGIES MAPPED USING GAMMA RAY AND NEUTRON SPECTROSCOPY

The differentiated asteroid 4 Vesta was recently orbited and mapped by the gamma ray and neutron detector onboard the Dawn spacecraft. Vesta is the parent body for the most voluminous group of differentiated meteorites, the howardite, eucrite and diogenite (HED) meteorite clan. We conducted laboratory simulations of 250 HEDs to establish gamma ray and neutron behavior in numerous HED lithologies. Using those established behaviors, we map the distribution of pure primary (igneous) HED lithologies on Vesta and note some observations:

1. Only 3% of the surface is pure basaltic eucrite. Basaltic eucrite formed the primordial crust on Vesta ~4.56 Ga ago and its present paucity is likely due to extensive impact mixing with other lithologies over time and a lack of subsequent basaltic resurfacing.
2. 0% of the surface is pure orthopyroxenitic diogenite, an observation that is corroborated by other Dawn instruments. This supports the hypothesis that ejecta from Rheasilvia (south-polar impact thought to have excavated and introduced orthopyroxenite to the surface) is much older than 1 Ga (one proposed date for Rheasilvia formation).
3. 11% of the total surface of Vesta, all localized to the north-polar region, is pure Yamato type B diogenite (evolved gabbronorite). The north-polar gabbronoritic regions show a strong correlation with young (post-Rheasilvia), relatively shallow craters. The association of 1) an evolved meteorite lithology that has been hypothesized to possibly form from re-melting of vestan mantle with 2) post-Rheasilvia craters in an area that is 3) antipodal to Rheasilvia, raises the possibility that type B lithologies in the north resulted from antipodal melting of the vestan mantle associated with the Rheasilvia impact event. At the very least, the identification of gabbronoritic diogenite exposed at shallow depths in the crust and localized to the north polar region supports more complicated differentiation models for Vesta (i.e. not a simple magma ocean).