Vesta’s Pinaria Region- A window on Vesta’s Ancient Crust

Abstract:

Asteroid 4 Vesta was the first of the NASA Dawn mission’s two asteroid targets. After its 14-month orbit its instruments and science team produced maps from which geology, mineralogy, elemental composition and internal structure of Vesta were characterized. Using data from both the high altitude and low altitude orbits, we examine visible and infrared spectra from the Pinaria region which is located between 21°-66° S lat and 0°-90° E long in the Claudia coordinate system. We present data from the visible and near-infrared spectrometer to derive band parameter maps that are interpreted in terms of mineralogy and petrology. Within the Pinaria quadrangle is Matronalia Rupes, a rim of the ~505 km south polar basin, Rheasilvia. Spectra from this rim wall have band parameters of low-iron orthopyroxene that are a marker of diogenite meteorites. Studies of diogenites support their origin from the lower crust of the basaltic achondrite parent body, which has been demonstrated via analysis of Dawn’s data sets, to be Vesta. Regolith on the smooth plains Northwest of the rim has band parameters indicative of howardites, a mixture of lower crust diogenites and crustal eucrites. The regolith here has more of a diogenitic component than eucritic, suggesting that basin ejecta mixed with the eucritic crust after formation of Rheasilvia. Another aspect of the Pinaria region is that the spectral signature of 2.8 µm OH absorption is most often absent, making this region anhydrous compared to other regions more northerly and further east of Pinaria. Current analysis suggests that hydrated material is exogenic to Vesta. Examination of the spectra of the Pinaria crater’s floor compared to its wall shows a broader 1.9 µm band of varying absorption depth that indicates a range of pyroxene composition that is expected in a regolith consisting of mixed components from different depths. The lower, anhydrous crust of Vesta is revealed in the Pinaria region.