Neutron absorption constraints on the composition of 4 Vesta

Abstract

Abstract-Global maps of the macroscopic thermal neutron absorption cross section of Vesta's regolith by the Gamma Ray and Neutron Detector (GRaND) on board the NASA Dawn spacecraft provide constraints on the abundance and distribution of Fe, Ca, Al, Mg, and other rock-forming elements. From a circular, polar low-altitude mapping orbit, GRaND sampled the regolith to decimeter depths with a spatial resolution of about 300 km. At this spatial scale, the variation in neutron absorption is about seven times lower than that of the Moon. The observed variation is consistent with the range of absorption for howardite whole-rock compositions, which further supports the connection between Vesta and the howardite, eucrite, and diogenite meteorites. We find a strong correlation between neutron absorption and the percentage of eucritic materials in howardites and polymict breccias, which enables petrologic mapping of Vesta's surface. The distribution of basaltic eucrite and diogenite determined from neutron absorption measurements is qualitatively similar to that indicated by visible and near infrared spectroscopy. The Rheasilvia basin and ejecta blanket has relatively low absorption, consistent with Mg-rich orthopyroxene. Based on a combination of Fe and neutron absorption measurements, olivine-rich lithologies are not detected on the spatial scales sampled by GRaND. The sensitivity of GRaND to the presence of mantle material is described and implications for the absence of an olivine signature are discussed. High absorption values found in Vesta's "dark" hemisphere, where exogenic hydrogen has accumulated, indicate that this region is richer in basaltic eucrite, representative of Vesta's ancient upper crust.

The chronostratigraphy of protoplanet Vesta

“…Application of crater statistical techniques were applied to mapped terrains to determine relative and absolute model ages of portions of the surface (Marchi et al, 2012a(Marchi et al, , 2012bO'Brien et al, 2014;Schmedemann et al, 2014). Hyperspectral and thermophysical analyses of multi-wavelength visible and near-infrared data from Dawn's Visible and Infrared Spectrometer (VIR: De Sanctis et al, 2012;Ammannito et al, 2013a), and correlation of geology with geochemical distributions derived from Dawn's Gamma Ray and Neutron Detector (GRaND: Prettyman et al, 2012Prettyman et al, , 2013Yamashita et al, 2013), were used to provide information on the mineralogical and physical composition of surface materials. Finally, assessment of Vesta's gravity, mass distributions, and crustal thickness were obtained from Dawn's Radio Science experiment (Konopliv et al, 2013;Park et al, 2014;Raymond et al, 2013Raymond et al, , 2014.…”

Imprint of the Rheasilvia impact on Vesta – Geologic mapping of quadrangles Gegania and Lucaria

“…This lobe has a low iron counting rate and possesses almost the same spatial distribution as the region found in FC images. Another comparison between GRaND and FC data is given by Prettyman et al (2013), where a diogenite rich region with low Percentage of Eucritic Material (POEM) has been found by GRaND in nearly the same area.…”

Asteroid (4) Vesta II: Exploring a geologically and geochemically complex world with the Dawn Mission

“…Those data were presented and discussed in Yamashita et al (2013b), and the data shown in this work were taken from the NASA Planetary Data Systems (PDS) . Vestan thermal neutron absorption data presented here also are from the NASA PDS (Prettyman, 2013), and were first presented and discussed in Prettyman et al (2013). Fe concentrations and thermal neutron macroscopic absorption for cross sections for HEDs are part of a As can be seen in Fig. 14, basaltic eucrite, cumulate eucrite and orthopyroxenitic diogenites (the three main vestan igneous lithologies, as per the HEDs) establish three distinct regions in Fe vs. eff -space.…”

“…Surprisingly, reflectance, gamma ray and neutron spectral data thus far show no signs of olivine in the Rheasilvia basin Prettyman et al, 2013;Ammannito et al, 2013a). Given the size of the Rheasilvia basin, it was anticipated that the mantle of Vesta would be exposed, likely including olivine-rich materials.…”

“…In contrast, pure basaltic eucrite terrains are more difficult to establish using VIR due to overlaps in band centers with howardite compositions. However, the most eucrite-rich portion of the surface can be found near the equator (e.g., Reddy et al, 2012a,b;De Sanctis et al, 2012;Ammannito et al, 2013a;Prettyman et al, 2013), and corresponds to most heavily cratered, oldest terrains on Vesta . This suggests that the initial vestan upper crust was nearly pure eucrite, a hypothesis supported by HED meteorites.…”