Concentrations of Potassium and Thorium within Vesta’s Regolith

The globally-averaged concentrations of radioelements K and Th within Vesta’s regolith are determined from gamma ray spectra acquired by Dawn’s Gamma Ray and Neutron Detector (GRaND). Spectra measured by GRaND’s bismuth germinate (BGO) scintillator, while in close proximity to Vesta, are analyzed. Improvements in data reduction and analysis methods enable detection and quantification of K and Th. Ample precision is achieved using the entire data set acquired by Dawn during 5 months of low-altitude operations. A simple, analytic model, which can be applied to measurements of Vesta and Ceres, is used to determine radioelement concentrations from measured counting rates. Systematic errors in the analysis are evaluated using simulated gamma ray spectra for representative vestan meteorite compositions. Concentrations of K and Th within Vesta’s global regolith, measured by GRaND, are consistent with eucrite-rich howardite, and are distinct from most achondrites, all chondrites, and Mars meteorites. The K/Th ratio of Vesta (900 400) is similar to the average ratio for howardite (approximately 1200). These radioelement data, along with major element ratios determined by nuclear spectroscopy, strongly support the hypothesis that Vesta is the parent body of the HEDs. The depletion of moderately-volatile elements implied by the measured K/Th ratio is consistent with early accretion of Vesta from a hot, incompletely condensed solar nebula and/or, less likely, subsequent removal of volatiles by energetic collisions or degassing of magmas.