|Author and Affiliation:
|Prettyman, T. H.||(Planetary Science Inst., Tucson, AZ, United States);|
|Mittlefehldt, D. W.||(NASA Johnson Space Center, Houston, TX, United States);|
|Feldman, W. C.||(Planetary Science Inst., Tucson, AZ, United States);|
|Hendricks, J. S.||(TechSource, Inc., Santa Fe, NM, United States);|
|Lawrence, D. J.||(Johns Hopkins Univ., Applied Physics Lab., Laurel, MD, United States);|
|Peplowski, P. N.||(Johns Hopkins Univ., Applied Physics Lab., Laurel, MD, United States);|
|Toplis, M. J.||(Tennessee Univ., TN, United States);|
|Yamashita, N.||(Planetary Science Inst., Tucson, AZ, United States);|
|Beck, A.||(Smithsonian Institution, United States);|
|LeCorre, L.||(Max-Planck-Inst. fuer Sonnensystemforschung, Lindau, Germany);|
|McCoy, T. J.||(Smithsonian Institution, United States);|
|McSween, H. Y.||(Tennessee Univ., TN, United States);|
|Reedy, R. C.||(Planetary Science Inst., Tucson, AZ, United States);|
|Titus, T. N.||(Geological Survey, Astrogeology Science Center, United States);|
|Mizzon, H.||(Toulouse Univ., France);|
|Reddy, V.||(California Univ., Los Angeles, CA, United States);|
Raymond, C. A.
|Joy, S. P.||(Jet Propulsion Lab., California Inst. of Tech., Pasadena, CA, United States);|
Russell, C. T.
The NASA Dawn Mission s Gamma Ray and Neutron Detector (GRaND)  acquired mapping data during 5 months in a polar, low altitude mapping orbit (LAMO) with approx.460-km mean radius around main-belt asteroid Vesta (264-km mean radius) . Neutrons and gamma rays are produced by galactic cosmic ray interactions and by the decay of natural radioelements (K, Th, U), providing information about the elemental composition of Vesta s regolith to depths of a few decimeters beneath the surface. From the data acquired in LAMO, maps of vestan neutron and gamma ray signatures were determined with a spatial resolution of approx.300 km full-width-at-half-maximum (FWHM), comparable in scale to the Rheasilvia impact basin (approx.500 km diameter). The data from Vesta encounter are available from the NASA Planetary Data System. Based on an analysis of gamma-ray spectra, Vesta s global-average regolith composition was found to be consistent with the Howardite, Eucrite, and Diogenite (HED) meteorites, reinforcing the HED-Vesta connection [2-7]. Further, an analysis of epithermal neutrons revealed variations in the abundance of hydrogen on Vesta s surface, reaching values up to 400 micro-g/g . The association of high concentrations of hydrogen with equatorial, low-albedo surface regions indicated exogenic delivery of hydrogen by the infall of carbonaceous chondrite (CC) materials. This finding was buttressed by the presence of minimally-altered CC clasts in howardites, with inferred bulk hydrogen abundances similar to that found by GRaND, and by studies using data from Dawn s Framing Camera (FC) and VIR instruments [8-10]. In addition, from an analysis of neutron absorption, spatial-variations in the abundance of elements other than hydrogen were detected .