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Neutron absorption constraints on the composition of 4 Vesta

November 1, 2013

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.

Publication Year 2013
Title Neutron absorption constraints on the composition of 4 Vesta
DOI 10.1111/maps.12244
Authors Thomas H. Prettyman, David W. Mittlefehldt, Naoyuki Yamashita, Andrew W. Beck, William C. Feldman, John S. Hendricks, David J. Lawrence, Timothy J. McCoy, Harry Y. McSween, Patrick N. Paplowski, Robert C. Reedy, Michael J. Toplis, Lucille Le Corre, Hugau Mizzon, Vishnu Reddy, Timothy N. Titus, Carol A. Raymond, Christopher T. Russell
Publication Type Article
Publication Subtype Journal Article
Series Title Meteoritics and Planetary Science
Index ID 70058462
Record Source USGS Publications Warehouse
USGS Organization Astrogeology Science Center