Neutron absorption constraints on the composition of 4 Vesta |
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| 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 Peplowski Robert C Reedy Michael J Toplis Lucille Le Corre Hugau Mizzon Vishnu Reddy Timothy N Titus Carol A Raymond Christopher T Russell |
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| Institution: | 1. Planetary Science Institute, , Tucson, Arizona, 85719‐2395 USA;2. NASA Johnson Space Center, , Houston, Texas, 77058 USA;3. Smithsonian Institution, National Museum of Natural History, , Washington, District of Columbia, 20560‐0112 USA;4. TechSource, Inc., , Los Alamos, New Mexico, 87544 USA;5. Applied Physics Laboratory, Johns Hopkins University, , Laurel, Maryland, 20723 USA;6. Planetary Geoscience Institute and Department of Earth & Planetary Sciences, University of Tennessee, , Knoxville, Tennessee, 37996‐1410 USA;7. IRAP (UMR 5277), CNRS–Observatoire Midi‐Pyrénées, University of Toulouse, , Toulouse, France;8. Max Planck Institute for Solar System Research, , Lindau, Germany;9. United States Geological Survey, Astrogeology Science Center, , Flagstaff, Arizona, 86001 USA;10. Jet Propulsion Laboratory, California Institute of Technology, , Pasadena, California, 91109‐8099 USA;11. Institute of Geophysics and Planetary Physics, University of California, , Los Angeles, California, 90095‐1567 USA |
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| 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. |
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