Olivine or impact melt: Nature of the “Orange” material on Vesta from Dawn |
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| Authors: | Lucille Le Corre Vishnu Reddy Nico Schmedemann Kris J Becker David P O’Brien Naoyuki Yamashita Patrick N Peplowski Thomas H Prettyman Jian-Yang Li Edward A Cloutis Brett W Denevi Thomas Kneissl Eric Palmer Robert W Gaskell Andreas Nathues Michael J Gaffey David W Mittlefehldt William B Garry Holger Sierks Christopher T Russell Carol A Raymond Maria C De Sanctis Eleonora Ammanito |
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| Institution: | 1. Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA;2. Max-Planck-Institute for Solar System Research, 37191 Katlenburg-Lindau, Germany;3. Institute of Geological Sciences, Freie Universitaet Berlin, 12249 Berlin, Germany;4. Astrogeology Science Center, USGS, Flagstaff, AZ 86001, USA;5. Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA;6. Department of Geography, University of Winnipeg, Manitoba, Canada;g Department of Space Studies, University of North Dakota, Room 518, Box 9008, Grand Forks, ND 58202, USA;h Astromaterials Research Office, NASA Johnson Space Center, Houston, TX 77058, USA;i NASA Goddard Spaceflight Center, Greenbelt, MD 20771, USA;j Institute of Geophysics and Planetary Physics, University of California Los Angeles, Los Angeles, CA 90095, USA;k Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA;l Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, Rome, Italy |
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| Abstract: | NASA’s Dawn mission observed a great variety of colored terrains on asteroid (4) Vesta during its survey with the Framing Camera (FC). Here we present a detailed study of the orange material on Vesta, which was first observed in color ratio images obtained by the FC and presents a red spectral slope. The orange material deposits can be classified into three types: (a) diffuse ejecta deposited by recent medium-size impact craters (such as Oppia), (b) lobate patches with well-defined edges (nicknamed “pumpkin patches”), and (c) ejecta rays from fresh-looking impact craters. The location of the orange diffuse ejecta from Oppia corresponds to the olivine spot nicknamed “Leslie feature” first identified by Gaffey (Gaffey, M.J. 1997]. Icarus 127, 130–157) from ground-based spectral observations. The distribution of the orange material in the FC mosaic is concentrated on the equatorial region and almost exclusively outside the Rheasilvia basin. Our in-depth analysis of the composition of this material uses complementary observations from FC, the visible and infrared spectrometer (VIR), and the Gamma Ray and Neutron Detector (GRaND). Several possible options for the composition of the orange material are investigated including, cumulate eucrite layer exposed during impact, metal delivered by impactor, olivine–orthopyroxene mixture and impact melt. Based on our analysis, the orange material on Vesta is unlikely to be metal or olivine (originally proposed by Gaffey (Gaffey, M.J. 1997]. Icarus 127, 130–157)). Analysis of the elemental composition of Oppia ejecta blanket with GRaND suggests that its orange material has ∼25% cumulate eucrite component in a howarditic mixture, whereas two other craters with orange material in their ejecta, Octavia and Arruntia, show no sign of cumulate eucrites. Morphology and topography of the orange material in Oppia and Octavia ejecta and orange patches suggests an impact melt origin. A majority of the orange patches appear to be related to the formation of the Rheasilvia basin. Combining the interpretations from the topography, geomorphology, color and spectral parameters, and elemental abundances, the most probable analog for the orange material on Vesta is impact melt. |
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| Keywords: | Asteroid Vesta Asteroids composition Asteroids surfaces Mineralogy Spectroscopy |
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