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Psrd he ds-vesta

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  • 1. Overview: Scientists are primed with geochemical data from HED meteoritesfor Dawns encounter with asteroid 4 VestaReference: Usui, T. and McSween, Jr., H. Y. (2007) Geochemistry of 4 Vestabased on HED meteorites: Prospective study for interpretation of gamma rayand neutron spectra for the Dawn mission. v. 42, Meteoritics and PlanetaryScience, v. 42, p. 255-269.Abstract: The howardite-eucrite-diogenite class of meteorites (called theHEDs) are rocks formed from basaltic magmas. What makes them special isthat the HEDs have reflectance spectra in the visible and near-infrared thatmatch spectra from asteroid 4 Vesta, implying Vesta is their parent body. Wewill soon have new data from Vesta from NASAs Dawn orbiting spacecraft,which carries a gamma ray and neutron detector, dubbed the GRaNDinstrument. GRaND will orbit asteroid 4 Vesta and dwarf planet Ceres andmap the near-surface abundances of major and minor elements, and volatilesfound in ices (in the case of Ceres) such as hydrogen, carbon, nitrogen, andoxygen. Tomohiro Usui and Harry Y. (Hap) McSween, Jr. (University ofTennessee) have proposed a way to interpret the upcoming GRaND data fromVesta based on well-analyzed samples of HED meteorites and a mixing model 1
  • 2. GRaND uses 21 sensors to measure the energy from gamma rays andneutrons that are reflected or emitted by the different elements on the targetbodys surface and down to a depth of one meter. Scientists will make a globalelemental map of Vesta to show where elements exist and in whatabundances. They are particularly interested in mapping major elements (Si,Fe, Ti, Mg, Al, Ca) and minor elements (K, Th, U) on Vesta. The beauty of theminor elements is that they track the melting that led to the formation of thecrust of the body during differentiation.Because of GRaNDs coarse spatial resolution, its data is expected to mimicthe mixing of HED meteorites.An interesting aspect of this approach by Usui and McSween is that the mixingrelations of the three end-member rock types can be plotted in an appropriatetwo-dimensional diagram. 2
  • 3. The dashed line represents perfect agreement between measured andcalculated concentrations. Elements are not labeled to avoid cluttering thediagram. 3
  • 4. Usui and McSween show their mixing model can accurately estimate theabundances of all the major elements that can be measured by GRaND aswell as predict abundances of minor elements (Na, Cr, and Mn) not analyzedby the instrument.The use of the vast amount of data on HED meteorites to devise a mixingmodel to understand data from a spacecraft orbiting Vesta shows the value ofan integrated approach to solar system exploration.Both laboratory analysis of samples like that funded by the CosmochemistryProgram and remote sensing data like that to be radioed back by the Dawnmission are crucial bits of information about the igneous histories of thebuilding blocks of the planets.In neutron activation analysis, rock samples are irradiated by neutrons in aresearch nuclear reactor (like the one shown on this slide from the Universityof Missouri, Columbia). Nuclear interactions take place, producing radioactiveisotopes. When the isotopes decay, they produce gamma rays with energiescharacteristic of the isotope, giving a measurement of the abundance of anelement. The samples are placed into shielded detectors for gamma raymeasurements. Typically, the abundances of 25 to 30 elements are 4