¹Andrew W. Beck et al. (>10)*
¹The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
*Find the extensive, full author and affiliation list on the publishers website

Here, we construct a comprehensive howardite, eucrite, and diogenite (HED) bulk chemistry data set to compare with Dawn data. Using the bulk chemistry data set, we determine four gamma-ray/neutron parameters in the HEDs (1) relative fast neutron counts (fast counts), (2) macroscopic thermal neutron absorption cross section (absorption), (3) a high-energy gamma-ray compositional parameter (Cp), and (4) Fe abundance. These correspond to the four measurements of Vesta made by Dawn’s Gamma Ray and Neutron Detector (GRaND) that can be used to discern HED lithologic variability on the Vestan surface. We investigate covariance between fast counts and average atomic mass in the meteorite data set, where a strong correlation (r2 = 0.99) is observed, and we demonstrate that systematic offsets from the fast counttrend are linked to changes in Fe and Ni concentrations. To compare the meteorite and GRaND data, we investigate and report covariance among fast counts, absorption, Cp, and Fe abundance in the HED meteorite data set. We identify several GRaND measurement spaces where the Yamato type B diogenites are distinct from all other HED lithologies, including polymict mixtures. The type B’s are diogenites that are enriched in Fe + pigeonite + diopside ± plagioclase, relative to typical, orthopyroxenitic diogenites. We then compare these results to GRaND data and demonstrate that regions north of ~70°N latitude on Vesta (including the north pole) are consistent with type B diogenites. We propose two models to explain type B diogenite compositions in the north (1) deposition as Rheasilvia ejecta, or (2) type B plutons that were emplaced at shallow depths in the north polar region and sampled by local impacts. Lastly, using principal component (PC) analysis, we identify unique PC spaces for all HED lithologies, indicating that the corresponding GRaND measurables may be used to produce comprehensive lithologic maps for Vesta.

Reference
Andrew W. Beck et al. (2015) Using HED meteorites to interpret neutron and gamma-ray data from asteroid 4 Vesta. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12467]

Published by arrangement with John Wiley&Sons

¹T. K. Croat, ¹C. Floss, ¹B. A. Haas, ²M. J. Burchell, ^2,3A. T. Kearsley
¹Laboratory for Space Sciences and Department of Physics, Washington University, St. Louis, Missouri, USA
²Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent, Canterbury, CT2 7NH, UK
³Department of Earth Sciences, Natural History Museum, London, UK

We present results of FIB–TEM studies of 12 Stardust analog Al foil craters which were created by firing refractory Si and Ti carbide and nitride grains into Al foils at 6.05 km s−1 with a light-gas gun to simulate capture of cometary grains by the Stardust mission. These foils were prepared primarily to understand the low presolar grain abundances (both SiC and silicates) measured by SIMS in Stardust Al foil samples. Our results demonstrate the intact survival of submicron SiC, TiC, TiN, and less-refractory Si3N4 grains. In small (5 μm) are typically fragmented and are somewhat flattened in the direction of impact due to partial melting and/or plastic deformation. The low presolar grain abundance estimates derived from SIMS measurements of large craters (mostly >50 μm) likely result from greater modification of these impactors (i.e., melting and isotopic dilution), due to higher peak temperatures/pressures in these crater impacts. The better survivability of grains in smaller craters suggests that more accurate presolar grain estimates may be achievable through measurement of such craters. It also suggests small craters can provide a complementary method of study of the Wild 2 fine fraction, especially for refractory CAI-like minerals.

Reference
Croat TK, Floss C, Haas BA, Burchell MJ, Kearsley AT (2015) Survival of refractory presolar grain analogs during Stardust-like impact into Al foils: Implications for Wild 2 presolar grain abundances and study of the cometary fine fraction. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12474]

Published by arrangement with John Wiley&Sons