Tomoki Nakamura¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Division of Earth and Planetary Materials Science, Laboratory for Early Solar System Evolution, Graduate School of Science, Tohoku University, Aoba, Sendai, Miyagi, Japan

The mineralogy and mineral chemistry of Itokawa dust particles captured during the first and second touchdowns on the MUSES-C Regio were characterized by synchrotron-radiation X-ray diffraction and field-emission electron microprobe analysis. Olivine and low- and high-Ca pyroxene, plagioclase, and merrillite compositions of the first-touchdown particles are similar to those of the second-touchdown particles. The two touchdown sites are separated by approximately 100 meters and therefore the similarity suggests that MUSES-C Regio is covered with dust particles of uniform mineral chemistry of LL chondrites. Quantitative compositional properties of 48 dust particles, including both first- and second-touchdown samples, indicate that dust particles of MUSES-C Regio have experienced prolonged thermal metamorphism, but they are not fully equilibrated in terms of chemical composition. This suggests that MUSES-C particles were heated in a single asteroid at different temperatures. During slow cooling from a peak temperature of approximately 800 °C, chemical compositions of plagioclase and K-feldspar seem to have been modified: Ab and Or contents changed during cooling, but An did not. This compositional modification is reproduced by a numerical simulation that modeled the cooling process of a 50 km sized Itokawa parent asteroid. After cooling, some particles have been heavily impacted and heated, which resulted in heterogeneous distributions of Na and K within plagioclase crystals. Impact-induced chemical modification of plagioclase was verified by a comparison to a shock vein in the Kilabo LL6 ordinary chondrite where Na-K distributions of plagioclase have been disturbed.

Reference
Nakamura T et al. (in press) Mineral chemistry of MUSES-C Regio inferred from analysis of dust particles collected from the first- and second-touchdown sites on asteroid Itokawa. Meteoritics & Planetary Science
[doi:10.1111/maps.12247]
Published by arrangement with John Wiley & Sons

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Bo Reipurth

Astronomy, University of Hawaii, Hilo, Hawaii, USA

No abstract is available for this article.

Reference
Reipurth B (in press) Road-map to the Indian’s treasure—on the Chilean meteorite Vaca Muerta and its early mistake for silver, by Holger Pedersen. Norderstedt, Germany: Books on Demand, 2012, 312 p., paperback (ISBN-13: 9788771144406). Available through German book dealers. Meteoritics & Planetary Science
[doi:10.1111/maps.12253]
Published by arrangement with John Wiley & Sons

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Ottaviano Ruesch^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany

The Dawn Framing Camera (FC) has imaged the northern hemisphere of the asteroid (4) Vesta at high spatial resolution and coverage. This study represents the first investigation of the overall geology of the northern hemisphere (22°N-90°N, quadrangles Av-1, 2, 3, 4 and 5) using these unique Dawn mission observations. We have compiled a morphologic map and performed crater size-frequency distribution (CSFD) measurements to date the geologic units. The hemisphere is characterized by a heavily cratered surface with a few highly subdued basins up to ~200 km in diameter. The most widespread unit is a plateau (cratered highland unit), similar to, although of lower elevation than the equatorial Vestalia Terra plateau. Large-scale troughs and ridges have regionally affected the surface. Between ~180° and ~270°E, these tectonic features are well developed and related to the south pole Veneneia impact (Saturnalia Fossae trough unit), elsewhere on the hemisphere they are rare and subdued (Saturnalia Fossae cratered unit). In these pre-Rheasilvia units we observed an unexpectedly high frequency of impact craters up to ~10 km in diameter, whose formation could in part be related to the Rheasilvia basin-forming event. The Rheasilvia impact has potentially affected the northern hemisphere also with S-N small-scale lineations, but without covering it with an ejecta blanket. Post-Rheasilvia impact craters are small (<60 km in diameter) and show a wide range of degradation states due to impact gardening and mass wasting processes. Where fresh, they display an ejecta blanket, bright rays and slope movements on walls. In places, crater rims have dark material ejecta and some crater floors are covered by ponded material interpreted as impact melt.

Reference
Ruesch O et al. (in press) Geologic map of the northern hemisphere of Vesta based on Dawn FC images. Icarus
[doi:10.1016/j.icarus.2014.01.035]
Copyright Elsevier

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