¹Toru Matsumoto et al. (>10)*
¹Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
*Find the extensive, full author and affiliation list on the publishers website

The morphological properties of 26 regolith particles from asteroid Itokawa were observed using scanning electron microscopes in combination with an investigation of their three-dimensional shapes obtained through X-ray microtomography. Surface observations of a cross section of the LL5 chondrite, and of crystals of olivine and pyroxene, were also performed for comparison. Some Itokawa particles have surfaces corresponding to walls of microdruses in the LL chondrite, where concentric polygonal steps develop and euhedral or subhedral grains exist. These formed through vapor growth owing to thermal annealing, which might have been caused by thermal metamorphism or shock-induced heating in Itokawa’s parent body. Most of the Itokawa particles have more or less fractured surfaces, indicating that they were formed by disaggregation, probably caused by impacts. Itokawa particles with angular and rounded edges observed in computed tomography images are associated with surfaces exhibiting clear and faint structures, respectively. These surfaces can be interpreted by invoking different degrees of abrasion after regolith formation. A possible mechanism for the abrasion process is grain migration caused by impact-driven seismic waves. Space-weathered rims with blisters are distributed heterogeneously across the Itokawa regolith particles. This heterogeneous distribution can be explained by particle motion and fracturing, combined with solar-wind irradiation of the particle surfaces. The regolith activity—including grain motion, fracturing, and abrasion—might effectively act as refreshing process of Itokawa particles against space-weathered rim formation. The space-weathering processes affecting Itokawa would have developed simultaneously with space-weathered rim formation and regolith particle refreshment.

Reference
Matsumoto T et al. (2016) Nanomorphology of Itokawa regolith particles:Application to space-weathering processes affecting the Itokawa asteroid. Geochimica et Cosmochmica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.05.011]
Copyright Elsevier

^1,2,3Levke Kööp, ^1,2,3,4Andrew M. Davis, ^5,6Daisuke Nakashima, ^7,8Changkun Park, ⁷Alexander N. Krot, ⁷Kazuhide Nagashima, ⁵Travis J. Tenner, ^1,2,3Philipp R. Heck, ⁵Noriko T. Kita
¹Department of the Geophysical Sciences, The University of Chicago, Chicago, IL, USA
²Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL, United States
³Robert A. Pritzker Center for Meteoritics and Polar Studies, Field Museum of Natural History, Chicago, IL, USA
⁴Enrico Fermi Institute, The University of Chicago, Chicago, IL, USA
⁵Dept. Geoscience, University of Wisconsin-Madison, WI, USA
⁶Division of Earth and Planetary Materials Science, Tohoku University, Sendai, Japan
⁷HIGP/SOEST University of Hawai‘i at Mānoa, Honolulu, HI, USA
⁸Korea Polar Research Institute, Incheon, Korea

PLACs (platy hibonite crystals) and related hibonite-rich calcium-, aluminum-rich inclusions (CAIs; hereafter collectively referred to as PLAC-like CAIs) have the largest nucleosynthetic isotope anomalies of all materials believed to have formed in the solar system. Most PLAC-like CAIs have low inferred initial 26Al/27Al ratios and could have formed prior to injection or widespread distribution of 26Al in the solar nebula. In this study, we report 26Al-26Mg systematics combined with oxygen, calcium, and titanium isotopic compositions for a large number of newly separated PLAC-like CAIs from the Murchison CM2 chondrite (32 CAIs studied for oxygen, 26 of these also for 26Al-26Mg, calcium and titanium). Our results confirm (1) the large range of nucleosynthetic anomalies in 50Ti and 48Ca (our data range from –70 to 170‰ and –60 to 80‰, respectively), (2) the substantial range of Δ17O values (–28 to –17‰, with Δ17O = δ17O–0.52×δ18O), and (3) general 26Al-depletion in PLAC-like CAIs.

The multielement approach reveals a relationship between Δ17O and the degree of variability in 50Ti and 48Ca: PLAC-like CAIs with the highest Δ17O (∼–17‰) show large positive and negative 50Ti and 48Ca anomalies, while those with the lowest Δ17O (∼–28‰) have small to no anomalies in 50Ti and 48Ca. These observations could suggest a physical link between anomalous 48Ca and 50Ti carriers and an 16O-depleted reservoir. We suggest that the solar nebula was isotopically heterogeneous shortly after collapse of the protosolar molecular cloud, and that the primordial dust reservoir, in which anomalous carrier phases were heterogeneously distributed, was 16O-depleted (Δ17O ⩾ –17‰) relative to the primordial gaseous (CO + H2O) reservoir (Δ17O < –35‰). However, other models such as CO self-shielding in the protoplanetary disk are also considered to explain the link between oxygen and calcium and titanium isotopes in PLAC-like CAIs.

Reference
Kööp L, Davis AM, Nakashima D, Park C,Krot AN, Nagashima K, Tenner TJ, Heck PR, Kita NT (2016) A link between oxygen, calcium and titanium isotopes in 26Al-depleted hibonite-rich CAIs from Murchison and implications for the heterogeneity of dust reservoirs in the solar nebula. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.05.014]
Copyright Elsevier