Abstract
We investigated the surface texture and chemical compositions of two ~40-μm particles returned from the surface regolith of asteroid Itokawa (RB-DQ04-0062 and RB-DQ04-0091) by the Japan Aerospace Exploration Agency’s Hayabusa mission. We identified splash melts, surface blistering, and many small adhering particles. Seven focused ion beam sections were extracted from both Itokawa particles, targeting one splash melt and ten adhering particles to investigate their composition and provenance and the role of micrometeoroid impacts on Itokawa’s surface. Based on the particle’s structure, mineralogy, and interface between the adhering particle and host grain, we identified lithic fragments and particles deposited by impact. These have morphologies and compositions consistent with impact-generated deposits: two have morphologies and compositions that are consistent with impact-generated silica glass, and one was a Ni-free, metallic Fe, and S-rich assemblage that was likely generated by vapor recondensation during a micrometeoroid impact. This study shows that, even though its regolith is young, micrometeoroid impacts have altered the regolith of asteroid Itokawa.
Highlights
Analyses of returned samples provide the opportunity to overcome the constraints imposed by remote-sensing and in situ studies of bodies in our solar system (Gaffey et al 1989)
The Itokawa particles analyzed in this study (RBDQ04-0062—Naoko and RB-DQ04-0091—Mizuki) are both irregular, angular grains with the longest dimension ~40 μm in length (Fig. 1)
In the particle Mizuki (Fig. 1b), we identified plagioclase (An0-18Ab75-95, no exsolution or twinning), Al2O3-rich silica glass (22 wt.% Al2O3 and 78 wt.% SiO2, the Al/Si ratio is consistent with feldspar), and FeO-rich olivine (~Fa25)
Summary
Analyses of returned samples provide the opportunity to overcome the constraints imposed by remote-sensing and in situ studies of bodies in our solar system (Gaffey et al 1989). The Hayabusa mission collected surface regolith samples (unconsolidated surface deposits) from the S-type near-Earth asteroid 25143 Itokawa and returned them to Earth for laboratory study. More than 1500 particles were identified, ranging in size from 3 to 180 μm (Nakamura et al 2011). Mineralogical and oxygen isotope analyses revealed that the composition of the particles is consistent with LL5-6 chondrite composition (Nakamura et al 2011; Yurimoto et al 2011; Nakashima et al 2013). The return of the Hayabusa samples has provided us the strongest evidence that the most common meteorites in our collections, the ordinary chondrites, are derived from the S-type asteroids (Nakamura et al 2011; Thompson et al 2014). The Hayabusa samples are the second extraterrestrial regolith, after the lunar samples, which can give us information about surface modification
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