Surface morphology of Itokawa regolith particles related tospace weathering on Itokawa.
T. Matsumoto1, A. Tsuchiyama2, A. Miyake2, K. Yasuda3, N.
Watanabe4, A. Kouchi4, A. Takigawa2, T. Noguchi5,Y. Nakata3,
T. Nakamura6, J. Matsuno2, A. Shimada1, 1Osaka Univ., 2Kyoto
Univ., 3The Wakasa Wan energy research center, 4Hokkaido
Univ., 5Ibaraki Univ., 6Tohoku Univ. (tmatsumoto@kueps.kyotou.ac.jp)
Introduction: Surface morphology of Itokawa regolithparticles has important information on interactions between spaceenvironments and solid mineral surfaces on the asteroid [1]. In
preliminary examinations, solar wind He implantation to Itokawa
particles are confirmed by noble gas isotope analysis [2]. Space
weathering rims were also found on the particle surfaces within
100 nm in depth by TEM/STEM, wchich may be primarily
caused by solar wind H and He irradiation [3, 4]. Especially,
vesicles and hillorocks (blister structures) on the space
weathering rims [4] indicate that space weathering modified
surface nano-morphologies of the particles. In this study, in order
to examine the surface nano-morphology related to space
weathering, the surfaces of Itokawa particles were observed by
FE-SEM and TEM/STEM. He+ irradiation experiments were also
performed to compare the Itoakawa particles.
Experiments: Surfaces of nineteen Itokawa particles, whichhave been already examined by microtomography [1], wereobserved using an FE-SEM (JSM-7001F). After FE-SEM
observations, one Itokawa particle (RB-QD04-0043) wassectioned by FIB (Quanta 200 3DS) and were observed by
TEM/STEM (JEM-2100F). He+ ions of 4-50 keV were irradiatedon olivine fragments (Fa30) at the Wakasa Wan Energy ResearchCenter and Institute of Low Temperature Science (HokkaidoUniversity) [5]. The irradiated sample surfaces were also
observed by FE-SEM and TEM/STEM.
Results and Discussion: Numerous convex blister structures ofseveral tens nm in size were observed by FE-SEM on most Itokawa particle surfaces. We observed FIB sections of these blister
structures with TEM/STEM and found that there are vesicularrims similar to the space weathering rims observed by [4].
Twelve of nineteen Itokawa particles have the blister structures,
which indicates that space weathering has been commonly occurred on Itokawa particles. The blister structures are distributed
heterogeneously on each Itokawa particle, suggesting that thedegree of space weathering varies with location on a single grainsurface. Roundness of grain edges, which indicates grain abrasion[6], does not correlate with the blister distribution.
Vesicles and blister structures were also observed on thesurfaces of the He+ irradiated olivine samples. The irradiation
duration for the blister formation (~500 and ~5000 years) seems
to be consistent with solar wind irradiation duration of vesicularrims (the order of 103 years) estimated by solar flare track density[4]. Effects of ion flux difference between cosmic environments
and the experiments should be understood for further quantitativeevaluation for space weathering.
References: [1] Tsuchiyama A. et al. 2013. This volume. [2]Nagao K. et al. 2011. Science 333:1128-1131. [3] Noguchi T.
et al. 2011. Science 333:1121-1125. [4] Noguchi T. et al. 2013.
Met. & Planet. Sci. in print. [5] Matsumoto T. et al. 2013.

LPSC XLIV:1441. [6] Tsuchiyama A. et al. 2011. Science,
333:1125-1128.