Distribution of space weathered rims on Itokawa regolithparticles and implication to space weathering of Asteroid
Itokawa.
T. Matsumoto1, A. Tsuchiyama1, T. Nakamura2, M. Nakamura2,K. Uesugi3, T. Nakano4. 1Kyoto University, Japan. 2Tohoku
University, Japan. 3JASRI/SPring-8, Japan. 4AIST/GSJ, Japan.
(tmatsumoto@kueps.kyoto-u.ac.jp.)
Introduction: Asteroidal regolith particles have informations
about surface evolusion porcesses of the asteroids. Analysis byTEM and STEM of regolith particles of S-type asteroid 25143
Itokawa revealed that space weathered rims including nano-phaseiron surround the regolith particles, which were probably formedby micrometeoroid bombardment and solar wind irradiation [1,2]. Previous study shows that space weathered rims with vesicles(blisters) on regolith particles can be identified non-destructively
by observation of the particle surfaces with FE-SEM [3]. So far,
distribution of space weathered rims on the particle surfaces havenot been investigated. In this study, distributions of the space
weathered rims were examined and discuss the evolution processes of the individual particle surfaces and space weathering
processes of Itokawa.
Methods: Three-dimensional (3D) external shapes of twenty
Itokawa regolith particles were analyzed by X-ray microtomography (µ-CT) at SPring-8, Japan. After µ-CT experiments, observations of surface micromorphologies of the Itokawa regolithparticles were performed by FE-SEM at Kyoto University.
Results and discussions: Itokawa particle surfaces are composed of mainly two types: fractured surfaces formed by impact
and surfaces of micro-druses, which were covered by subhedral
to euhedral mineral grains formed by probably thermal metamorphism of fine materials or by vapor condensation. The surfaces ofItokawa particles can be also classified in two types; fresh surfaces having sharp edges, and matured surfaces having roundededges. Previous study by µ-CT reported that the matured surfaces
were considered to be formed from fresh surfaces by abrasion
processes on Itokawa [4]. The fresh and matured surfaces wereobserved in both surfaces formed by impact and surfaces of micro-druses in this study.
Eleven out of twenty regolith particles have space weatheredrims with blisters. These rims are heterogeneously distributed andthe rims often present in opposite surfaces of the same particle,
suggesting migration of regolith particles on Itokawa. In addition,
the blister distribution and the roundness of the particle surface
are not correlated, indicating that main mechanism of the abrasion process is not solar wind sputtering but mechanical abrasion.
The picture of space weathering processes of Asteroid Itokawa was speculated as follows. Space weathered rims were developed on local surfaces of individual regolith particles, which
promotes the spectral change of Asteroid Itokawa. On the otherhand, refreshment of the regolith surfaces occurred, which suppresses the spectral change, by mechanical abrasion due to grain
migration and fragmentation due to impact.
References: [1] Noguchi T. et al. 2011. Science, 333, 11211125. [2] Noguchi T. et al. 2013. Meteoritics & Planetary Science. 27, 1-27. [3] Matsumoto T. et al. 2013. Goldschmidt 2013
Conf. Abstract pp.1711 [4] Tsuchiyama A. et al. 2011. Science,333, 1125-1128.