RECENT PROGRESS IN HAYABUSA SAMPLE ANALYSIS ON 3D MICROSTRUCTURE AND SURFACE MORPHOLOGY: COMPARISON WITH LL CHONDRITE AND
PROCESSES ON THE ASTEROID SURFACE.
A. Tsuchiyama1, T. Matsumoto2, K. Uesugi3, T. Nakano4, M.
Uesugi5, J. Matsuno1, A. Shimada2 A. Takeuchi3, Y. Suzuki3, T.
Nakamura6, T. Noguchi7, M. Abe5, T. Yada5, Y. Karouji5, Y.
Ishibashi5, K. Shirai5, T. Okada5, A. Fujimura5. 1Kyoto Univ. atsuchi@kueps.kyoto-u.ac.jp, 2Osaka Univ., 3SPring-8/JASRI,4GSJ/AIST, 5JAXA/ISAS, 6 Tohoku Univ., 7Ibaraki Univ.
Introduction: The 3D structures of Itokawa regolith particles
have been revealed by SXCT in 1st PE [1,2]. The mineral abundances and the 3D texture indicate that the particles represent LL
chondrite materials. Their size and shape features and surface
micromorphology suggest that meteoroid impacts on the asteroid
surface primarily form the regolith particle, and that seismicinduced grain motion in the smooth terrain abrades them overtime. In addition to 40 particles from Room A of the sample
chamber examined by 1st PE, 8 and 12 particles were newly examined by 2nd PE and 1st AO, respectively (11 and 9 particles
from Rooms A and B). Their surface nanomorphology were examined by FE-SEM as well as 3D structures by SXCT. The 3D
texture was compared with that of LL5 and 6 chondrite fragments[3]. Here, we report the results of these investigations.
3D texture: comparison with LL chondrites: The mineralabundances, bulk density, porosity, and void and crack featuresare similar between the both samples, showing that the Itokawa
particles are consistent with the LL chondrite materials in termsof not only elemental and isotopic composition of the minerals
but also 3D microtexture [3]. There was not any significant difference between the particles collected from Rooms A and B.
Particle shape and size features: The shape distribution of
the Itokawa particles is consistent with the results of mechanicaldisaggregation [3], primarily as a response to impacts [1]. The
cumulative size distribution confirms the log slope of –2 [3], indicating lower abundance of the fine particles on Itokawa [1].
Surface micro/nanomorphology and space microerosion:
SXCT shows that some particles have rounded edges, whichindicate grain abrasion as proposed by [1]. FE-SEM observationshows a variety of nanomorphologies; fractured structures (e.g.,cleavage steps), convex and concave (pits) structures, whichsometimes have concentric parallel steps probably due to thermal
metamorphism, blisters by solar wind implantation [4], and meltsplash by impacts. Particles with rounded edges do not have clearsteps. They do not correlate with the degree of space weathering
rim development observed by TEM/STEM [5]. The abrasion process has a longer time scale than the space weathering rim formation, and can be regarded as a different type of space weathering in a broad sense, called “space microerosion”.
The present results and studies on space weathering [5] andnoble gas [6] indicate the following surface processes: (1) regolith formation by meteoroid impacts, (2) formation of space
weathering rim by solar wind implantation (~103 yr.), (3) space
microerosion by seismic-induced grain motion (~>103 yr.), andfinal escape by impact after repeating (2) and (3) (<<~8x106 yr.).
References: [1] Tsuchiyama A. et al. 2011. Science,333:1125-1128. [2] Tsuchiyama A. et al. 2013. Geochimica etCosmochimica Acta 116:5-16. [3] Tsuchiyama A. et al. 2013.
Meteoritics & Planetary Science, in print. [4] Matsumoto T. 2013.
This volume. [5] Noguchi T. et al. 2013. Meteoritics & Planetary
Science in print. [6] Nagao K. et al. 2011. Science 333:1128-1131.