Measuring Shock Stage of Itokawa Regolith Grains by Electron Back-Scattered Diffraction and Synchrotron X-ray
Diffraction
Michael Zolensky1, Takashi Mikouchi2, Kenji Hagiya3, Kazumasa Ohsumi4, James Martinez1, Kenji Hagiya3, Scott Sitzman5,
Yasuko Terada4, Naoto Yagi4, Mutsumi Komatsu6, Hikaru Ozawa3, Yuta Taki3, Yuta Yamatsuta3, Atsushi Takenouchi2, Hikari
Hasegawa2, Haruka Ono2, Kotaro Higashi2, Masaki Takata4, Arashi Hirata3, Ayaka Kurokawa3, Shoki Yamaguchi3.
1
ARES, NASA JSC, Houston, TX (USA); 2School of Science, Univ. of Tokyo (Japan); 3Graduate School of Life Science, Univ. ofHyogo (Japan); 4Japan Synchrotron Radiation Research Institute (Japan); 5Aerospace Corporation, El Segundo, CA, (USA);
6SOKENDAI (Japan).
We have been analyzing Itokawa samples in order to definitively establish the degree of shock experienced by theregolith of asteroid Itokawa, and to devise a bridge between shock determinations by standard light optical petrography, crystal
structures as determined by electron and X-ray diffraction techniques [1,2,3,4]. We are making measurements of olivine crystal
structures and using these to elucidate critical regolith impact processes. We use electron back-scattered diffraction (EBSD)
and synchrotron X-ray diffraction (SXRD). We are comparing the Itokawa samples to L and LL chondrite meteorites chosen tospan the shock scale experienced by Itokawa, specifically Chainpur (LL3.4, Shock Stage 1), Semarkona (LL3.00, S2), Kilabo(LL6, S3), NWA100 (L6, S4) and Chelyabinsk (LL5, S4). In SXRD we measure the line broadening of olivine reflections as ameasure of shock stage. In this presentation we concentrate on the EBSD work. We employed JSC’s Supra 55 variable
pressure FEG-SEM and Bruker EBSD system. We are not seeking actual strain values, but rather indirect strain-related
measurements such as extent of intra-grain lattice rotation, and determining whether shock state “standards” (meteorite
samples of accepted shock state, and appropriate small grain size) show strain measurements that may be statisticallydifferentiated, using a sampling of particles (number and size range) typical of asteroid regoliths.
Using our system we determined that a column pressure of 9 Pa and no C-coating on the sample was optimal. We varied
camera exposure time and gain to optimize mapping performance, concluding that 320x240 pattern pixilation, frame averagingof 3, 15 kV, and low extractor voltage yielded an acceptable balance of hit rate (>90%), speed (11 fps) and map quality using
an exposure time of 30 ms (gain 650). We found that there was no strong effect of step size on Grain Orientation Spread(GOS) and Grain Reference Orientation Deviation angle (GROD-a) distribution; there was some effect on grain average
Kernel Average Misorientation (KAM) (reduced with smaller step size for the same grain), as expected. We monitored GOS,
Maximum Orientation Spread (MOS) and GROD-a differences between whole olivine grains and sub-sampled areas, andfound that there were significant differences between the whole grain dataset and subsets, as well as between subsets, likely
due to sampling-related “noise”. Also, in general (and logically) whole grains exhibit greater degrees of cumulative lattice
rotation. Sampling size affects the apparent strain character of the grain, at least as measured by GOS, MOS and GROD-a.
There were differences in the distribution frequencies of GOS and MOS between shock stages, and in plots of MOS and GOS
vs. grain diameter. These results are generally consistent with those reported this year [5]. However, it is unknown whether thedifferences between samples of different shock states exceeds the clustering of these values to the extent that shock stage
determinations can still be made with confidence. We are investigating this by examination of meteorites with higher shockstage 4 to 5.
Our research will improve our understanding of how small, primitive solar system bodies formed and evolved, andimprove understanding of the processes that determine the history and future of habitability of environments on other solarsystem bodies. The results will directly enrich the ongoing asteroid and comet exploration missions by NASA and JAXA, andbroaden our understanding of the origin and evolution of small bodies in the early solar system, and elucidate the nature ofasteroid and comet regolith.
References: [1] Zolensky et al. (2012) Lunar and Planetary Science, XLIII, #1477, Lunar Planet. Inst., Houston (CD-ROM);[2] Zolensky et al. (2014) Hayabusa 2014: 2nd Symposium of Solar System Materials. Abstracts.; [3] Zolensky et al. (2016)
Hayabusa 2016 Abstracts; [4] Hagiya et al. (2016) Abstracts, 79th Annual Meeting of the Meteoritical Society; Ruzicka andHugo (2017) 80th Annual Meeting of the Meteoritical Society, 6368.