Measuring the shock stage of Itokawa and asteroid regolith grains by electron backscattered diffraction, optical petrography, and synchrotron X-ray diffraction

Our goal was to devise a bridge between shock determinations of asteroid regolith grains by standard light optical petrography, synchrotron X-ray diffraction (SXRD), and electron backscattered diffraction (EBSD). We determined the optimal conditions under which to measure the shock stage of olivine crystals in astromaterial grains by EBSD. We applied this EBSD procedure to the shock stage determination of four regolith grains from asteroid Itokawa, returned to earth by the Hayabusa spacecraft. Interpretation of these data required a parallel examination of three ordinary chondrite standards that exhibited shock histories ranging from stage 2 to stage 4, using all three techniques. Standard light optical petrography indicated shock stage of S2/3 for the 24 Itokawa grains analyzed. SXRD results for seven Itokawa grains indicate a shock stage of S2. EBSD maps of four Itokawa grains indicate shock stage S3. Thus, the different techniques indicate slightly different shock stages, probably due to small sampling populations for EBSD and SXRD. We therefore recommend that significantly more than seven regolith grains should be separately analyzed by any shock determination technique, probably between 10 and 20. In any case, Itokawa regolith grains have been shocked to stage S2/3, or approximately 5–10 GPa. Finally, we investigated the crystallinity of one Itokawa olivine by SXRD, determining that the 5–10 GPa shock it had experienced did not appreciably alter the size of the unit cell, contrary to some previous suggestions.