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[SGC51-09] Relationships between morphological features and isotopic signatures of noble gases of four Itokawa particles
Samples and methods:. We fixed four Itokawa particles on tantalum plates by acetone-soluble glue. We observed the surface of the particles and prepared thin foils by FIB-SEM at Ibaraki University. After preparation of FIB foils, all the Ta plates with samples rinsed by acetone in a N2 filled glove box. They were transferred to Kyushu University and noble gas mass spectrometry was performed for all the noble gas elements using a modified VG-5400. Noble gas extraction from each sample was conducted stepwise at 50, 100, 200, 300, 600 and 1400 ℃.
Results and discussion:. SEM observation of the four Itokawa grains investigated shows various morphology and surface features. Edges observed on the surfaces of three grains vary from sharp to dull. Additionally, one grain is composed of two subgrains with stepped surfaces. Morphology and surface features are related to the He release patterns. The three particles have similar 4He release patterns; they release most 4He below 200 ℃ fractions and a small amount of 4He above 600 ℃. Although the gas-release profiles are similar to each other, there is a difference in the bulk concentrations of 4He; one of the three grain having dull edges contain more abundant 4He than the others having sharp edges. Because bulk 3He/4He ratios in the three grains are 2.5 to 6.5 x 10-4, which is comparable with that in the solar wind of 4.57 x 10-4 , it is likely that the implanted He in the grains increased as the surfaces were degraded by solar wind sputtering. On the other hand, the grain composed of subgrains with stepped surfaces shows a different 4He release pattern; 4He was released only below 200 ℃. This fact suggests that deeply implanted He, which is probably related to He released above 600 ℃, was completely reset by severe heating. The steps on the surfaces of this grain may have been formed by evaporation and/or evaporation and subsequent recondensation by a heating event, which released He from the interior of the grain. These results strongly suggest that micrometer-size surface features are related to the amounts and the release patterns of the implanted He.
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