We developed a unique analytical procedure, which enables us to obtain several kinds of mineralogical data (SEM observation on the morphology and surface textures of the Itokawa particles and TEM observation of the <100-nm scale surface modification probably formed by solar wind irradiation) and noble gas isotopic compositions from a <100-um-sized sample. We have already applied the procedure to the four Itokawa particles allocated in the 2nd opportunity for Hayabusa sample investigation. In the previous investigation, we found that an Itokawa particle had growth macrosteps on its surface and obtained the noble gas composition. The release pattern and the 4He concentration of the particle with macrosteps revealed that the particle had once completely released 4He from its deep interior and later been experienced a short exposure to solar wind. The other three Itokawa particles without macrosteps have not experienced severe heating on their history based on their 4He release patterns (Noguchi et al., 2014). Although similar particles with macrosteps have been already reported by Tsuchiyama et al. (2013) and Matsumoto et al. (2013), the particles have been treated using adhesive resin that precludes noble gas analyses.
In this proposal, we would like to focus on the Itokawa particles with macrosteps on their surfaces. We obtain data on morphology, nanometer-scale surface modification formed by solar wind irradiation, and exposure history by using the analytical procedure applied in Noguchi et al. (2014). Based on these data, we would like to discuss if there is a stark difference in the exposure history between the particles with macrosteps and the others without macrosteps, what kind of process formed the unique stepped surfaces, and what the formation process of the macrosteps means on the history of the asteroid Itokawa.