Space weathered rims found on the surfaces of the Itokawa dust particles

On the basis of observations using Cs-corrected STEM, we identified three types of surface modificationprobably formed by space weatheringon the surfaces of Itokawa particles. They are (1) redeposition rims (2–3 nm), (2) composite rims (30–60 nm), and (3) composite vesicular rims (60–80 nm). These rims are characterized by a combination of three zones. ZoneI occupies the outermost part of the surface modification, which contains elements that are not included in the unchanged substrate minerals, suggesting that this zoneis composed of sputter deposits and/or impact vapor deposits originating from the surrounding minerals. Redeposition rims are composed only of ZoneI and directly attaches to the unchanged minerals ( ZoneIII). ZoneI of composite and composite vesicular rims often contains nanophase (Fe,Mg)S. The composite rims and the composite vesicular rims have a two-layered structure: a combination of ZoneI and ZoneII, below which ZoneIII exists. ZoneII is the partially amorphized zone. ZoneII of ferromagnesian silicates contains abundant nanophase Fe. Radiation-induced segregation and in situ reduction are the most plausible mechanisms to form nanophase Fe in ZoneII. Their lattice fringes indicate that they contain metallic iron, which probably causes the reddening of the reflectance spectra of Itokawa. ZoneII of the composite vesicular rims contains vesicles. The vesicles in ZoneII were probably formed by segregation of solar wind He implanted in this zone. The textures strongly suggest that solar wind irradiation damage and implantation are the major causes of surface modificationand space weatheringon Itokawa.