12:00 PM - 12:15 PM
[PPS06-12] Space weathered rims on iron sulfide of Itokawa regolith particles.
Keywords:Itokawa, Space weathering , Iron sulfide
We observed surface features of eleven Itokawa particles focusing on troilite (FeS) using a field-emission scanning electron microscope (SEM). Two particles were investigated for further study. Regions of interest were selected based on the SEM observation, and electron-transparent sections including particle surface were prepared using a focused ion beam (FIB). We observed the sections using a field-emission transmission electron microscope (TEM).
From SEM observation of Itokawa particles, submicron sized craters and blisters, which are evidences of micrometeorite bombardment and solar wind implantation , were often recognized on troilite as well as silicate surfaces. Some troilite surfaces have whiskers ranging from several tens nm to 2 μm in length. We lifted out FIB sections from troilite surfaces where the whiskers develop on its surface. TEM analysis shows that the whiskers have a structure of body-centered cubic iron. Fe whiskers were found to be elongated nearly along the low-index crystallographic axis. Numerous vesicles were observed beneath the troilite surface to a depth of 90 nm.
The vesicular rim of troilite might have formed by hydrogen and helium gases after the accumulation of solar wind hydrogen ions and helium ions. In this study, we also conducted hydrogen ion irradiation experiments in order to evaluate the alteration of iron sulfide by solar wind ion irradiation. The hydrogen ion irradiation experiment results in the formation of similar vesicular rim. The iron whiskers on Itokawa particles might have been produced through selective sputtering of sulfur atoms by solar wind irradiation  and/or reduction of iron sulfide with the production of Fe metal and H2S as the gas phase . Characteristic diffusion distance of iron atoms in iron sulfide reaches 1μm for ten years  under radiative equilibrium temperature at Itokawa's perihelion (400K). Therefore, iron atoms can sufficiently diffuse from troilite into iron whiskers during short stay of regolith particles on Itokawa's surface. The whisker growth along low-index axis may minimize its surface energy. Fe whisker growth mechanism could be similar to the stress relief mechanism as previously reported for various metal whiskers, such as Ag whiskers on Ag2S .
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