In order to understand the regolith processes occurring on small asteroids such as Itokawa, it is essential to know noble gas isotopic compositions in the regolith materials. Solar noble gases implanted into thin surface layer of regolith grains give direct evidence that the grains resided on the uppermost surface of asteroids, being irradiated by solar wind (SW) particles. The grains are also bombarded by more energetic galactic cosmic-rays (GCRs), which produce noble gas isotopes inside the grains locating in the surface layers at a depth of ~1 m. The SW and GCR-produced noble gas compositions represent dynamic gardening processes occurring on the asteroid's surface such as migration of the grains in regolith and their residence time on the asteroidal surface.
Noble gas extraction from the samples will be conducted by stepped heating using a Nd-YAG CW laser, then He, Ne, and Ar will be measured with a modified VG5400 (MS-III) mass spectrometer at the University of Tokyo. Krypton and Xe in some extraction steps will be stored in small metal tubes to be measured at the University of Manchester with the ultra-high sensitive mass spectrometers. Half of the samples allocated will be heated up to 1600 oC, while the remaining ones will be heated mildly up to 800 oC, followed by TEM observation. Reflectance spectroscopy (in the visible/NIR region) will be done for every particle. The TEM and spectroscopic data will be compared with noble gas data to examine relationship between SW implantation and space-weathering products to evaluate mechanism for the weathering.