Abstract
Cometary dust of Wild2 was recovered by NASA Stardust mission. Tracks were made in aerogel by impacting particles at hyper velocity of 6.1 km/s. Variety of the impact tracks shows variety of materials composed of the cosmic dust. Among these tracks, tracks with unique shapes, such as curved and spiral tracks can be recognized. If a spiral track with a pitch of about 1.5 mm was formed by spinning of an impacting particle, this particle must rotate at about 4 million rps. In this study, 3-D structures of two tracks (C2017,2,100,0 A and B), which seem to be spiral under an optical microscope, were obtained by microtomography. Imaging experiments were performed at BL47XU of SPring-8 at 8 keV with 3600 projections and voxel size of 1.05 microns, which gives the effective spatial resolution of about a few microns. Track cavity, condensed aerogel and individual particles were recognized in CT images, and their 3-D distributions were obtained. The spiral features correspond to cracks, which develop outwards from the track walls. However, the spiral cracks develop only partially. This strongly suggests that the hypervelocity rotation was not responsible for the spiral features. Heterogeneity of the aerogel may cause the spiral features.
