Three-dimensional structures and elemental distributions of Stardust impacttracks using synchrotron microtomography and X-ray fluorescence analysis

摘要

Three-dimensional structures and elemental abundances of four impact tracks in silicaaerogel keystones of Stardust samples from comet 81P/Wild 2 (bulbous track 67 and carrot-typetracks 46, 47, and 68) were examined non-destructively by synchrotron radiation-basedmicrotomography and X-ray fluorescence analysis. Track features, such as lengths, volumes andwidth as a function of track depth, were obtained quantitatively by tomography. A bulbous portionwas present near the track entrance even in carrot-type tracks. Each impact of a cometary dust particleresults in the particle disaggregated into small pieces that were widely distributed on the track wallsas well as at its terminal. Fe, S, Ca, Ni, and eight minor elements are concentrated in the bulbous portionof track 68 as well as in terminal grains. It was confirmed that bulbous portions and thin tracks wereformed by disaggregation of very fine fragile materials and relatively coarse crystalline particles,respectively. The almost constant ratio of whole Fe mass to track volume indicates that the trackvolume is almost proportional to the impact kinetic energy. The size of the original impactor wasestimated from the absolute Fe mass by assuming its Fe content (CI) and bulk density. Relationsbetween the track sizes normalized by the impactor size and impact conditions are roughly consistentwith those of previous hypervelocity impact experiments.