A study of three-dimensional porous structures and fractal dimension of micrometeorites using X-ray microtomography.

Abstract

Micrometeorites (MMs) of likely cometary origin have been discovered in Antarctic ice and snow. The porosities and fractal dimensions of MMs would provide clues about dust evolution in the early stages of the solar system. Using X-ray microtomography, we investigate 3-D structures of MMs recovered from Antarctic snow. The samples were imaged at beamline BL20XU and BL47XU in SPring-8 (10keV, voxel size: 500nm). Most of pores in the MMs are connected to their exteriors (i.e., open pores). We have developed a new method to extract open pores and to obtain effective porosities, P, of the samples. Their fractal dimensions, D, are also obtained using a box counting method. Our analysis shows that P of porous MMs decreases with increasing D. The same method is applied to examine 3-D structures of model aggregates made by simulation of collisions between dust aggregates (Wada et al., 2007). With increasing the collision velocity D increases, while P decreases first, increases and finally decreases again. The D-P relation apparently shows a link between the MMs and the model aggregates. The MMs might preserve information on dust aggregation and accumulation processes, despite of subsequent alterations of the dust such as compaction in comets and heating during atmospheric entry.