SEM and TEM study of some Wild II cometary particles returned by the NASA Stardust Mission

Department of Earth and Planetary Science, University of Tokyo* Graduate School of Life Science, University of Hyogo** High Energy Accelerator Research Org. (KEK)*** NASA Johnson Space Center****
â—‹Takashi Mikouchi* Osamu Tachikawa* Kenji Hagiya** Kazumasa Ohsumi*** Michael Zolensky****

Stardust is a NASA comet sample return mission that successfully returned the Wild II cometary particles in Utah, USA on Jan. 15, 2006. We studied several naked particles and microtomed sections by Hitachi S-4500 SEM and JEOL JEM-2010 TEM for mineralogy and crystallography as a part of preliminary examination teams. The size of Wild II particles ranges from <1 to 30 micrometers. Most analyzed samples were contaminated with silica aerogel used for the capture of cometary particles in space. These samples are composed of an amorphous silica-rich phase with scattered ~100 nm particles of iron metal and iron sulfides. It is likely that these particles were formed by high temperature during the capture process. There are some areas enriched in Mg, suggesting the presence of mafic silicates. We could also find some crystalline materials such as olivine and pyroxene that were identified by electron diffraction of TEM and electron back-scatter diffraction of SEM. Most of crystalline phases are small (up to 1 micrometer), but their diffraction patterns are sharp, suggesting good crystallinity. It is interesting that both olivine and pyroxene show a fairly wide range in Mg-Fe ratios (atomic Mg/(Mg+Fe) = 0.6-1.0). Although most pyroxenes were low-Ca pyroxenes (orthopyroxene), small amounts of Ca-rich pyroxene (augite) were also detected. The presence of mafic silicates suggests that some Comet Wild II particles were formed near the sun at high temperature and then transformed to a cold comet formation region away from the sun by some process such as X-wind in the solar nebula.