• Smelting of Fe-bearing glass during hypervelocity capture in aerogel

      Marcus, M. A.; Fakra, S.; Westphal, A. J.; Snead, C. J.; Keller, L. P.; Kearsley, A.; Burchell, M. J. (The Meteoritical Society, 2008-01-01)
      Hypervelocity capture of material in aerogel can expose particles to high transient temperatures. We tested some of the possible effects of capture by using a light-gas gun to shoot particles of basalt glass into aerogel at 6.1 km s^(-1). Using synchrotron-based micro-X-ray absorption spectroscopy (micro-XAS), we find that the starting material, in which the Fe was trivalent, is chemically reduced to divalent. In addition, some fragments were chemically reduced so that they contained Fe0 in a form spectroscopically consistent with a mixture of two forms of iron carbide (cohenite and haxonite). The carbon presumably originated from organic impurities in the aerogel. High-resolution transmission electron microscopy (HRTEM) imaging shows the presence of Fe-rich crystalline nanoparticles. A similar species has been found in actual Stardust material, suggesting that smelting effects occurred during capture and should be taken into account when interpreting data on Stardust samples.
    • TOF-SIMS analysis of cometary matter in Stardust aerogel tracks

      Stephan, T.; Rost, D.; Vicenzi, E. P.; Bullock, E. S.; MacPherson, G. J.; Westphal, A. J.; Snead, C. J.; Flynn, G. J.; Sandford, S. A.; Zolensky, M. E. (The Meteoritical Society, 2008-01-01)
      Cometary matter in aerogel samples from the Stardust mission was investigated with TOF-SIMS for its elemental and organic composition. While single grains >1 micrometer are highly variable in their chemical composition, nanometer-scale material found in the wall of one track has within a factor of 1.22 bulk CI chondritic element ratios relative to Fe for Na, Mg, Al, Ti, Cr, Mn, and Co. Compared to CI, a depletion in Ca by a factor of four and an enrichment in Ni by a factor of two was observed. These results seem to confirm recent reports of a CI-like bulk composition of Wild 2. The analysis of organic compounds in aerogel samples is complicated by the presence of contaminants in the capture medium. However, polycyclic aromatic hydrocarbons that could possibly be attributed to the comet were observed.