• A TEM study of thermally modified comet 81P/Wild 2 dust particles by interactions with the aerogel matrix during the Stardust capture process

      Leroux, H.; Rietmeijer, F. J. M.; Velbel, M. A.; Brearley, A. J.; Jacob, D.; Langenhorst, F.; Bridges, J. C.; Zega, T. J.; Stroud, R. M.; Cordier, P.; et al. (The Meteoritical Society, 2008-01-01)
      We report the results of high-resolution, analytical and scanning transmission electron microscopy (STEM), including intensive element mapping, of severely thermally modified dust from comet 81P/Wild 2 caught in the silica aerogel capture cells of the Stardust mission. Thermal interactions during capture caused widespread melting of cometary silicates, Fe-Ni-S phases, and the aerogel. The characteristic assemblage of thermally modified material consists of a vesicular, silica-rich glass matrix with abundant Fe-Ni-S droplets, the latter of which exhibit a distinct core-mantle structure with a metallic Fe,Ni core and a iron-sulfide rim. Within the glassy matrix, the elemental distribution is highly heterogeneous. Localized amorphous dust-rich patches contain Mg, Al, and Ca in higher abundances and suggest incomplete mixing of silicate progenitors with molten aerogel. In some cases, the element distribution within these patches seems to depict the outlines of ghost mineral assemblages, allowing the reconstruction of the original mineralogy. A few crystalline silicates survived with alteration limited to the grain rims. The Fe- and CI-normalized bulk composition derived from several sections show CI-chondrite relative abundances for Mg, Al, S, Ca, Cr, Mn, Fe, and Ni. The data indicate a 5 to 15% admixture of fine-grained chondritic comet dust with the silica glass matrix. These strongly thermally modified samples could have originated from a finegrained primitive material, loosely bound Wild 2 dust aggregates, which were heated and melted more efficiently than the relatively coarse-grained material of the crystalline particles found elsewhere in many of the same Stardust aerogel tracks (Zolensky et al. 2006).
    • Carbon investigation of two Stardust particles: A TEM, NanoSIMS, and XANES study

      Matrajt, G.; Ito, M.; Wirick, S.; Messenger, S.; Brownlee, D. E.; Joswiak, D.; Flynn, G.; Sandford, S.; Snead, C.; Westphal, A. (The Meteoritical Society, 2008-01-01)
      In this work we present the results of a systematic search for cometary organics in 14 Stardust particles (particles from comet 81P/Wild 2, captured by NASAs Stardust mission) by TEM and multidisciplinary studies (XANES and NanoSIMS) of Febo and Ada, two of the organic-bearing particles identified. The combination of the three analytical techniques has established the presence of organic, cometary degrees C in both particles. Using energy-filtered and high-resolution imaging it was shown that the degrees C is amorphous and rare, given that it is found in grains less than or equal to 200 nm in size that are not abundant throughout the particles. The XANES maps and spectra of the carbonaceous areas identified with the TEM have shown that the carbonaceous material is organic due to the presence of carbonyl (C=O) functional groups and the overlapping of degrees C and N on the same grains. In addition, several different C-XANES spectra were obtained from the same particle, suggesting that there is diversity in the types of carbonaceous phases present in these particles, as well as a heterogeneous distribution of the carbonaceous phases within these particles. The C-XANES spectra obtained are different from C-XANES spectra of carbonaceous chondrites and IDPs. In the particle Febo we found five spots showing a pronounced enrichment in the isotope 15N (delta-15N from 420 to 639 +/- 20 to 70 ppm, 1-sigma) that were clearly associated with the C-rich regions. The carbonaceous material has approximately solar C and D/H isotopic compositions, and the bulk O isotopic composition was found to be delta-17O = -18 +/- 13 ppm and delta-18O = -37 +/- 12 ppm (1-sigma). In the particle Ada we found a C-rich phase with enrichments in the isotope 15N (delta-15N = 550 +/- 70 ppm, 1-sigma) and the isotope D (delta-D = 610 +/- 254 ppm, 1-sigma). The C isotopic composition at this phase is solar (delta-13C = -4 +/-29 ppm, 1 sigma). The bulk O isotopic composition of Ada was found to be delta-17O = 9 +/- 14.6 ppm and delta-18O = -7.3 +/- 8.1 ppm (2-sigma).
    • Origin and formation of iron silicide phases in the aerogel of the Stardust mission

      Rietmeijer, F. J. M.; Nakamura, T.; Tsuchiyama, A.; Uesugi, K.; Nakano, T.; Leroux, H. (The Meteoritical Society, 2008-01-01)
      Suessite along with hapkeite and more Fe-rich iron-silicides up to Fe7Si2 formed near the entrance of aerogel track #44. These phases are ~100 nm, quenched-melt spheres, but the post-impact cooling regime was such that melt vitrification produced a polycrystalline mixture of Fe silicides and kamacite. The compositional similarities of the impact-produced Fe-Si phases and the Fe-Ni-S phases scattered throughout the aerogel capture medium strongly supports the idea that Fe silicides resulted from a reaction between molten Fe-Ni-S phases and aerogel at very high heating and cooling rates. Temperatures of around 1500 degrees C are inferred from the observed compositions had the silicide spheres formed at thermodynamic equilibrium, which seems unlikely. When the conditions were kinetically controlled, they could have been similar to those leading to the formation of solids with predictable deep metastable eutectic compositions in laboratory condensation experiments.
    • Silicate minerals and Si-O glass in comet Wild 2 samples: Transmission electron microscopy

      Tomeoka, K.; Tomioka, N.; Ohnishi, I. (The Meteoritical Society, 2008-01-01)
      A transmission electron microscope (TEM) study of seven comet Wild 2 samples shows that three samples consist mainly of olivine and pyroxene and four samples consist of Mg-Fe-bearing Si-O glass with minor amounts of Fe-Ni sulfide and metal. The olivine in the silicate-rich samples differs in fayalite content between the samples and shows a wide range of fayalite content within individual samples, indicating that the degree of thermal metamorphism on the comet, if any, was extremely low. One olivine grain has a high density of dislocations with Burgers vector b = [001], suggesting that the Wild 2 particles experienced hypervelocity impacts before capture. The structural type and composition of pyroxene differ between the samples and within individual samples. Both low-Ca and high-Ca pyroxenes are present. Enstatite occurs as ortho- and clinoenstatite, suggesting that the Wild 2 particles contain materials that went through distinct high-temperature and cooling histories. One silicate-rich sample exhibits a zone texture consisting of a core of low-Ca pyroxene surrounded by an inner rim of Mg-Fe-bearing Si-O-rich glass and an outer rim of melted aerogel. The texture suggests that the inner rim was formed by the mixing of melted cometary low-Ca pyroxene and melted aerogel during capture heating. The four Mg-Fe-bearing, Si-O glassrich samples show close similarities in mineralogy and texture to the inner rim of the zoned silicate-rich sample. The four samples are probably secondary products formed by interaction between melted cometary silicates and melted aerogel during the capture process.
    • 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.