• 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).
    • Characteristics of cometary dust tracks in Stardust aerogel and laboratory calibrations

      Burchell, M. J.; Fairey, S. A. J.; Wozniakiewicz, P.; Brownlee, D. E.; Hörz, F.; Kearsley, A. T.; See, T. H.; Tsou, P.; Westphal, A.; Green, S. F.; et al. (The Meteoritical Society, 2008-01-01)
      The cometary tray of the NASA Stardust spacecraft’s aerogel collector was examined to study the dust captured during the 2004 flyby of comet 81P/Wild 2. An optical scan of the entire collector surface revealed 256 impact features in the aerogel (width >100 micrometers). Twenty aerogel blocks (out of a total of 132) were removed from the collector tray for a higher resolution optical scan and 186 tracks were observed (track length >50 micrometers and width >8 micrometers). The impact features were classified into three types based on their morphology. Laboratory calibrations were conducted that reproduced all three types. This work suggests that the cometary dust consisted of some cohesive, relatively strong particles as well as particles with a more friable or low cohesion matrix containing smaller strong grains. The calibrations also permitted a particle size distribution to be estimated for the cometary dust. We estimate that approximately 1200 particles bigger than 1 micrometer struck the aerogel. The cumulative size distribution of the captured particles was obtained and compared with observations made by active dust detectors during the encounter. At large sizes (>20 micrometers) all measures of the dust are compatible, but at micrometer scales and smaller discrepancies exist between the various measurement systems that may reflect structure in the dust flux (streams, clusters etc.) along with some possible instrument effects.
    • Discovery of non-random spatial distribution of impacts in the Stardust cometary collector

      Westphal, A. J.; Bastien, R. K.; Borg, J.; Bridges, J.; Brownlee, D. E.; Burchell, M. J.; Cheng, A. F.; Clark, B. C.; Djouadi, Z.; Floss, C.; et al. (The Meteoritical Society, 2008-01-01)
      We report the discovery that impacts in the Stardust cometary collector are not distributed randomly in the collecting media, but appear to be clustered on scales smaller than ~10 cm. We also report the discovery of at least two populations of oblique tracks. We evaluate several hypotheses that could explain the observations. No hypothesis is consistent with all the observations, but the preponderance of evidence points toward at least one impact on the central Whipple shield of the spacecraft as the origin of both clustering and low-angle oblique tracks. High-angle oblique tracks unambiguously originate from a non-cometary impact on the spacecraft bus just forward of the collector.