• Bulbous tracks arising from hypervelocity capture in aerogel

      Trigo-Rodríguez, J. M.; Domínguez, G.; Burchell, M. J.; Hörz, F.; Llorca, J. (The Meteoritical Society, 2008-01-01)
      The capture of 81P/Wild 2 cometary particles in aerogel with a well-defined impact velocity (6.1 km s^(1)) has provided a wealth of data concerning the composition of Jupiter-family comets. To interpret this data we must understand the capture processes in the aerogel. A major category of tracks are those with bulbous cavities lined with particle fragments. We present a new model to account for the production of these turnip-shaped impact cavities. The model uses a thermodynamic approach in order to account for the likely expansion of vapors from particles rich in volatile species. Volume measurements of some of the largest Stardust tracks analysed so far, together with theoretical considerations, indicate that for the majority of Stardust cometary aggregate particle impacts, fragmentation of relatively weak impactors (combined with radial expansion of the resulting subgrains) is the leading cause of bulbous track production, while volatile release of vapors played a secondary role.
    • Comparing Wild 2 particles to chondrites and IDPs

      Zolensky, M.; Nakamura-Messenger, K.; Rietmeijer, F.; Leroux, H.; Mikouchi, T.; Ohsumi, K.; Simon, S.; Grossman, L.; Stephan, T.; Weisberg, M.; et al. (The Meteoritical Society, 2008-01-01)
      We compare the observed composition ranges of olivine, pyroxene, and Fe-Ni sulfides in Wild 2 grains with those from chondritic interplanetary dust particles (IDPs) and chondrite classes to explore whether these data suggest affinities to known hydrous materials in particular. Wild 2 olivine has an extremely wide composition range, from Fa096, with a pronounced frequency peak at Fa1. The composition range displayed by the low-calcium pyroxene is also very extensive, from Fs48 to Fs0, with a significant frequency peak centered at Fs5. These ranges are as broad or broader than those reported for any other extraterrestrial material. Wild 2 Fe-Ni sulfides mainly have compositions close to that of FeS, with less than 2 atom% Ni; to date, only two pentlandite grains have been found among the Wild grains, suggesting that this mineral is not abundant. The complete lack of compositions between FeS and pentlandite (with intermediate solid solution compositions) suggests (but does not require) that FeS and pentlandite condensed as crystalline species, i.e., did not form as amorphous phases, which later became annealed. While we have not yet observed any direct evidence of water-bearing minerals, the presence of Ni-bearing sulfides, and magnesium-dominated olivine and low-Ca pyroxene does not rule out their presence at low abundance. We do conclude that new investigations of major- and minorelement compositions of chondrite matrix and IDPs are required.
    • 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.
    • Stardust in Stardust—The C, N, and O isotopic compositions of Wild 2 cometary matter in Al foil impacts

      Stadermann, F. J.; Hoppe, P.; Floss, C.; Heck, P. R.; Hörz, F.; Huth, J.; Kearsley, A. T.; Leitner, J.; Marhas, K. K.; McKeegan, K. D.; et al. (The Meteoritical Society, 2008-01-01)
      In January 2006, the Stardust mission successfully returned dust samples from the tail of comet 81P/Wild 2 in two principal collection media, low-density silica aerogel and Al foil. While hypervelocity impacts at the Stardust encounter velocity of 6.1 km/s into Al foils are generally highly disruptive for natural, silicate-dominated impactors, previous studies have shown that many craters retain sufficient residue to allow a determination of the elemental and isotopic compositions of the original projectile. We have used two NanoSIMS ion microprobes to perform degrees C, N, and O isotope imaging measurements on four large (59-295 micrometers in diameter) and on 47 small (0.32-1.9 micrometers in diameter) Al-foil impact craters as part of the Stardust preliminary examination (PE). Most analyzed residues in and around these craters are isotopically normal (solar) in their degrees C, N, and O isotopic compositions. However, the debris in one large crater shows an average 15N enrichment of ~450, which is similar to the bulk composition of some isotopically primitive interplanetary dust particles (IDPs) and to components of some primitive meteorites. A 250 nm grain in another large crater has an 17O enrichment with ~2.65 times the solar 17O/16O ratio. Such an O isotopic composition is typical for circumstellar oxide or silicate grains from red giant or asymptotic giant branch stars. The discovery of this circumstellar grain clearly establishes that there is authentic stardust in the cometary samples returned by the Stardust mission. However, the low apparent abundance of circumstellar grains in Wild 2 samples and the preponderance of isotopically normal material indicates that the cometary matter is a diverse assemblage of presolar and solar system materials.