• 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.
    • Dust from comet Wild 2: Interpreting particle size, shape, structure, and composition from impact features on the Stardust aluminum foils

      Kearsley, A. T.; Borg, J.; Graham, G. A.; Burchell, M. J.; Cole, M. J.; Leroux, H.; Bridges, J. C.; Hörz, F.; Wozniakiewicz, P. J.; Bland, P. A.; et al. (The Meteoritical Society, 2008-01-01)
      Aluminum foils of the Stardust cometary dust collector are peppered with impact features of a wide range of sizes and shapes. By comparison to laboratory shots of known particle dimensions and density, using the same velocity and incidence geometry as the Stardust Wild 2 encounter, we can derive size and mass of the cometary dust grains. Using scanning electron microscopy (SEM) of foil samples (both flown on the mission and impacted in the laboratory) we have recognized a range of impact feature shapes from which we interpret particle density and internal structure. We have documented composition of crater residues, including stoichiometric material in 3 of 7 larger craters,by energy dispersive X-ray microanalysis. Wild 2 dust grains include coarse (>10 micrometers) mafic silicate grains, some dominated by a single mineral species of density around 34 g cm^(-3) (such as olivine). Other grains were porous, low-density aggregates from a few nanometers to 100 micrometers, with an overall density that may be lower than 1 g cm^(-3), containing mixtures of silicates and sulfides and possibly both alkali-rich and mafic glass. The mineral assemblage is very similar to the most common species reported from aerogel tracks. In one large aggregate crater, the combined diverse residue composition is similar to CI chondrites. The foils are a unique collecting substrate, revealing that the most abundant Wild 2 dust grains were of sub-micrometer size and of complex internal structure. Impact residues in Stardust foil craters will be a valuable resource for future analyses of cometary dust.
    • 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.
    • 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.
    • TOF-SIMS analysis of cometary particles extracted from Stardust aerogel

      Stephan, T.; Flynn, G. J.; Sandford, S. A.; Zolensky, M. E. (The Meteoritical Society, 2008-01-01)
      Sections of seven cometary fragments extracted from the aerogel collector flown on the Stardust mission to comet 81P/Wild 2 were investigated with TOF-SIMS. These grains showed a rather heterogeneous chemical and mineralogical composition on a submicrometer scale. However, their average chemical composition is close to bulk CI chondritic values, which is consistent with analyses of numerous Stardust samples using various techniques. As a result, the TOF-SIMS analyses support the conclusion that Wild 2 has a CI-like bulk composition. The cometary particles resemble anhydrous chondritic porous interplanetary dust particles, which have previously been suggested to originate from comets. For one of the fragments, polycyclic aromatic hydrocarbons that could possibly be attributed to the comet were observed.
    • TOF-SIMS analysis of crater residues from Wild 2 cometary particles on Stardust aluminum foil

      Leitner, J.; Stephan, T.; Kearsley, A. T.; Hörz, F.; Flynn, G. J.; Sandford, S. A. (The Meteoritical Society, 2008-01-01)
      Impact residues of cometary particles on aluminum foils from the Stardust mission were investigated with TOF-SIMS for their elemental and organic composition. The residual matter from comet 81P/Wild 2 shows a wide compositional range, from nearly monomineralic grains to polymict aggregates. Despite the comparably small analyzed sample volume, the average element composition of the investigated residues is similar to bulk CI chondritic values. Analysis of organic components in impact residues is complicated, due to fragmentation and alteration of the compounds during the impact process and by the presence of contaminants on the aluminum foils. Nevertheless, polycyclic aromatic hydrocarbons (PAHs) that are unambiguously associated with the impact residues were observed, and thus are most likely of cometary origin.