• 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).
    • Bulk mineralogy and three-dimensional structures of individual Stardust particles deduced from synchrotron X-ray diffraction and microtomography analysis

      Nakamura, T.; Tsuchiyama, A.; Akaki, T.; Uesugi, K.; Nakano, T.; Takeuchi, A.; Suzuki, Y.; Noguchi, T. (The Meteoritical Society, 2008-01-01)
      During preliminary examination of particles released from 81P/Wild 2 short-period comet, we analyzed 28 particles by nondestructive means, high-sensitive X-ray diffraction and high-resolution X-ray tomography, in order to characterize bulk mineralogy and three-dimensional structures of individual particles. The analyses were performed at synchrotron facilities, KEK and SPring-8 in Japan. Twenty-eight particles from 5 to 25 micrometers in size, including 25 particles from Track 35 and 3 particles from Track 44, were first analyzed by X-ray diffraction and then 4 out of 28 particles were analyzed by X-ray tomography. All particles are classified into two groups based on silicate crystallinity: crystalline type and amorphous-rich type. The abundance of the former is approximately 10% of the particles investigated. Crystalline type shows very sharp reflections of olivine and low-Ca pyroxene, while amorphous-rich type shows no or very weak silicate reflections, suggesting that silicates are mostly amorphous. Broad reflections of Fe sulfides and Fe silicides are detected from most of amorphous-rich type particles. Subsequent tomography analysis revealed that the crystalline type is non-porous material consisting of coarse silicate crystals larger than 1 micrometer in size, while the amorphous-rich type is very porous aggregates with amorphous silicates and small Fe sulfide and Fe metallic grains. All characteristics of amorphousrich type particles indicate that most of them are melted and rapidly solidified during capture in the silica aerogel. On the other hand, the crystalline type is indigenous cometary particle formed through high-temperature heating episodes that have taken place prior to formation of comet Wild 2. One of the crystalline-type particles (C2054,0,35,6,0) consists of Mg-rich olivine, pyroxene, and kamacite and exhibits porphyritic or poikilitic texture very similar to chondrules.
    • 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.
    • Combined micro-Raman, micro-infrared, and field emission scanning electron microscope analyses of comet 81P/Wild 2 particles collected by Stardust

      Rotundi, A.; Baratta, G. A.; Borg, J.; Brucato, J. R.; Busemann, H.; Colangeli, L.; D'Hendecourt, L.; Djouadi, Z.; Ferrini, G.; Franchi, I. A.; et al. (The Meteoritical Society, 2008-01-01)
      We report combined micro-infrared, micro-Raman, and field emission scanning electron microscope (FESEM) analyses of particles collected by the Stardust spacecraft during its flyby of comet 81P/Wild 2 on 2 January 2004 and successfully returned back to Earth on 15 January 2006. We present mid-infrared (IR) spectra of six of these particles. The CH2/CH3 ratios inferred from the infrared data are greater than those seen in organics in the diffuse interstellar medium, possibly indicating the presence of longer or less branched aliphatic chains. The micro-Raman data offer insights into the state of the order of the carbonaceous component present in the particles. Raman parameters for most of the particles span a similar range to that observed in interplanetary dust particles (IDPs) and the most primitive meteorites. Both the IR and Raman data imply the presence of a very labile carbonaceous component. Hydrated silicates may be present in two particles of Track 35, one of which may also contain carbonates, but further investigations with other techniques need to be performed to confirm these findings. In some cases, the analyses are difficult to interpret because of the presence of compressed aerogel mixed with the grains.
    • 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.
    • Recovering the elemental composition of comet Wild 2 dust in five Stardust impact tracks and terminal particles in aerogel

      Ishii, H. A.; Brennan, S.; Bradley, J. P.; Luening, K.; Ignatyev, K.; Pianetta, P. (The Meteoritical Society, 2008-01-01)
      The elemental (non-volatile) composition of five Stardust impact tracks and terminal particles left from capture of comet 81P/Wild 2 dust were mapped in a synchrotron X-ray scanning microprobe with full fluorescence spectra at each pixel. Because aerogel includes background levels of several elements of interest, we employ a novel "dual threshold" approach to discriminate against background contaminants: an upper threshold, above which a spectrum contains cometary material plus aerogel and a lower threshold below which it contains only aerogel. The difference between normalized cometary-plus-background and background-only spectra is attributable to cometary material. The few spectra in-between are discarded since misallocation is detrimental: cometary material incorrectly placed in the background spectrum is later subtracted from the cometary spectrum, doubling the loss of reportable cometary material. This approach improves accuracy of composition quantification. We present the refined whole impact track and terminal particle elemental abundances for the five impact tracks. One track shows mass increases in Cr and Mn (1.4x), Cu, As and K (2x), Zn (4x), and total mass (13%) by dual thresholds compared to a single threshold. Major elements Fe and Ni are not significantly affected. The additional Cr arises from cometary material containing little Fe. We exclude Au intermixed with cometary material because it is found to be a localized surface contaminant carried by comet dust into an impact track. The dual threshold technique can be used in other situations where elements of interest in a small sample embedded in a matrix are also present in the matrix itself.
    • 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.
    • 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.