• 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.
    • Identification of mineral impactors in hypervelocity impact craters in aluminum by Raman spectroscopy of residues

      Burchell, M. J.; Foster, N. J.; Kearsley, A. T.; Creighton, J. A. (The Meteoritical Society, 2008-01-01)
      Here we demonstrate the use of Raman spectroscopy techniques to identify mineral particle fragments after their impact into aluminum foil at ~6 km s^(-1). Samples of six minerals (olivine, rhodonite, enstatite, diopside, wollastonite, and lizardite) were fired into aluminum foil and the resulting impact craters were studied with a HeNe laser connected to a Raman spectrometer. Raman spectra similar to those of the raw mineral grains were obtained from the craters for impacts by olivine, rhodonite, enstatite, wollastonite, and diopside, but no Raman signals were found from lizardite after impact. In general, the impactors do not survive completely intact, but are fragmented into smaller fractions that retain the structure of the original body. Combined with evidence for SEM and FIB studies, this suggests that in most cases the fragments are relatively unaltered during impact. The survival of identifiable projectile fragments after impact at ~6 km s^(-1) is thus established in general, but may not apply to all minerals. Where survival has occurred, the use of Raman spectroscopic techniques for identifying minerals after hypervelocity impacts into a metallic target is also demonstrated.
    • Transmission electron microscopy of cometary residues from micron-sized craters in the Stardust Al foils

      Leroux, H.; Stroud, R. M.; Dai, Z. R.; Graham, G. A.; Troadec, D.; Bradley, J. P.; Teslich, N.; Borg, J.; Kearsley, A. T.; Hörz, F. (The Meteoritical Society, 2008-01-01)
      We report transmission electron microscopy (TEM) investigations of micro-craters that originated from hypervelocity impacts of comet 81P/Wild 2 dust particles on the aluminium foil of the Stardust collector. The craters were selected by scanning electron microscopy (SEM) and then prepared by focused ion beam (FIB) milling techniques in order to provide electron transparent crosssections for TEM studies. The crater residues contain both amorphous and crystalline materials in varying proportions and compositions. The amorphous component is interpreted as resulting from shock melting during the impact and the crystalline phases as relict minerals. The latter show evidence for shock metamorphism. Based on the residue morphology and the compositional variation, the impacting particles are inferred to have been dominated by mixtures of submicron olivine, pyroxene and Fe sulfide grains, in agreement with prior results of relatively coarse-grained mineral assemblages in the aerogel collector.