• Penetration tracks in aerogel produced by Al2O3 spheres

      Hörz, F.; Cintala, M. J.; See, T. H.; Nakamura-Messenger, K. (The Meteoritical Society, 2009-01-01)
      We conducted impact experiments into SiO2-based aerogel of uniform density (0.02 g cm^(-3)) with spherical corundum projectiles. The highly refractory nature and mechanical strength of corundum minimizes projectile deformation and continuous mass loss by ablation that might have affected earlier experiments with soda-lime glass (SLG) impactors into aerogel targets. We find that corundum is a vastly superior penetrator producing tracks a factor of 2.5 longer, yet similar in diameter to those made by SLG. At velocities > 4 km s^(-1) a cylindrical cavity forms, largely by melting of aerogel. The diameter and length of this cavity increase with velocity and impactor size, and its volume dominates total track volume. A continuously tapering, exceptionally long and slender stylus emerges from this cavity and makes up some 80-90% of the total track length; this stylus is characterized by solid-state deformations. Tracks formed below 4 km s^(-1) lack the molten cavity and consist only of a stylus. Projectile residues recovered from a tracks terminus substantially resemble the initial impactors at V < 4 km s^(-1), yet they display two distinct surfaces at higher velocities, such as a blunt, forward face and a well-preserved, hemispherical trailing side; a pronounced, circumferential ridge of compressed and molten aerogel separates these two surfaces. Stringers and patches of melt flow towards the impactors rear where they accumulate in a characteristic melt tip. SEM-EDS analyses indicate the presence of Al in these melts at velocities as low as 5.2 km s^(-1), indicating that the melting point of corundum (2054 degrees C) was exceeded. The thermal model of aerogel impact by Anderson and Cherne (2008) suggests actual aerogel temperatures >5000 K at comparable conditions. We therefore propose that projectile melting occurs predominantly at those surfaces that are in contact with this very hot aerogel, at the expense of viscous heating and associated ablation. Exposure to superheated aerogel may be viewed as extreme form of "flash heating." This seems consistent with observations from the Stardust mission to comet Wild 2, such as relatively pristine interiors of rather large, terminal particles, yet total melting of most fine-grained dust components.
    • Petrography, mineralogy, and trace element geochemistry of lunar meteorite Dhofar 1180

      Zhang, A.; Hsu, W. (The Meteoritical Society, 2009-01-01)
      Here we report the petrography, mineralogy, and trace element geochemistry of the Dhofar 1180 lunar meteorite. Dhofar 1180 is predominantly composed of fine-grained matrix with abundant mineral fragments and a few lithic and glassy clasts. Lithic clasts show a variety of textures including cataclastic, gabbroic, granulitic, ophitic/subophitic, and microporphyritic. Both feldspathic and mafic lithic clasts are present. Most feldspathic lithic clasts have a strong affinity to ferroan anorthositic suite rocks and one to magnesian suite rocks. Mafic lithic clasts are moderately to extremely Fe-rich. The Ti/[Ti+Cr]-Fe/[Fe+Mg] compositional trend of pyroxenes in mafic lithic clasts is consistent with that of low-Ti mare basalts. Glasses display a wide chemical variation from mafic to feldspathic. Some glasses are very similar to those from Apollo 16 soils. KREEP components are essentially absent in Dhofar 1180. One glassy clast is rich in K, REE and P, but its Mg/[Mg+Fe] is very low (0.25). It is probably a last-stage differentiation product of mare basalt. Molar Fe/Mn ratios of both olivine and pyroxene are essentially consistent with a lunar origin. Dhofar 1180 has a LREE-enriched (La 18 x CI, Sm 14 x CI) pattern with a small positive Eu anomaly (Eu x 15 CI). Th concentration is 0.7 ppm in Dhofar 1180. Petrography, mineralogy, and trace element geochemistry of Dhofar 1180 are different from those of other lunar meteorites, indicating that Dhofar 1180 represents a unique mingled lunar breccia derived from an area on the lunar nearside but far away from the center of the Imbrium Basin.
    • Pyroxene mineralogies of near-Earth vestoids

      Burbine, T. H.; Buchanan, P. C.; Dolkar, T.; Binzel, R. P. (The Meteoritical Society, 2009-01-01)
      We have calculated pyroxene mineralogies of seven near-Earth asteroids (NEAs) with reflectance spectra similar to HEDs (howardites, eucrites, and diogenites). Two different sets of formulas (Gaffey et al. 2002; Burbine et al. 2007) are used to calculate the pyroxene mineralogies of the NEAs from their Band I and II centers. The band centers have been adjusted to compensate for the low temperatures on the asteroid surfaces. All of the derived mineralogies from the Gaffey et al. (2002) formulas and the Burbine et al. (2007) formulas overlap. The derived wollastonite (Wo) contents are very similar with differences being only approximately 1 mol%. The derived ferrosilite (Fs) contents differ by only 3 to 8 mol%. The determined pyroxene mineralogies for all seven near- Earth vestoids are consistent with eucrites or howardites. None of the objects have pyroxene mineralogies consistent with diogenites. The absence of near-Earth vestoids with pyroxene mineralogies similar to diogenites may indicate that it is difficult to produce sizeable (km-sized or larger) bodies that are predominantly composed of diogenitic material, suggesting these objects are rubble piles of mixed ejecta.
    • The Meteoritical Bulletin, No. 96, September 2009

      Weisberg, M. K.; Smith, C.; Benedix, G.; Herd, C. D. K.; Righter, K.; Haack, H.; Yamaguchi, A.; Chennaoui Aoudjehane, H.; Grossman, J. N. (The Meteoritical Society, 2009-01-01)
      The Meteoritical Bulletin No. 96 contains a total of 1590 newly approved meteorite names with their relevant data. These include 12 from specific locations within Africa, 76 from northwest Africa, 9 from the Americas, 13 from Asia, 1 from Australia, 2 from Europe, 950 from Antarctica recovered by the Chinese Antarctic Research Expedition (CHINARE), and 527 from the American Antarctic program (ANSMET). Among these meteorites are 4 falls, Almahata Sitta (Sudan), Sulagiri (India), Ash Creek (United States), and Maribo (Denmark). Almahata Sitta is an anomalous ureilite and is debris from asteroid 2008 TC3 and Maribo is a CM2 chondrite. Other highlights include a lunar meteorite, a CM1 chondrite, and an anomalous IVA iron.
    • Weston: Revised position for a historically significant meteorite fall

      Robson, M.; Pagliaro, F. (The Meteoritical Society, 2009-01-01)
      Weston is the first well-documented meteorite fall in the New World. The fall occurred on December 14, 1807. The Weston event began the study of meteoritics in the United States in the decade that science accepted that stones do, in fact, fall from the sky. It is unfortunate that much of the literature regarding this historically significant fall is erroneous. This paper will deal with the geographic position of the fall site. One of us (Robson 2007) proposed a new set of coordinates for Weston that was accepted by the Meteoritical Society. At the societys 70th annual meeting, new Weston coordinates were suggested; with the caveat that research was ongoing. However, it was also stated in the presentation that it was unlikely that either coordinate would change by as much as a minute. Further research indicates a final revision is required. Our revised mean fall position of the seven documented fragments of Weston is: 41 degrees 16' N, 73 degrees 16' W (WGS 84 coordinates, to the nearest minute). A quirk of history is a main factor in the derivation of faulty positions for Weston. The historically changing positions given for the fall are explored. Our methodology is discussed and the newly discovered Weston manuscripts, maps, and communications of Yales foremost meteoric astronomer, Professor H. A. Newton, support our findings.