• Cosmic impact versus terrestrial origin of the Azuara structure (Spain): A review

      Cortés, Angel L.; Díaz-Martínez, Enrique; Sanz-Rubio, Enrique; Martínez-Frías, Jésus; Fernández, Cristina (The Meteoritical Society, 2002-01-01)
      The Azuara structure is the largest one proposed so far in Spain as possibly related to a cosmic impact event. A review of the evidence set forward in favor of and against its cosmic origin indicates that the discussion is not yet finished. Some megascopic features (inverted stratigraphy, megabreccia, negative gravity anomalies) and shock-metamorphic effects (planar deformational features) have been described in relation with the structure, although their real significance has been questioned and is still being debated. Comparison with other similar-sized verified impacts suggests that unequivocal impactogenic features are yet to be found before the Azuara structure can be related to a cosmic impact. Until then, the Azuara structure should be considered as an unverified impact structure, and should not be included in global comprehensive maps of terrestrial impact structures.
    • Radiocarbon on Titan

      Lorenz, Ralph D.; Jull, A. J. Timothy; Swindle, Timothy D.; Lunine, Jonathan I. (The Meteoritical Society, 2002-01-01)
      We explore the likely production and fate of 14C in the thick nitrogen atmosphere of Saturn's moon Titan and investigate the constraints that measurements of 14C might place on Titan's photochemical, atmospheric transport and surface-atmosphere interaction processes. Titan's atmosphere is thick enough that cosmic-ray flux limits the production of 14C: absence of a strong magnetic field and the increased distance from the Sun suggest production rates of ~9 atom/cm^2/s, ~4x higher than Earth. The fate and detectability of 14C depends on the chemical species into which it is incorporated: as methane it would be hopelessly diluted even in only the atmosphere. However, in the more likely case that the 14C attaches to the haze that rains out onto the surface (as tholin, HCN or acetylene and their polymers), haze in the atmosphere or recently deposited on the surface would be quite radioactive. Such radioactivity may lead to a significant enhancement in the electrical conductivity of the atmosphere which will be measured by the Huygens probe. Measurements with simple detectors on future missions could place useful constraints on the mass deposition rates of photochemical material on the surface and identify locations where surface deposits of such material are "freshest".
    • Planar deformation features and impact glass in inclusions from the Vredefort Granophyre, South Africa

      Buchanan, P. C.; Reimold, W. U. (The Meteoritical Society, 2002-01-01)
      The Vredefort Granophyre represents impact melt that was injected downward into fractures in the floor of the Vredefort impact structure, South Africa. This unit contains inclusions of country rock that were derived from different locations within the impact structure and are predominantly composed of quartzite, feldspathic quartzite, arkose, and granitic material with minor proportions of shale and epidiorite. Two of the least recrystallized inclusions contain quartz with single or multiple sets of planar deformation features. Quartz grains in other inclusions display a vermicular texture, which is reminiscent of checkerboard feldspar. Feldspars range from large, twinned crystals in some inclusions to fine-grained aggregates that apparently are the product of decomposition of larger primary crystals. In rare inclusions, a mafic mineral, probably biotite or amphibole, has been transformed to very fine-grained aggregates of secondary phases that include small euhedral crystals of Fe-rich spinel. These data indicate that inclusions within the Vredefort Granophyre were exposed to shock pressures ranging from <5 to 8-30 GPa. Many of these inclusions contain small, rounded melt pockets composed of a groundmass of devitrified or metamorphosed glass containing microlites of a variety of minerals, including K-feldspare, quartz, augite, low-Ca pyroxene, and magnetite. The composition of this devitrified glass varies from inclusion to inclusion, but is generally consistent with a mixture of quartz and feldspare with minor properties of mafic minerals. In the case of granitoid inclusions, melt pockets commonly occur at the boundaries between feldspar and quartz grains. In metasedimentary inclusions, some of these melt pockets contain remnants of partially melted feldspare grains. These melt pockets may have formed by eutectic melting caused by inclusion of these fragments in the hot (650 to 1610 degrees C) impact melt that crystallized to form the Vredefort Granophyre.
    • Itqiy: A study of noble gases and oxygen isotopes including its terrestrial age and a comparison with Zakłodzie

      Patzer, A.; Hill, D. H.; Boynton, W. V.; Franke, L.; Schultz, L.; Jull, A. J. T.; McHargue, L. R.; Franchi, I. A. (The Meteoritical Society, 2002-01-01)
      We report noble gas, oxygen isotope, 14C and 10Be data of Itqiy as well as noble gas, 14C and 10Be results for Zakłodzie. Both samples have been recently classified as anomalous enstatite meteorites and have been compared in terms of their mineralogy and chemical composition. The composition of enstatite and kamacite and the occurrence of specific sulfide phases in Itqiy indicate it formed under similar reducing conditions to those postulated for enstatite chondrites. The new results now seem to point at a direct spatial link. The noble gas record of Itqiy exhibits the presence of a trapped subsolar component, which is diagnostic for petrologic types 4-6 among enstatite chondrites. The concentration of radiogenic 4He is very low in Itqiy and indicates a recent thermal event. Its 21Ne cosmic-ray exposure age is 30.1 +/- 3.0 Ma and matches the most common age range of enstatite chondrites (mostly EL6 chondrites) but not that of Zakłodzie. Itqiy's isotopic composition of oxygen is in good agreement with that observed in Zakłodzie as well as those found in enstatite meteorites suggesting an origin from a common oxygen pool. The noble gas results, on the other hand, give reason to believe that the origin and evolution of Itqiy and Zakłodzie are not directly connected. Itqiy's terrestrial age of 5800 +/- 500 years sheds crucial light on the uncertain circumstances of its recovery and proves that Itqiy is not a modern fall, whereas the 14C results from Zakłodzie suggest it hit Earth only recently.
    • Small hypervelocity particles captured in aerogel collectors: Location, extraction, handling and storage

      Westphal, Andrew J.; Snead, Christopher; Borg, Janet; Quirico, Eric; Raynal, Pierre-Ivan; Zolensky, Michael E.; Ferrini, Gianluca; Colangeli, Luigi; Palumbo, Pasquale (The Meteoritical Society, 2002-01-01)
      It has now been about a decade since the first demonstrations that hypervelocity particles could be captured, partially intact, in aerogel collectors. But the initial promise of a bonanza of partially-intact extraterrestrial particles, collected in space, has yet to materialize. One of the difficulties that investigators have encountered is that the location, extraction, handling and analysis of very small (10 micrometers and less) grains, which constitute the vast majority of the captured particles, is challenging and burdensome. Furthermore, current extraction techniques tend to be destructive over large areas of the collectors. Here we describe our efforts to alleviate some of these difficulties. We have learned how to rapidly and efficiently locate captured particles in aerogel collectors, using an automated microscopic scanning system originally developed for experimental nuclear astrophysics. We have learned how to precisely excavate small access tunnels and trenches using an automated micromanipulator and glass microneedles as tools. These excavations are only destructive to the collector in a very small area-this feature may be particularly important for excavations in the precious Stardust collectors. Using actuatable silicon microtweezers, we have learned how to extract and store "naked" particles--essentially free of aerogel--as small as 3 m in size. We have also developed a technique for extracting particles, along with their terminal tracks, still embedded in small cubical aerogel blocks. We have developed a novel method for storing very small particles in etched nuclear tracks. We have applied these techniques to the extraction and storage of grains captured in aerogel collectors (Particle Impact Experiment, Orbital Debris Collector Experiment, Comet-99) in low Earth orbit.
    • Sayh al Uhaymir 094: A new martian meteorite from the Oman desert

      Gnos, E.; Hofmann, B.; Franchi, I. A.; Al-Kathiri, A.; Hauser, M.; Moser, L. (The Meteoritical Society, 2002-01-01)
      Sayh al Uhaymir (SaU) is a 223.3 g, partially crusted, strongly to very strongly shocked melanocratic olivine-porphyric rock of the shergottite group showing a microgabbroic texture. The rock consists of pyroxene (52.0-58.2 vol%)--dominantly prismatic pigeonite (En60-68Fs20-27Wo7-9) associated with minor augite (En46-49Fs15-16Wo28-31)--brown (shock-oxidized) olivine (Fo65-69; 22.1-31%), completely isotropic interstitial plagioclase glass (maskelynite; An50-64Or0.3-0.9; 8.6-13.0%), chromite and titanian magnesian chromite (0.9-1.0%), trace of ilmenite (Ilm80-86), pyrrhotite (Fe92-100; 0.1-0.2%), merrillite (<<0.1%), and pockets (4.8-6.7%) consisting of green basaltic to basaltic andesitic shock glass that is partially devitrified into a brown to black product along boundaries with the primary minerals. The average maximum dimesions of minerals are: olivine (1.5 mm), pyroxene (0.3 mm) and maskelynite (0.3 mm). Primary melt inclusions in olivine and chromite are common and account for 0.1-0.6% of the rock. X-ray tomography revealed that the specimen contains ~0.4 vol% of shock-melt associated vesicles, up to 3 mm in size, which show a preferred orientation. Fluidizatio of the maskelynite, melting and recrystallization of pyroxene, olivine and pyrrhotite indicate shock stage S6. Minor terrestrial weathering resulted in calcite-veining and minor oxidation of sulfides. The meteorite is interpreted as paired with SaU 005/008/051. The modal composition is similar to Dar al Gani 476/489/670/735/876, with the exception that neither mesostasis nor titanomagnetite nor apatite are present and that all phases show little zonation. The restricted mineral composition, predominance of chromite among the oxides, and abundance of olivine indicate affinities to the lherzolitic shergottites.
    • Radar observations of asteroid 1999 JM8

      Benner, Lance A. M.; Ostro, Steven J.; Nolan, Michael C.; Margot, Jean-Luc; Giorgini, Jon D.; Hudson, R. Scott; Jurgens, Raymond F.; Slade, Martin A.; Howell, Ellen S.; Campbell, Donald B.; et al. (The Meteoritical Society, 2002-01-01)
      We report results of delay-Doppler observations of 1999 JM8 with the Goldstone 8560 MHz (3.5 cm) and Arecibo 2380 MHz (13 cm) radars over 18 days in July-August 1999. The images place thousands of pixels on the asteroid and achieve range resolutions as fine as 15 m/pixel. The images reveal an asymmetric, irregularly shaped object with a typical overall dimension within 20% of 7 km. If we assume that 1999 JM8's effective diameter is 7 km, then the absolute magnitude, 15.15, and the average Goldstone radar cross section, 2.49 km^2, correspond to optical and radar albedos of 0.02 and 0.06, establishing that 1999 JM8 is a dark object at optical and radar wavelengths. The asteroid is in a non-principal axis spin state that, although not yet well determined, has a dominant periodicity of ~7 days. However, images obtained between July 31 and August 9 show apparent regular rotation of features from day to day, suggesting that the rotation state is not far from principal axis rotation. 1999 JM8 has regions of pronounced topographic relief, prominent facets several kilometers in extent, numerous crater-like features between ~100 m and 1.5 km in diameter, and features whose structural nature is peculiar. Arecibo images provide the strongest evidence to date for a circular polarization ratio feature on any asteroid. Combined optical and radar observations from April 1990 to December 2000 permit computation of planetary close approach times to within 10 days over the interval from 293 to at least 2907, one of the longest spans for any potentially hazardous asteroid. Integration of the orbit into the past and future shows close approaches to Earth, Mars, Ceres, and Vesta, but the probability of the object impacting Earth is zero for at least the next nine centuries.
    • Thermoluminescence sensitivity and thermal history of type 3 ordinary chondrites: Eleven new type 3.0-3.1 chondrites and possible explanations for differences among H, L, and LL chondrites

      Benoit, P. H.; Akridge, G. A.; Ninagawa, K.; Sears, D. W. G. (The Meteoritical Society, 2002-01-01)
      We review induced thermoluminescence (TL) data for 102 unequilibrated ordinary chondrites (UOCs), many data just published in abstracts, in order to identify particularly primitive UOCs and further explore TL systematics that may have implications for the history of the chondrites and their parent body. We have identified 11 UOCs of petrologic types 3.0-3.1: Adrar 003, Elephant Moraine (EET) 90066, EET 90161, Grosvenor Mountains (GRO) 95502, Lewis Cliff (LEW) 88477, Meteorite Hills (MET) 96503, Yamato (Y)-790787, Y-791324, Y-791558, Y-793565, and Y-793596. These samples represent an important new resource for researchers interested in the nature of primitive solar system materials. Previously reported trends in which TL sensitivity increases with TL peak temperature and TL peak width, which we interpret in terms of crystallization of feldspar in the ordered or disordered forms during metamorphism, are confirmed by the new data. Importantly, the present data strengthen the trend described earlier in which the mean level of metamorphism experienced by UOCs increases along the series LL, L and H. This suggests either different burial depths for the UOCs from each class, or formation at similar depths in regoliths of different thickness.