• Fabric analysis of Allende matrix using EBSD

      Watt, Lauren E.; Bland, Phil A.; Prior, Dave J.; Russell, Sara S. (The Meteoritical Society, 2006-01-01)
      Fabric analysis of the interstitial matrix material in primitive meteorites offers a novel window on asteroid formation and evolution. Electron backscatter diffraction (EBSD) has allowed fabrics in these fine-grained materials to be visualized in detail for the first time. Our data reveal that Allende, a CV3 chondrite, possesses a uniform, planar, short-axis alignment fabric that is pervasive on a broad scale and is probably the result of deformational shortening related to impact or gravitational compaction. Interference between this matrix fabric and the larger, more rigid components, such as dark inclusions (DIs) and calcium-aluminium-rich inclusions (CAIs), has lead to the development of locally oriented and intensified matrix fabrics. In addition, DIs possess fabrics that are conformable with the broader matrix fabric. These results suggest that DIs were in situ prior to the deformational shortening event responsible for these fabrics, thus providing an argument against dark inclusions being fragments from another lithified part of the asteroid (Kojima and Tomeoka 1996; Fruland et al. 1978). Moreover, both DIs and Allende matrix are highly porous (~25%) (Corrigan et al. 1997). Mobilizing a highly porous DI during impact-induced brecciation without imposing a fabric and incorporating it into a highly porous matrix without significantly compacting these materials is improbable. We favor a model that involves Allende DIs, CAIs, and matrix accreting together and experiencing the same deformation events.
    • Focused ion beam recovery of hypervelocity impact residue in experimental craters on metallic foils

      Graham, G. A.; Teslich, N.; Dai, Z. R.; Bradley, J. P.; Kearsley, A. T.; Hörz, F. (The Meteoritical Society, 2006-01-01)
      The Stardust sample return capsule returned to Earth in January 2006 with primitive debris collected from Comet 81P/Wild-2 during the fly-by encounter in 2004. In addition to the cometary particles embedded in low-density silica aerogel, there are microcraters preserved in the aluminum foils (1100 series; 100 micrometers thick) that are wrapped around the sample tray assembly. Soda lime spheres (~49 micrometers in diameter) have been accelerated with a light gas gun into flight-grade aluminum foils at 6.35 km s^(-1) to simulate the capture of cometary debris. The experimental craters have been analyzed using scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDX) to locate and characterize remants of the projectile material remaining within the craters. In addition, ion beam-induced secondary electron imaging has proven particularly useful in identifying areas within the craters that contain residue material. Finally, high-precision focused ion beam (FIB) milling has been used to isolate and then extract an individual melt residue droplet from the interior wall of an impact. This has enabled further detailed elemental characterization that is free from the background contamination of the aluminum foil substrate. The ability to recover "pure" melt residues using FIB will significantly extend the interpretations of the residue chemistry preserved in the aluminum foils returned by Stardust.
    • Formation of the binary near-Earth object 1996 FG3: Can binary NEOs be the source of short-CRE meteorites?

      Morbidelli, A.; Levison, H. F.; Bottke, W. F. (The Meteoritical Society, 2006-01-01)
      1996 FG3 is a binary near-Earth object (NEO) that was likely formed during a tidal disruption event. Our results indicate that the formation of this binary object was unlikely to have occurred when the progenitor had a encounter velocity with the Earth significantly smaller than its current value (10.7 km/s); The formation of the binary object on an orbit similar to the present one is possible, and the survival of the satellite constrains this to have happened less than 1.6 Ma ago. However, the binary object could also have been formed when the progenitor's encounter velocity with Earth was >12 km/s, and in this case we cannot constrain its formation age. Our results indicate that tidal disruptions occurring among NEOs with low velocity encounters with Earth are unlikely to produce long-lasting NEO binaries. Thus, tidal disruption may not be able to completely re-supply the observed population. This would imply that a significant fraction of the observed NEO binaries evolved out of the main asteroid belt. Overall, our results suggest to us that the CM2 meteorites having cosmic ray exposure (CRE) ages of ~200,000 yr were likely liberated by the tidal disruption of a primitive NEO with a relative velocity with the Earth significantly smaller than that of 1996 FG3. We propose a list of such objects, although as far as we know, none of the candidates is a binary for the reasons described above.
    • Formation of TiC core-graphitic mantle grains from CO gas

      Kimura, Yuki; Nuth, Joseph A.; Ferguson, Frank T. (The Meteoritical Society, 2006-01-01)
      We demonstrate a new formation route for TiC core-graphitic mantle spherules that does not require carbon-atom addition and the very long time scales associated with such growth (Bernatowicz et al. 1996). Carbonaceous materials can be formed from C2H2 and its derivatives, as well as from CO gas. In this paper, we will demonstrate that large-cage-structure carbon particles can be produced from CO gas by the Boudouard reaction. Since the sublimation temperature for such fullerenes is low, the large cages can be deposited onto previously nucleated TiC and produce TiC core-graphitic mantle spherules. New constraints for the formation conditions and the time scale for the formation of TiC core-graphitic mantle spherules are suggested by the results of this study. In particular, TiC core-graphitic mantle grains that are found in primitive meteorites that have never experienced hydration could be mantled by fullerenes or carbon nanotubes rather than by graphite. In situ observations of these grains in primitive anhydrous meteoritic matrix could confirm or refute this prediction and would demonstrate that the graphitic mantle on such grains is a metamorphic feature due to interaction of the presolar fullerenes with water within the meteorite matrix.
    • Fracture-related intracrystalline transformation of olivine to ringwoodite in the shocked Sixiangkou meteorite

      Chen, Ming; Li, Hui; El Goresy, Ahmed; Liu, Jing; Xie, Xiande (The Meteoritical Society, 2006-01-01)
      Magnesium-iron olivine in the Sixiangkou L6 chondrite contains abundant fractures induced by plastic deformation during shock metamorphism. This study reports the discovery of lamellar ringwoodite that incoherently nucleated and grew along planar and irregular fractures in olivine. Magnesium-iron interdiffusion took place between olivine matrix and crystallizing ringwoodite at high pressures and high temperatures, which resulted in higher FeO content in ringwoodite lamellae than in olivine. This suggests that a quasi-hydrostatic high pressure lasting for several minutes should have been produced in the shock veins of the meteorite. The intracrystalline transformation of olivine to ringwoodite also has implications for phase transitions in subducting lithospheric slabs because planar and irregular fractures are commonly produced in olivine that suffered plastic deformation.
    • Free dicarboxylic and aromatic acids in the carbonaceous chondrites Murchison and Orgueil

      Martins, Z.; Watson, J. S.; Sephton, M. A.; Botta, O.; Ehrenfreund, P.; Gilmour, I. (The Meteoritical Society, 2006-01-01)
      We have analyzed an important fraction of the free carboxylic acids present in water extracts of the CM2 chondrite Murchison and the CI1 chondrite Orgueil using gas chromatography-mass spectrometry (GC-MS). The free nature of the carboxylic acids analyzed was ensured by employing a single-step water extraction. Analyses revealed the presence of a structurally diverse suite of both aliphatic and aromatic acids in Murchison, while Orgueil exhibits a simpler distribution of exclusively aromatic acids. Within the Murchison aromatic acids, there are previously unreported phthalic acids, methyl phthalic acids, and hydroxybenzoic acids. In Orgueil, benzoic acid and very small amounts of methylbenzoic acids and methylhydroxybenzoic acids were detected. For the aromatic acids in both Murchison and Orgueil, most structural isomers were identified, suggesting an origin by abiotic processes. Quantitative differences are evident between acids in the two meteorites; carboxylic acids are much more abundant in Murchison than in Orgueil. The data suggest that differing levels of aqueous alteration on the meteorite parent body(ies) has produced dissimilar distributions of carboxylic acids.
    • Frontier Mountain 93001: A coarse-grained, enstatite-augite-oligoclase-rich, igneous rock from the acapulcoite-lodranite parent asteroid

      Folco, L.; D'Orazio, M.; Burroni, A. (The Meteoritical Society, 2006-01-01)
      The Frontier Mountain (FRO) 93001 meteorite is a 4.86 g fragment of an unshocked, medium-to coarse-grained rock from the acapulcoite-lodranite (AL) parent body. It consists of anhedral orthoenstatite (FS13.3 +/- .04 WO 3.1 +/- 0.2), augite (FS6.1 +/- 0.7 WO42.3 +/- 0.9; Cr2O3 = 1.54 +/- 0.03), and oligoclase (Ab80.5 +/- 3.3 Or 3.1 +/- 0.6) up to >1 cm in size enclosing polycrystalline aggregates of fine-grained olivine (average grain size: 460 +/- 210 micrometers) showing granoblastic textures, often associated with Fe,Ni metal, troilite, chromite (cr# = 0.91 +/- 0.03; fe# = 0.62 +/ 0.04), schreibersite, and phosphates. Such aggregates appear to have been corroded by a melt. They are interpreted as lodranitic xenoliths. After the igneous (the term "igneous" is used here strictly to describe rocks or minerals that solidified from molten material) lithology intruding an acapulcoite host in Lewis Cliff (LEW) 86220, FRO 93001 is the second-known silicate-rich melt from the AL parent asteroid. Despite some similarities, the silicate igneous component of FRO 930011 (i.e., the pyroxene-plagioclase mineral assemblage) differs in being coarser-grained and containing abundant enstatite. Melting-crystallization modeling suggests that FRO 93001 formed through high-degree partial melting (greater than or equal to 35 wt%; namely, greater than or equal to 15 wt% silicate melting and ~20 wt% metal melting) of an acapulcoite source rock, or its chondritic precursor, at temperatures greater than or equal to 1200 degrees C, under reducing conditions. The resulting magnesium-rich silicate melt then underwent equilibrium crystallization; prior to complete crystallization at ~1040 degrees C, it incorporated lodranitic xenoliths. FRO 93001 is the highest-temperature melt from the AL parent-body so far available in laboratory. The fact that FRO 93001 could form by partial melting and crystallization under equilibrium conditions, coupled with the lack of quench-textures and evidence for shock deformation in xenoliths, suggests that FRO 93001 is a magmatic rock produced by endogenic heating rather than impact melting.
    • FTIR 2–16 micron spectroscopy of micron-sized olivines from primitive meteorites

      Morlok, A.; Bowey, J.; Köhler, M.; Grady, M. M. (The Meteoritical Society, 2006-01-01)
      Infrared spectra of mineral grains from primitive meteorites could be useful for comparison with astronomical infrared spectra since some of their grains might be similar to those formed in the planet-forming disks around young stars or in the envelopes surrounding late-typestars. To assess the usefulness of meteorite spectra, olivine grains separated from primitive meteorites have been analyzed using FTIR microscope techniques in the 2-16 micrometers wavelength range. The sub-micron sizes of the grains made a complex preparation process necessary.Five characteristic bands were measured near 11.9, 11.2, 10.4, 10.1, and 10.0 micrometers. The results of 59 analyses allow the calculation of band positions for meteoritic olivines as a function of their iron and magnesium contents. Comparison of the meteoritic results with astronomical data for comets and dust around young and old stars, which exhibit bands similar to the strongest infrared bands observed in the grains (at 11.2 micrometers), show that the spectral resolution of the astronomical observations is too low to ascertain the exact iron and magnesium (Mg:Fe) ratio of the dust in the 8-13 micrometers wavelength range.
    • Geochemical identification of projectiles in impact rocks

      Tagle, R.; Hecht, L. (The Meteoritical Society, 2006-01-01)
      The three major geochemical methods for impactor identification are evaluated with respect to their potential and limitations with regards to the precise detection and identification of meteoritic material in impactites. The identification of a projectile component in impactites can be achieved by determining certain isotopic and elemental ratios in contaminated impactites. The isotopic methods are based on Os and Cr isotopic ratios. Osmium isotopes are highly sensitive for the detection of minute amounts of extraterrestrial components of even <<0.05 wt% in impactites. However, this only holds true for target lithologies with almost no chemical signature of mantle material or young mantle-derived mafic rocks. Furthermore, this method is not currently suitable for the precise identification of the projectile type. The Cr-isotopic method requires the relatively highest projectile contamination (several wt%) in order to detect an extraterrestrial component, but may allow the identification of three different groups of extraterrestrial materials, ordinary chondrites, an enstatite chondrites, and differentiated achondrites. A significant advantage of this method is its independence of the target lithology and post-impact alteration. The use of elemental ratios, including platinum group elements (PGE: Os, Ir, Ru, Pt, Rh, Pd), in combination with Ni and Cr represents a very powerful method for the detection and identification of projectiles in terrestrial and lunar impactites. For most projectile types, this method is almost independent of the target composition,especially if PGE ratios are considered. This holds true even in cases of terrestrial target lithologies with a high component of upper mantle material. The identification of the projectile is achieved by comparison of the “projectile elemental ratio” derived from the slope of the mixing line (target-projectile) with the elemental ratio in the different types of possible projectiles (e.g., chondrites). However, this requires a set of impactite samples of various degree of projectile contamination.
    • Geological and geochemical data from the proposed Sirente crater field: New age dating and evidence for heating of target

      Ormö, Jens; Koeberl, Christian; Rossi, Angelo Pio; Komatsu, Goro (The Meteoritical Society, 2006-01-01)
      The proposed Sirente crater field consists of a slightly oblong main structure (main crater) 120 m in width and about 30 smaller structures (satellite craters), all in unconsolidated but stiff carbonate mud. Here we focus on the subsurface structure of the satellite craters and compare the Sirente field with known meteorite crater fields. We present a more complete outline of the crater field than previously reported, information on the subsurface morphology of a satellite crater (C8) 8 m in width, radiocarbon and thermoluminescence (TL) ages of material from this crater, and evidence for heated material in both crater C8 and the rim of the main crater. Crater C8 has a funnel shape terminating downwards, and evidence for soil injection from the surface to a depth of 9 m. The infill contained dispersed charcoal and small, irregular, porous fragments of heated clay with a calibrated age of B.P. 1712 (13C-corrected radiocarbon age: B.P. 1800 +/- 100) and a TL age of B.P. 1825 (calculated error +/- 274). Together with previous radiocarbon age (B.P. 1538) of the formation of the main crater (i.e., target surface below rim), a formation is suggested at the beginning of the first millennium A.D. Although projectile vaporization is not expected in Sirente-sized craters in this type of target material, we used geochemistry in an attempt to detect a meteoritic component. The results gave no unequivocal evidence of meteoritic material. Nevertheless, the outline of the crater field, evidence of heated material within the craters, and subsurface structure are comparable with known meteorite crater fields.
    • High-latitude cold-based glacial deposits on Mars: Multiple superposed drop moraines in a crater interior at 70 degrees N latitude

      Garvin, James B.; Head, James W.; Marchant, David R.; Kreslavsky, Mikhail A. (The Meteoritical Society, 2006-01-01)
      An impact crater 26.8 km in diameter, located in the northern lowlands (70.32 degrees N, 266.45 degrees E) at the base of the flanking slopes of the shield volcano Alba Patera, is characterized by highly unusual deposits on its southeastern floor and interior walls and on its southeastern rim. These include multiple generations of distinctive arcuate ridges about 115-240 m in width and lobate deposits extending down the crater wall and across the crater floor, forming a broad, claw-like, ridged deposit around the central peak. Unusual deposits on the eastern and southeastern crater rim include frost, dunes, and a single distal arcuate ridge. Based on their morphology and geometric relationships, and terrestrial analogs from the Mars-like Antarctic Dry Valleys, the floor ridges are interpreted to represent drop moraines, remnants of the previous accumulation of snow and ice, and formation of cold-based glaciers on the crater rim. The configuration and superposition of the ridges indicate that the accumulated snow and ice formed glaciers that flowed down into the crater and across the crater floor, stabilized, covering an area of about 150 km^2, and produced multiple individual drop moraines due to fluctuation in the position of the stable glacier front. Superposition of a thin mantle and textures attributed to a recent ice-age period (~0.5-2 Myr ago) suggest that the glacial deposits date to at least 4-10 Myr before the present. At least five phases of advance and retreat are indicated by the stratigraphic relationships, and these may be related to obliquity excursions. These deposits are in contrast to other ice-related modification and degradation processes typical of craters in the northern lowlands, and may be related to the distinctive position of this crater in the past atmospheric circulation pattern, leading to sufficient preferential local accumulation of snow and ice to cause glacial flow.
    • Hydrocode modeling of the Sierra Madera impact structure

      Goldin, T. J.; Wünnemann, K.; Melosh, H. J.; Collins, G. S. (The Meteoritical Society, 2006-01-01)
      We present the first hydrocode simulations of the formation of the Sierra Madera structure (west Texas, USA), which was caused by an impact into a thick sedimentary target sequence. We modeled Sierra Madera using the iSALE hydrocode, and here we present two best-fit models: 1) a crater with a rim (final crater) diameter of ~12 km, in agreement with previous authors interpretations of the original structure, and 2) a crater ~16 km in diameter with increased postimpact erosion. Both models fit some of the geologic observational data, but discrepancies with estimates of peak shock pressure, extent of deformation, and stratigraphic displacement remain. This study suggests that Sierra Madera may be a larger crater than previously reported and illustrates some of the challenges in simulating impact deformation of sedimentary lithologies. As many terrestrial craters possess some amount of sedimentary rocks in the target sequence, numerical models of impacts into sedimentary targets are essential to our understanding of target rock deformation and the mechanics of crater formation.
    • Identification of minerals and meteoritic materials via Raman techniques after capture in hypervelocity impacts on aerogel

      Burchell, M. J.; Mann, J.; Creighton, J. A.; Kearsley, A. T.; Graham, G.; Franchi, I. A. (The Meteoritical Society, 2006-01-01)
      Mineral particles analogous to components of cosmic dust were tested to determine if their Raman signatures can be recognized after hypervelocity capture in aerogel. The mineral particles were accelerated onto the silica aerogel by light-gas-gun shots. It was found that all the individual minerals captured in aerogel could be identified using Raman (or fluorescence) spectra. The laser beam spot size was 5 micrometers, and in some cases the captured particles were of a similar small size. In some samples fired into aerogel, a broadening and a shift in the wave numbers of some of the Raman bands was observed, a result of the trapped particles being at elevated temperatures due to laser heating. Temperatures of samples were also estimated from the relative intensities of Stokes and anti-Stokes Raman bands, or, in the case of corundum particles, from the wave number of fluorescence bands excited by the laser. The temperature varied greatly, dependent upon laser power and the nature of the particle. Most of the mineral particles examined had temperatures below 200 degrees C at a laser power of about 3 mW at them sample. This temperature is sufficiently low enough not to damage most materials expected to be found captured in aerogel in space. In the worst case, some particles were shown to have temperatures of 500-700 degrees C. In addition, selected meteorite samples were examined to obtain Raman signatures of their constituent minerals and were then shot into aerogel. It was possible to find Raman signatures after capture in aerogel and obtain a Raman map of a whole grain in situ in the aerogel. It is concluded that Raman analysis is indeed well suited for an in situ analysis of micrometer-sized materials captured in aerogel.
    • Impact craters in the northern hemisphere of Mars: Layered ejecta and central pit characteristics

      Barlow, Nadine G. (The Meteoritical Society, 2006-01-01)
      Mars Global Surveyor (MGS) and Mars Odyssey data are being used to revise the Catalog of Large Martian Impact Craters. Analysis of data in the revised catalog provides new details on the distribution and morphologic details of 6795 impact craters in the northern hemisphere of Mars. This report focuses on the ejecta morphologies and central pit characteristics of these craters. The results indicate that single-layer ejecta (SLE) morphology is most consistent with impact into an ice-rich target. Double-layer ejecta (DLE) and multiple-layer ejecta (MLE) craters also likely form in volatile-rich materials, but the interaction of the ejecta curtain and target-produced vapor with the thin Martian atmosphere may be responsible for the large runout distances of these ejecta. Pancake craters appear to be a modified form of double-layer craters where the thin outer layer has been destroyed or is unobservable at present resolutions. Pedestal craters are proposed to form in an ice-rich mantle deposited during high obliquity periods from which the ice has subsequently sublimated. Central pits likely form by the release of vapor produced by impact into ice-soil mixed targets. Therefore, results from the present study are consistent with target volatiles playing a dominant role in the formation of crater morphologies found in the Martian northern hemisphere.
    • In situ identification, pairing, and classification of meteorites from Antarctica through magnetic susceptibility measurements

      Folco, L.; Rochette, P.; Gattacceca, J.; Perchiazzi, N. (The Meteoritical Society, 2006-01-01)
      We report on the effectiveness of using magnetic measurements in the search for meteorites on the Antarctic ice sheet, which is thus far the Earth's most productive terrain. Magnetic susceptibility measurements carried out with a pocket meter (SM30) during the 2003/04 PNRA meteorite collection expedition to northern Victoria Land (Antarctica) proved to be a rapid, sensitive, non-destructive means for the in situ identification, pairing, and classification of meteorites. In blue ice fields characterized by the presence of moraines and glacial drifts (e.g., Miller Butte, Roberts Butte, and Frontier Mountain), magnetic susceptibility measurements allowed discrimination of meteorites from abundant terrestrial stones that look like meteorites thanks to the relatively high magnetic susceptibility of the former with respect to terrestrial rocks. Comparative measurements helped identify 16 paired fragments found at Johannessen Nunataks, thereby reducing unnecessary duplication of laboratory analyses and statistical bias. Following classifications schemes developed by us in this and previous works, magnetic susceptibility measurements also helped classify stony meteorites directly in the field, thereby providing a means for selecting samples with higher research priority. A magnetic gradiometer capable of detecting perturbations in the Earth's magnetic field induced by the presence of meteorites was an efficient tool for locating meteorites buried in snow along the downwind margin of the Frontier Mountain blue ice field. Based on these results, we believe that magnetic sensors should constitute an additional payload for robotic search for meteorites on the Antarctic ice sheet and, by extension, on the surface of Mars where meteorite accumulations are predicted by theoretical works. Lastly, magnetic susceptibility data was successfully used to cross-check the later petrographic classification of the 123 recovered meteorites, allowing the detection of misclassified or peculiar specimens.
    • Investigation of Shuttle Radar Topography Mission data of the possible impact structure at Serra da Cangalha, Brazil

      Reimold, Wolf U.; Cooper, Gordon R. J.; Romano, Rafael; Cowan, Duncan R.; Koeberl, Christian (The Meteoritical Society, 2006-01-01)
      The Serra da Cangalha crater structure in northeast Brazil, ~13 km in diameter, has long been widely considered to be a confirmed impact structure, based on reports of shatter cone findings. Only very limited field work has been carried out at this crater structure. Landsat Thematic Mapper (TM) and Shuttle Radar Topography Mission (SRTM) data sets for the region around this crater structure are compared here with regard to their suitability to determine first-order structural detail of impact crater structures. The SRTM data provide very detailed information regarding drainage patterns and topography. A pronounced central ring of up to 300 m elevation above the surrounding area, two comparatively subdued intermediate rings of 6 and 10.5 km diameter, respectively, and the broad, complex crater rim of up to >100 m elevation can be distinguished in the Serra da Cangalha data. The maximum cratering-related regional deformation (radial and concentric features) seems to be limited to a radial distance of 16-18 km from the center of the structure. A first comparison of macrostructural information from several impact structures with that from Serra da Cangalha does not yield firm trends, but the database is still very small at this stage. The varied nature of the target geology strongly influences the development of structural features in any impact event.
    • Laboratory simulation of impacts on aluminum foils of the Stardust spacecraft: Calibration of dust particle size from comet Wild-2

      Kearsley, A. T.; Burchell, M. J.; Hörz, F.; Cole, M. J.; Schwandt, C. S. (The Meteoritical Society, 2006-01-01)
      Metallic aluminum alloy foils exposed on the forward, comet-facing surface of the aerogel tray on the Stardust spacecraft are likely to have been impacted by the same cometary particle population as the dedicated impact sensors and the aerogel collector. The ability of soft aluminum alloy to record hypervelocity impacts as bowl-shaped craters offers an opportunistic substrate for recognition of impacts by particles of a potentially wide size range. In contrast to impact surveys conducted on samples from low Earth orbit, the simple encounter geometry for Stardust and Wild-2, with a known and constant spacecraft-particle relative velocity and effective surface-perpendicular impact trajectories, permits closely comparable simulation in laboratory experiments. For a detailed calibration program, we have selected a suite of spherical glass projectiles of uniform density and hardness characteristics, with well-documented particle size range from 10 micrometers to nearly 100 micrometers. Light gas gun buckshot firings of these particles at approximately 6 km s^(-1) onto samples of the same foil as employed on Stardust have yielded large numbers of craters. Scanning electron microscopy of both projectiles and impact features has allowed construction of a calibration plot, showing a linear relationship between impacting particle size and impact crater diameter. The close match between our experimental conditions and the Stardust mission encounter parameters should provide another opportunity to measure particle size distributions and fluxes close to the nucleus of Wild-2, independent of the active impact detector instruments aboard the Stardust spacecraft.
    • Laboratory simulation of terrestrial meteorite weathering using the Bensour (LL6) ordinary chondrite

      Lee, M. R.; Smith, C. L.; Gordon, S. H.; Hodson, M. E. (The Meteoritical Society, 2006-01-01)
      Laboratory dissolution experiments using the LL6 ordinary chondrite Bensour demonstrate that meteoritic minerals readily react with distilled water at low temperatures, liberating ions into solution and forming reaction products. Three experiments were performed, all for 68 days and at atmospheric fO2 but using a range of water/rock ratios and different temperatures. Experiments 1 and 2 were batch experiments and undertaken at room temperature, whereas in experiment 3, condensed boiling water was dripped onto meteorite subsamples within a Soxhlet extractor. Solutions from experiment 1 were chemically analyzed at the end of the experiment, whereas aliquots were extracted from experiments 2 and 3 for analysis at regular intervals. In all three experiments, a very significant proportion of the Na, Cl, and K within the Bensour subsamples entered solution, demonstrating that chlorapatite and feldspar were especially susceptible to dissolution. Concentrations of Mg, Al, Si, Ca, and Fe in solution were strongly affected by the precipitation of reaction products and Mg and Ca may also have been removed by sorption. Calculations predict saturation of experimental solutions with respect to Al hydroxides, Fe oxides, and Fe (oxy)hydroxides, which would have frequently been accompanied by hydrous aluminosilicates. Some reaction products were identified and include silica, a Mg-rich silicate, Fe oxides, and Fe (oxy)hydroxides. The implications of these results are that even very short periods of subaerial exposure of ordinary chondrites will lead to dissolution of primary minerals and crystallization of weathering products that are likely to include aluminosilicates and silicates, Mg-Ca carbonates, and sulfates in addition to the ubiquitous Fe oxides and (oxy)hydroxides.
    • Macroscopic subdivision of silica aerogel collectors for sample return missions

      Ishii, H. A.; Bradley, J. P. (The Meteoritical Society, 2006-01-01)
      Silica aerogel collector tiles have been employed for the collection of particles in low Earth orbit and, more recently, for the capture of cometary particles by NASA's Stardust mission. Reliable, reproducible methods for cutting these and future collector tiles from sample return missions are necessary to maximize the science output from the extremely valuable embedded particles. We present a means of macroscopic subdivision of collector tiles by generating large-scale cuts over several centimeters in silica aerogel with almost no material loss. The cut surfaces are smooth and optically clear allowing visual location of particles for analysis and extraction. This capability is complementary to the smaller-scale cutting capabilities previously described (Westphal 2004; Ishii 2005a, 2005b) for removing individual impacts and particulate debris in tiny aerogel extractions. Macroscopic cuts enable division and storage or distribution of portions of aerogel tiles for immediate analysis of samples by certain techniques in situ or further extraction of samples suited for other methods of analysis. The capability has been implemented in the Stardust Laboratory at NASA's Johnson Space Center as one of a suite of aerogel cutting methods to be used in Stardust sample curation.
    • Magmatic cristobalite and quartz in the NWA 856 Martian meteorite

      Leroux, Hugues; Cordier, Patrick (The Meteoritical Society, 2006-01-01)
      Silica-rich late-stage crystallization pockets in the Martian meteorite Northwest Africa (NWA) 856 were investigated by transmission electron microscopy (TEM). The pockets occur as wedges between maskelynite laths or between maskelynite and pyroxene. They consist of elongated grains of cristobalite and quartz embedded in a silica-rich glass. Interstitial to the amorphous phase and silica minerals, a number of small accessory minerals have been identified, typical for late-stage crystallization products. They are ilmenite, tranquillityite, fayalite, troilite, baddeleyite, apatite, and chloroapatite. Cristobalite and quartz are shocked, as revealed by the occurrence of numerous amorphous lamellae. This assemblage suggests metastable dendritic crystallization under hydrous conditions. Cristobalite crystallization was probably facilitated by the presence of impurities such as Na or H2O. Our observations show that silica minerals can be formed under magmatic conditions on Mars.