• Table of Contents, Volume 43, Supplement, 2008 July

      The Meteoritical Society, 2008-01-01
    • Target rocks, impact glasses, and melt rocks from the Lonar impact crater, India: Petrography and geochemistry

      Osae, S.; Misra, S.; Koeberl, C.; Sengupta, D.; Ghosh, S. (The Meteoritical Society, 2005-01-01)
      The Lonar crater, India, is the only well-preserved simple crater on Earth in continental flood basalts; it is excavated in the Deccan trap basalts of Cretaceous-Tertiary age. A representative set of target basalts, including the basalt flows excavated by the crater, and a variety of impact breccias and impact glasses, were analyzed for their major and trace element compositions. Impact glasses and breccias were found inside and outside the crater rim in a variety of morphological forms and shapes. Comparable geochemical patterns of immobile elements (e.g., REEs) for glass, melt rock and basalt indicates minimal fractionation between the target rocks and the impactites. We found only little indication of post-impact hydrothermal alteration in terms of volatile trace element changes. No clear indication of an extraterrestrial component was found in any of our breccias and impact glasses, indicating either a low level of contamination, or a non-chondritic or otherwise iridium-poor impactor.
    • Tectonic influences on the morphometry of the Sudbury impact structure: Implications for terrestrial cratering and modeling

      Spray, J. G.; Butler, H. R.; Thompson, L. M. (The Meteoritical Society, 2004-01-01)
      Impact structures developed on active terrestrial planets (Earth and Venus) are susceptible to pre-impact tectonic influences on their formation. This means that we cannot expect them to conform to ideal cratering models, which are commonly based on the response of a homogeneous target devoid of pre-existing flaws. In the case of the 1.85 Ga Sudbury impact structure of Ontario, Canada, considerable influence has been exerted on modification stage processes by late Archean to early Proterozoic basement faults. Two trends are dominant: 1) the NNW-striking Onaping Fault System, which is parallel to the 2.47 Ga Matachewan dyke swarm, and 2) the ENE-striking Murray Fault System, which acted as a major Paleoproterozoic suture zone that contributed to the development of the Huronian sedimentary basin between 2.45-2.2 Ga. Sudbury has also been affected by syn- to post-impact regional deformation and metamorphism: the 1.9-1.8 Ga Penokean orogeny, which involved NNW-directed reverse faulting, uplift, and transpression at mainly greenschist facies grade, and the 1.16-0.99 Ga Grenville orogeny, which overprinted the SE sector of the impact structure to yield a polydeformed upper amphibolite facies terrain. The pre-, syn-, and post-impact tectonics of the region have rendered the Sudbury structure a complicated feature. Careful reconstruction is required before its original morphometry can be established. This is likely to be true for many impact structures developed on active terrestrial planets. Based on extensive field work, combined with remote sensing and geophysical data, four ring systems have been identified at Sudbury. The inner three rings broadly correlate with pseudotachylyte (friction melt) -rich fault systems. The first ring has a diameter of ~90 km and defines what is interpreted to be the remains of the central uplift. The second ring delimits the collapsed transient cavity diameter at ~130 km and broadly corresponds to the original melt sheet diameter. The third ring has a diameter of ~180 km. The fourth ring defines the suggested apparent crater diameter at ~260 km. This approximates the final rim diameter, given that erosion in the North Range is 6 km and the ring faults are steeply dipping. Impact damage beyond Ring 4 may occur, but has not yet been identified in the field. One or more rings within the central uplift (Ring 1) may also exist. This form and concentric structure indicates that Sudbury is a peak ring or, more probably, a multi-ring basin. These parameters provide the foundation for modeling the formation of thisrelatively large terrestrial impact structure.
    • Tectonics of complex crater formation as revealed by the Haughton impact structure, Devon Island, Canadian High Arctic

      Osinski, Gordon R.; Spray, John G. (The Meteoritical Society, 2005-01-01)
      The results of a systematic field mapping campaign at the Haughton impact structure have revealed new information about the tectonic evolution of mid-size complex impact structures. These studies reveal that several structures are generated during the initial compressive outward-directed growth of the transient cavity during the excavation stage of crater formation: (1) sub-vertical radial faults and fractures; (2) sub-horizontal bedding parallel detachment faults; and (3) minor concentric faults and fractures. Uplift of the transient cavity floor toward the end of the excavation stage produces a central uplift. Compressional inward-directed deformation results in the duplication of strata along thrust faults and folds. It is notable that Haughton lacks a central topographic peak or peak ring. The gravitational collapse of transient cavity walls involves the complex interaction of a series of interconnected radial and concentric faults. While the outermost concentric faults dip in toward the crater center, the majority of the innermost faults at Haughton dip away from the center. Complex interactions between an outward-directed collapsing central uplift and inward collapsing crater walls during the final stages of crater modification resulted in a structural ring of uplifted, intensely faulted (sub-) vertical and/or overturned strata at a radial distance from the crater center of ~5.0-6.5 km. Converging flow during the collapse of transient cavity walls was accommodated by the formation of several structures: (1) sub-vertical radial faults and folds; (2) positive flower structures and chaotically brecciated ridges; (3) rollover anticlines in the hanging-walls of major listric faults; and (4) antithetic faults and crestal collapse grabens. Oblique strike-slip (i.e., centripetal) movement along concentric faults also accommodated strain during the final stages of readjustment during the crater modification stage. It is clear that deformation during collapse of the transient cavity walls at Haughton was brittle and localized along discrete fault planes separating kilometer-size blocks.
    • Tellurium isotope compositions of calcium-aluminum-rich inclusions

      Fehr, M. A.; Rehkämper, M.; Halliday, A. N.; Hattendorf, B.; Günther, D. (The Meteoritical Society, 2009-01-01)
      A method for the precise and accurate determination of the tellurium (Te) isotope compositions of calcium-aluminum-rich inclusions (CAIs) has been developed. The technique utilizes multiple-collector inductively coupled plasma-mass spectrometry (MC-ICPMS) with either Faraday detectors or a dual ion-counting system. The external reproducibility (2-sigma) for 126Te/125Te was ~15 per mil and ~2 per mil when 3 pg and 65 pg of Te were analyzed with the electron multipliers. Measurements performed on 200 pg of Te using Faraday detectors and time-resolved software displayed an external reproducibility of ~8 per mil for 126Te/124Te, whereas 3 ng Te could be measured to a precision of about 0.6 per mil. Analyses of five CAIs from the Allende chondrite yielded Te concentrations that range from 12 to 537 ppb and the inclusions are therefore depleted in Te relative to bulk Allende by factors of about 2 to 86. The Sn/Te ratios of the CAIs are also fractionated compared to bulk Allende (which displays 124Sn/128Te is approximately equal to 0.1) with 124Sn/128Te ratios of about 0.1 to 2.5. The Te isotope measurements for these refractory inclusions yielded no 126Te excesses from the decay of the short-lived radionuclide 126Sn (tau-1/2 = 234,500 years) and the most precise analysis provided a epsilon-126Te value of 1 +/- 6 (epsilon-126Te = 126Te/ 124Te normalized to 122Te/124Te = 0.53594 and reported relative to the JMC Te standard). Minor differences in the Te isotope composition of the CAIs relative to the terrestrial standard and bulk Allende hint at the presence of small deficits in r-process Te isotopes or excess of s-process Te, but these nucleosynthetic anomalies are barely resolvable given the analytical uncertainties. Hence, it is also conceivable that these effects reflect small unresolved analytical artifacts.
    • TEM investigations on the monomict ureilites Jalanash and Hammadah al Hamra 064

      Weber, I.; Bischoff, A.; Weber, D. (The Meteoritical Society, 2003-01-01)
      We studied the petrography and mineralogy of two monomict ureilites, Hammadah al Hamra 064 (HH064) and Jalanash, by using reflected light and scanning electron microscopy. Quantitative analyses were performed by electron microprobe and the microstructures were investigated with transmission electron microscopy (TEM). HH064 features two different textures, a poikilitic and a typical one, whereas Jalanash shows only the typical ureilite texture. Our synergetic chemical and microstructural investigations reveal a complex cooling history for both ureilites. The temperature for the first equilibrium deduced from the pigeonite-augite assemblage in HH064 is ~1200 degrees C. The presence of antiphase domains in low-Ca pyroxenes proves that they are clearly pigeonite. The occurrences of tweed microstructure and orthopyroxene lamellae, which are incompletely developed, imply a faster cooling rate from the first equilibrium with a sudden end. Although both ureilites contain shock induced diamonds, dislocations in silicates are rare. This observation suggests that the meteorites were hot at the time of strong shock metamorphism or that they were heated after strong shock metamorphism. After this event, new microstructural features were generated by different cooling processes and were frozen by a final rapid decrease in temperature possibly due to excavation from the ureilite parent body, or bodies.
    • TEM studies and the shock history of a "mysterite" inclusion from the Krymka LL chondrite

      Weber, I.; Semenenko, V. P.; Stephan, T.; Jessberger, E. K. (The Meteoritical Society, 2006-01-01)
      The microstructure and composition of the matrix of one carbonaceous inclusion (K1) in the Krymka LL3.1 chondrite were studied using transmission electron microscopy (TEM). K1 has previously shown an enigmatic nature and similarities with volatile-rich, fine-grained, dark inclusions of Krymka called "mysterite."In the present study, four minerals were identified by TEM. Olivine, pyroxene, and pyrrhotite typically have grain sizes of one micrometer; graphite occurs as flakes of a similar size. Olivine shows a moderately high dislocation density most probably caused by shock. Pyroxene shows coexisting ortho- and clinoenstatite lamellae that probably originated from shear stress after a shock event or from the rapid cooling of the protoenstatite stability field. However, we demonstrate that in this case, a shock trigger is more likely. Pyrrhotite in the studied sample occurs as a 4C monoclinic superstructure. The graphite flakes in the fragment are well crystallized, as can be seen by discrete spots in the diffraction pattern. In graphite, the degree of crystallization increases with the metamorphic grade. Based on the microstructure of this mineral we conclude that after a first moderate shock event, the residual temperature between 300 degrees C and 500 degrees C led to thermal metamorphism. A second shock event, possibly at excavation from the parent body, is responsible for the shock features observed in olivine, pyroxene, and graphite.
    • Terrestrial ages, pairing, and concentration mechanism of Antarctic chondrites from Frontier Mountain, Northern Victoria Land

      Welten, K. C.; Nishiizumi, K.; Caffee, M. W.; Hillegonds, D. J.; Johnson, J. A.; Jull, A. J. T.; Wieler, R.; Folco, L. (The Meteoritical Society, 2006-01-01)
      We report concentrations of cosmogenic 10Be, 26Al, 36Cl, and 41Ca in the metal phase of 26 ordinary chondrites from Frontier Mountain (FRO), Antarctica, as well as cosmogenic 14C in eight and noble gases in four bulk samples. Thirteen out of 14 selected H chondrites belong to two previously identified pairing groups, FRO 90001 and FRO 90174, with terrestrial ages of ~40 and ~100 kyr, respectively. The FRO 90174 shower is a heterogeneous H3-6 chondrite breccia that probably includes more than 300 individual fragments, explaining the high H/L chondrite ratio (3.8) at Frontier Mountain. The geographic distribution of 19 fragments of this shower constrains ice fluctuations over the past 50-100 kyr to less than 40 m, supporting the stability of the meteorite trap over the last glacial cycle. The second H-chondrite pairing group, FRO 90001, is much smaller and its geographic distribution is mainly controlled by wind-transport. Most L-chondrites are younger than 50 kyr, except for the FRO 93009/01172 pair, which has a terrestrial age of ~500 kyr. These two old L chondrites represent the only surviving members of a large shower with a similar preatmospheric radius (~80 cm) as the FRO 90174 shower. The find locations of these two paired L-chondrite fragments on opposite sides of Frontier Mountain confirm the general glaciological model in which the two ice flows passing both ends of the mountain are derived from the same source area on the plateau. The 50 FRO meteorites analyzed so far represent 21 different falls. The terrestrial ages range from 6 kyr to 500 kyr, supporting the earlier proposed concentration mechanism.
    • Testing an integrated chronology: I-Xe analysis of enstatite meteorites and a eucrite

      Busfield, A.; Turner, G.; Gilmour, J. D. (The Meteoritical Society, 2008-01-01)
      We have determined initial 129I/127I ratios for mineral concentrates of four enstatite meteorites and a eucrite. In the case of the enstatite meteorites the inferred ages are associated with the pyroxene-rich separates giving pyroxene closure ages relative to the Shallowater standard of Indarch (EH4, -0.04 +/- 0.67 Ma), Khairpur (EL6, -4.22 +/- 0.67 Ma), Khor Temiki (aubrite, -0.06 Ma), and Itqiy (enstatite achondrite, -2.6 +/- 2.6 Ma), negative ages indicate closure after Shallowater. No separate from the cumulate eucrite Asuka (A-) 881394 yielded a consistent ratio, though excess 129Xe was observed in a feldspar separate, suggesting disturbance by thermal metamorphism within 25 Ma of closure in Shallowater. Iodine-129 ages are mapped to the absolute Pb-Pb time scale using the calibration proposed by Gilmour et al. (2006) who place the closure age of Shallowater at 4563.3 +/- 0.4 Ma. Comparison of the combined 129I-Pb data with associated 53Mn ages, for objects that have been dated by both systems, indicates that all three chronometers evolved concordantly in the early solar system. The enstatite chondrites are offset from the linear array described by asteroid-belt objects when 53Mn ages are plotted against combined 129I-Pb data, supporting the suggestion that 53Mn was radially heterogeneous in the early solar system.
    • Textural constraints on the formation of impact spherules: A case study from the Dales Gorge BIF, Paleoproterozoic Hamersley Group of Western Australia

      Sweeney, D.; Simonson, B. M. (The Meteoritical Society, 2008-01-01)
      Impact ejecta (about 2.5 Gyr old) in the DS4 layer of the Dales Gorge BIF (Hamersley Group, Western Australia) are so well preserved that many original textures such as vesicles and microlites are faithfully preserved. About 65% of the particles in the layer originated as impact ejecta, of which 81% are splash forms. The remaining 19% are angular, but the splash forms and angular particles have the same composition (mainly diagenetic stilpnomelane and K-feldspar) and share a common suite of internal textures. Some particles contain randomly oriented microlites texturally identical to plagioclase in basalts. Most splash forms have rims of inward-growing crystals that may have formed from the melt (perhaps nucleated by impinging dust) or via thermal devitrification. The rims clearly formed in flight because in broken particles (which make up about 13% of the splash forms) they are generally not present on broken surfaces. The origin of the angular particles is uncertain, but they may represent solid ejecta. Given the large sizes and variable shapes of the splash forms, they are probably droplets of impact melt emplaced ballistically. This is largely by analogy to the K-T boundary layer, but DS4 splash forms differ from K-T spherules in important ways suggesting the K-T model is not universal. The occurrence of basaltic ejecta from a large impact highlights its scarcity in the stratigraphic record despite the areal abundance of oceanic crust. The diverse textures formed via in-flight crystallization suggest particle paths in the plume are more complex than is generally appreciated.
    • The 410,000 year terrestrial age of eucrite Rio Cuarto 001

      Levine, Jonathan; Arazi, Andrés; Faestermann, Thomas; Fernández Niello, Jorge O.; Korschinek, Gunther; La Gamma, Ana María; Negri, Agustín; Rugel, Georg; Steier, Peter; Wallner, Anton (The Meteoritical Society, 2008-01-01)
      We have measured a surprisingly long terrestrial age of 410,000 +20,000/-45,000 years (410 +20/-45 ka) for basaltic eucrite Ro Cuarto 001 using accelerator mass spectrometry of 26Al, 36Cl, and 41Ca. Though many meteorites are known to have survived for tens or hundreds of ka in Antarctica or hot deserts, the mean annual precipitation of 815 mm in Ro Cuarto, Cordoba Province, Argentina, makes the long survival of this meteorite remarkable. We propose two explanations for the exceptional preservation of Ro Cuarto 001. First, the meteorite contains only trace amounts of metal, so the weathering and oxidation of metallic Fe, which commonly destroys chondrites, is ineffective in this case. Second, the meteorite was found in a relatively young deflation basin, and may have been exhumed only recently from beneath a protective layer of soil. Insofar as the survival on Earth of Ro Cuarto 001 is due to environmental factors, there may be other meteorites with comparably long terrestrial ages still to be discovered in the vicinity.
    • The activity of chromite in multicomponent spinels: Implications for T-fO2 conditions of equilibrated H chondrites

      Kessel, Ronit; Beckett, John R.; Huss, Gary R.; Stolper, Edward M. (The Meteoritical Society, 2004-01-01)
      Activities of chromite in multicomponent spinels with compositions similar to those of H chondrites were experimentally determined by equilibrating Pt-alloys with spinel at known temperature and fO2. Our results are consistent with predictions based on the spinel solid solution model incorporated into the MELTS program. Therefore, we combined literature formulations for the activities of components in spinel, the ferromagnesian silicates, and alloys with measured and literature (bulk alloy) compositions of the meteoritic phases to constrain T-fO2 conditions for the Hgroup chondrites Avanhandava (H4), Allegan (H5), and Guarea (H6). Log10fO2 values based on the assemblage of olivine + orthopyroxene + metal are 2.19-2.56 log units below the iron-wstite (IW) buffer for any equilibration temperature between 740 and 990 degrees C, regardless of petrographic type. Only lower limits on fO2 could be determined from spinel + metal equilibria because of the extremely low concentrations of Cr in the alloys of equilibrated H chondrites (3 ppb). Log10fO2 values required by spinel + metal equilibria are inconsistent with those for olivine + orthopyroxene + metal if equilibration temperatures were at or above those inferred from olivine-spinel thermometry. This probably indicates that the closure for spinel + metal equilibria occurred under retrograde conditions at temperatures below ~625 degrees C for Allegan and Guarea and below ~660 degrees C for Avanhandava.
    • The Asco meteorite (1805): New petrographic description, chemical data, and classification

      Gattacceca, J.; Bourot-Denise, M.; Brandstaetter, F.; Folco, L.; Rochette, P. (The Meteoritical Society, 2007-01-01)
      We present magnetic measurements, chemical analyses, and petrographic observations of the poorly studied Asco historical meteorite fall (1805). These new data indicate that this meteorite has been previously misclassified as an L6 ordinary chondrite. Asco is reclassified as an H6 ordinary chondrite with shock stage S3. An interesting feature of this meteorite is the presence of chromiteplagioclase assemblages with variable textures.
    • The Bagnone iron meteorite (Tuscany, Italy): History, mineralogy, and chemical classification

      D'Orazio, M.; Folco, L.; Perchiazzi, N. (The Meteoritical Society, 2004-01-01)
      Bagnone, the largest Italian iron meteorite (48 kg), was found as a single mass at the beginning of the 20th century while ploughing a field close to the Bagnone Castle (Massa Carrara, northern Tuscany). The morphology of the external surface suggests that Bagnone represents a complete individual. It is classified as a medium octahedrite (average bandwidth = 0.96 mm) of the IIIAB chemical group, based on inductively coupled plasma mass spectrometry analyses.
    • The basaltic shergottite Northwest Africa 856 (NWA 856): Petrology and chemistry

      Jambon, A.; Barrat, J. A.; Sautter, V.; Gillet, Ph.; Göpel, C.; Javoy, M.; Joron, J. L.; Lesourd, M (The Meteoritical Society, 2002-01-01)
      We report on the discovery of a new shergottite from South Morocco. This single stone weighing 320 g is referenced as Northwest Africa 856 (NWA 856) with Djel Ibone as a synonymous name. It is a fresh, fine-grained basaltic rock consisting mainly of 2 pyroxenes (total ~68 vol%: 45% pigeonite En61-16 Wo9-22 Fs26-68, 23% augite En46-26 Wo34-29 Fs21-43) and plagioclase converted to maskelynite (about 23 %, Ab43-57 Or1-5 An54-36). Accessory minerals include merrillite, Cl-apatite, pyrrhotite, ilmenite, ulvöspinel, silica (stishovite and glass), amorphous K-feldspar and baddeleyite. Amorphous mixtures of maskelynite and silica occur most commonly as median layers inside maskelynite laths. In addition, melt pockets (about 2%) were recognized with relics of maskelynite, pyroxene and both dense silica glass and stishovite occurring as both grains and sub-micrometer needles. The compositions of the melt pockets are consistent with mixtures of maskelynite and pyroxenes with an average of about 50 % maskelynite. The meteorite is highly fractured at all scales. The bulk composition of NWA 856 has been measured for 44 elements. It is an Al-poor ferroan basaltic rock which strongly resembles Shergotty and Zagami in its major and trace element composition. The nearly flat REE pattern (La/Lu)n= 0.9, is similar to that of Shergotty or Zagami and differs significantly from NWA 480, another Moroccan shergottite recently described. According to the U, Ba and Sr abundances, NWA 856 is not significantly weathered. The oxygen isotopes (delta-18O = +5.03 ppm, delta-17O = +3.09 ppm, and Delta-17O = +0.47 ppm) are in agreement with the Martian origin of this meteorite. On the basis of grain size, pyroxene zoning and composition, abundance of silica inclusions associated with maskelynite, trace element abundances, REE pattern and oxygen isotopes, pairing with NWA 480 is excluded. The similarity with Shergotty and Zagami is striking. The only significant differences are a larger grain size, a greater abundance of silica and melt pockets, a slightly more restricted range of pyroxene compositions and the absence of significant mesostasis.
    • The beginning heights and light curves of high-altitude meteors

      Koten, Pavel; Spurný, Pavel; Borovička, Jiří; Evans, Stephen; Elliott, Andrew; Betlem, Hans; Štork, Rostislav; Jobse, Klaas (The Meteoritical Society, 2006-01-01)
      In this paper, we provide an overview of meteors with high beginning height. During the recent Leonid meteor storms, as well as within the regular double station video observations of other meteor showers, we recorded 164 meteors with a beginning height above 130 km. We found that beginning heights between 130 and 150 km are quite usual, especially for the Leonid meteor shower. Conversely, meteors with beginning heights above 160 km are very rare even among Leonids. From the meteor light curves, we are able to distinguish two different processes that govern radiation of the meteors at different altitudes. Light curves vary greatly above 130 km and exhibit sudden changes in meteor brightness. Sputtering from the meteoroid surface is the dominating process during this phase of the meteor luminous trajectory. Around 130 km, the process switches to ablation and the light curves become similar to the light curves of standard meteors. The sputtering model was successfully applied to explain the difference in the beginning heights of high-altitude Leonid and Perseid meteors. We show also that this process in connection with high altitude fragmentation could explain the anomalously high beginning heights of several relatively faint meteors.
    • The Benguerir meteorite: Report and description of a new Moroccan fall

      Chennaoui Aoudjehane, H.; Jambon, A.; Bourot Denise, M.; Rochette, P. (The Meteoritical Society, 2006-01-01)
      A meteorite fall was witnessed on November 22, 2004, at 11:45 A.M. (GMT) near the city of Benguerir, Morocco. This is one of the first falls from Morocco to be scientifically described. The total mass of the fall is estimated to be at least 40 and 80 kg. Three impact locations have been identified, two of them in soft ploughed ground and a third on a granite surface, which was apparently broken by the impact. The weight of complete pieces range between approximately 100 g and 10 kg. We have classified the stone as an LL6 ordinary chondrite, based on mineralogy and petrology, with shock grade S3 and alteration W0. The dark fusion crust is perfectly fresh, and a number of samples were collected shortly after the fall by local residents and authorities before rainfall, which occurred a few days later. We show that the magnetic susceptibility of Benguerir is homogeneously high, indicating a constant metal content despite brecciation, in the high range for LL6. The LL6 classification can also be confirmed magnetically, with a specific signature similar to other measured LL6 falls.
    • The Cali meteorite fall: A new H/L ordinary chondrite

      Trigo-Rodríguez, J. M.; Llorca, J.; Rubin, A. E.; Grossman, J. N.; Sears, D. W. G.; Naranjo, M.; Bretzius, S.; Tapia, M.; Guarín Sepúlveda, M. H. (The Meteoritical Society, 2009-01-01)
      The fall of the Cali meteorite took place on 6 July 2007 at 16 h 32 +/- 1 min local time (21 h 32 +/- 1 min UTC). A daylight fireball was witnessed by hundreds of people in the Cauca Valley in Colombia from which 10 meteorite samples with a total mass of 478 g were recovered near 3 degrees 24.3'N, 76 degrees 30.6'W. The fireball trajectory and radiant have been reconstructed with moderate accuracy. From the computed radiant and from considering various plausible velocities, we obtained a range of orbital solutions that suggest that the Cali progenitor meteoroid probably originated in the main asteroid belt. Based on petrography, mineral chemistry, magnetic susceptibility, thermoluminescence, and bulk chemistry, the Cali meteorite is classified as an H/L4 ordinary chondrite breccia.
    • The Canyon Diablo impact event: Projectile motion through the atmosphere

      Artemieva, N.; Pierazzo, E. (The Meteoritical Society, 2009-01-01)
      Meteor Crater is one of the first impact structures systematically studied on Earth. Its location in arid northern Arizona has been ideal for the preservation of the structure and the surviving meteoric material. The recovery of a large amount of meteoritic material in and around the crater has allowed a rough reconstruction of the impact event: an iron object 50 m in diameter impacted the Earths surface after breaking up in the atmosphere. The details of the disruption, however, are still debated. The final crater morphology (deep, bowl-shaped crater) rules out the formation of the crater by an open or dispersed swarm of fragments, in which the ratio of swarm radius to initial projectile radius Cd is larger than 3 (the final crater results from the sum of the craters formed by individual fragments). On the other hand, the lack of significant impact melt in the crater has been used to suggest that the impactor was slowed down to 12 km/s by the atmosphere, implying significant fragmentation and fragments separation up to 4 initial radii. This paper focuses on the problem of entry and motion through the atmosphere for a possible Canyon Diablo impactor as a first but necessary step for constraining the initial conditions of the impact event which created Meteor Crater. After evaluating typical models used to investigate meteoroid disruption, such as the pancake and separated fragment models, we have carried out a series of hydrodynamic simulations using the 3D code SOVA to model the impactor flight through the atmosphere, both as a continuum object and a disrupted swarm. Our results indicate that the most probable pre-atmospheric mass of the Meteor Crater projectile was in the range of 4x10^8 to 1.2x10^9 kg (equivalent to a sphere 4666 m in diameter). During the entry process the projectile lost probably 30% to 70% of its mass, mainly because of mechanical ablation and gross fragmentation. Even in the case of a tight swarm of particles (Cd <3), small fragments can separate from the crater-forming swarm and land on the plains (tens of km away from the crater) as individual meteorites. Starting from an impactor pre-atmospheric velocity of ~18 km/s, which represents an average value for Earth-crossing asteroids, we find that after disruption, the most probable impact velocity at the Earths surface for a tight swarm is around 15 km/s or higher. A highly dispersed swarm would result in a much stronger deceleration of the fragments but would produce a final crater much shallower than observed at Meteor Crater.
    • The Carancas meteorite impact crater, Peru: Geologic surveying and modeling of crater formation and atmospheric passage

      Kenkmann, T.; Artemieva, N. A.; Wünnemann, K.; Poelchau, M. H.; Elbeshausen, D.; Núñez del Prado, H. (The Meteoritical Society, 2009-01-01)
      The recent Carancas meteorite impact event caused a worldwide sensation. An H4-5 chondrite struck the Earth south of Lake Titicaca in Peru on September 15, 2007, and formed a crater 14.2 m across. It is the smallest, youngest, and one of two eye-witnessed impact crater events on Earth. The impact violated the hitherto existing view that stony meteorites below a size of 100 m undergo major disruption and deceleration during their passage through the atmosphere and are not capable of producing craters. Fragmentation occurs if the strength of the meteoroid is less than the aerodynamic stresses that occur in flight. The small fragments that result from a breakup rain down at terminal velocity and are not capable of producing impact craters. The Carancas cratering event, however, demonstrates that meter-sized stony meteoroids indeed can survive the atmospheric passage under specific circumstances. We present results of a detailed geologic survey of the crater and its ejecta. To constrain the possible range of impact parameters we carried out numerical models of crater formation with the iSALE hydrocode in two and three dimensions. Depending on the strength properties of the target, the impact energies range between approximately 100-1000 MJ (0.024-0.24 t TNT). By modeling the atmospheric traverse we demonstrate that low cosmic velocities (12- 1-4 kms^(-1)) and shallow entry angles (<20 degrees) are prerequisites to keep aerodynamic stresses low (<10 MPa) and thus to prevent fragmentation of stony meteoroids with standard strength properties. This scenario results in a strong meteoroid deceleration, a deflection of the trajectory to a steeper impact angle (40-60 degrees), and an impact velocity of 350-600 ms^(-1), which is insufficient to produce a shock wave and significant shock effects in target minerals. Aerodynamic and crater modeling are consistent with field data and our microscopic inspection. However, these data are in conflict with trajectories inferred from the analysis of infrasound signals.