• 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.
    • Major and trace element compositions of melt particles and associated phases from the Yaxcopoil-1 drill core, Chicxulub impact structure, Mexico

      Tuchscherer, Martin G.; Reimold, W. Uwe; Gibson, Roger L.; de Bruin, Deon; Späth, Andreas (The Meteoritical Society, 2006-01-01)
      Melt particles found at various depths in impactites from the Yaxcopoil-1 borehole into the Chicxulub impact structure (Yucatán) have been analyzed for their major and trace element abundances. A total of 176 electron microprobe and 45 LA-ICP-MS analyses from eight different melt particles were investigated. The main purpose of this work was to constrain the compositions of precursor materials and secondary alteration characteristics of these melt particles. Individual melt particles are highly heterogeneous, which makes compositional categorization extremely difficult. Melt particles from the uppermost part of the impactite sequence are Ca- and Na-depleted and show negative Ce anomalies, which is likely a result of seawater interaction. Various compositional groupings of melt particles are determined with ternary and binary element ratio plots involving major and trace elements. This helps distinguish the degree of alteration versus primary heterogeneity of melt phases. Comparison of the trace element ratios Sc/Zr, Y/Zr, Ba/Zr, Ba/Rb, and Sr/Rb with compositions of known target rocks provides some constraints on protolith compositions; however, the melt compositions analyzed exceed the known compositional diversity of possible target rocks. Normalized REE patterns are unique for each melt particle, likely reflecting precursor mineral or rock compositions. The various discrimination techniques indicate that the highly variable compositions are the products of melting of individual minerals or of mixtures of several minerals. Small, angular shards that are particularly abundant in units 2 and 3 represent rapidly quenched melts, whereas larger particles (>0.5 mm) that contain microlites and have fluidal, schlieric textures cooled over a protracted period. Angular, shard-like particles with microlites in unit 5 likely crystallized below the glass transition temperature or underwent fragmentation during or after deposition.
    • Martian perched craters and large ejecta volume: Evidence for episodes of deflation in the northern lowlands

      Meresse, Sandrine; Costard, Franois; Mangold, Nicolas; Baratoux, David; Boyce, Joseph M. (The Meteoritical Society, 2006-01-01)
      The northern lowland plains, such as those found in Acidalia and Utopia Planitia, have high percentages of impact craters with fluidized ejecta. In both regions, the analysis of crater geometry from Mars Orbiter Laser Altimeter (MOLA) data has revealed large ejecta volumes, some exceeding the volume of excavation. Moreover, some of the crater cavities and fluidized ejecta blankets of these craters are topographically perched above the surrounding plains. These perched craters are concentrated between 40 and 70 degrees N in the northern plains. The atypical high volumes of the ejecta and the perched craters suggest that the northern lowlands have experienced one or more episodes of resurfacing that involved deposition and erosion. The removal of material, most likely caused by the sublimation of ice in the materials and their subsequent erosion and transport by the wind, is more rapid on the plains than on the ejecta blankets. The thermal inertia difference between the ejecta and the surrounding plains suggests that ejecta, characterized by a lower thermal inertia, protect the underneath terrain from sublimation. This results in a decreased elevation of the plains relative to the ejecta blankets. Sublimation and eolian erosion can be particularly high during periods of high obliquity.
    • Martian subsurface properties and crater formation processes inferred from fresh impact crater geometries

      Stewart, Sarah T.; Valiant, Gregory J. (The Meteoritical Society, 2006-01-01)
      The geometry of simple impact craters reflects the properties of the target materials, and the diverse range of fluidized morphologies observed in Martian ejecta blankets are controlled by the near-surface composition and the climate at the time of impact.Using the Mars Orbiter Laser Altimeter (MOLA) data set, quantitative information about the strength of the upper crust and the dynamics of Martian ejecta blankets may be derived from crater geometry measurements. Here, we present the results fromgeometrical measurements of fresh craters 3-50 km in rim diameter in selected highland (Lunae and Solis Plana) and lowland (Acidalia, Isidis, and Utopia Planitiae) terrains. We find large, resolved differences between the geometrical properties of the freshest highland and lowland craters. Simple lowland craters are 1.5-2.0 times deeper (greater than or equal to 5-sigma difference) with >50% larger cavities (greater than or equal to 2-sigma) compared to highland craters of the same diameter. Rim heights and the volume of material above the preimpact surface are slightly greater in the lowlands over most of the size range studied. The different shapes of simple highland and lowland craters indicate that the upper ~6.5 km of the lowland study regions are significantly stronger than the upper crust of the highland plateaus. Lowland craters collapse to final volumes of 45-70% of their transient cavity volumes, while highland craters preserve only 25-50%. The effective yield strength of the upper crust in the lowland regions falls in the range of competent rock, approximately 9-12 MPa, and the highland plateaus may be weaker by a factor of 2 or more, consistent with heavily fractured Noachian layered deposits. The measured volumes of continuous ejecta blankets and uplifted surface materials exceed the predictions from standard crater scaling relationships and Maxwell's Z model of crater excavation by a factor of 3. The excess volume of fluidized ejecta blankets on Mars cannot be explained by concentration of ejecta through nonballistic emplacement processes and/or bulking. The observations require a modification of the scaling laws and are well fit using a scaling factor of ~1.4 between the transient crater surface diameter to the final crater rim diameter and excavation flow originating from one projectile diameter depth with Z = 2.7. The refined excavation model provides the first observationally constrained set of initial parameters for study of the formation of fluidized ejecta blankets on Mars.
    • Maximal size of chondrules in shock wave heating model: Stripping of liquid surface in a hypersonic rarefied gas flow

      Kato, Takaaki; Nakamoto, Taishi; Miura, Hitoshi (The Meteoritical Society, 2006-01-01)
      We examined partially molten dust particles that have a solid core and a surrounding liquid mantle, and estimated the maximal size of chondrules in a framework of the shock wave heating model for chondrule formation. First, we examined the dynamics of the liquid mantle by analytically solving the hydrodynamics equations for a core-mantle structure via a linear approximation. We obtained the deformation, internal flow, pressure distribution in the liquid mantle, and the force acting on the solid core. Using these results, we estimated conditions in which liquid mantle is stripped off from the solid core. We found that when the particle radius is larger than about 1-2 mm, the stripping is expected to take place before the entire dust particle melts. So chondrules larger than about 1-2 mm are not likely to be formed by the shock wave heating mechanism. Also, we found that the stripping of the liquid mantle is more likely to occur than the fission of totally molten particles. Therefore, the maximal size of chondrules may be determined by the stripping of the liquid mantle from the partially molten dust particles in the shock waves. This maximal size is consistent with the sizes of natural chondrules.
    • Microbial diversity on the Tatahouine meteorite

      Benzerara, K.; Chapon, V.; Moreira, D.; López-García, P. (The Meteoritical Society, 2006-01-01)
      Biological processes can alter the chemistry and mineralogy of meteorites in a very short time, even in cold or hot deserts. It is thus important to assess the diversity of microorganisms that colonize meteorites in order to better understand their physiological capabilities. Microscopy observations of Tatahouine meteorite fragments that were exposed for 70 years in the Sahara desert showed that they were colonized by morphologically diverse biomorphs. A molecular diversity study based on 16S rRNA gene amplification of DNA supported the conclusion that a huge taxonomic diversity of prokaryotes colonized the Tatahouine meteorite in less than 70 years in the Tatahouine sand. Eleven different bacterial divisions were evidenced, among which Cytophaga-Flexibacter- Bacteroides (CFB), Cyanobacteria, and Alpha-Proteobacteria were dominantly represented. Crenarcheota were also detected. Most of the Tatahouine meteorite phylotypes were related to sequences identified in the surrounding Tatahouine more generally to sequences detected in soils. Some of them, in particular many of the archaeal phylotypes, were detected in arid regions in association with desert varnish. The results suggest that the diversity of the clone library generated from the meteorite fraction was reduced compared with that of the Tatahouine sand clone library, which can be explained as the result of partial colonization of the meteorite and/or a specific selection of colonizing bacteria by the substrate. We discuss the possibility that several groups detected in this study may play a prominent role in the various alteration processes detected at the surface of the Tatahouine meteorite.
    • Microstructural study of micron-sized craters simulating Stardust impacts in aluminum 1100 targets

      Leroux, Hugues; Borg, Janet; Troadec, David; Djouadi, Zahia; Hörz, Friedrich (The Meteoritical Society, 2006-01-01)
      Various microscopic techniques were used to characterize experimental microcraters in aluminum foils to prepare for the comprehensive analysis of the cometary and interstellar particle impacts in aluminum foils to be returned by the Stardust mission. First, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were used to study the morphology of the impact craters and the bulk composition of the residues left by soda-lime glass impactors. A more detailed structural and compositional study of impactor remnants was then performed using transmission electron microscopy (TEM), EDS, and electron diffraction methods. The TEM samples were prepared by focused ion beam (FIB) methods. This technique proved to be especially valuable in studying impact crater residues and impact crater morphology. Finally, we also showed that infrared microscopy (IR) can be a quick and reliable tool for such investigations. The combination of all of these tools enables a complete microscopic characterization of the craters.
    • Microtextures and crystal chemistry of pigeonite in the ureilites ALHA77257, RKPA80239, Y-791538, and ALHA81101

      Tribaudino, Mario (The Meteoritical Society, 2006-01-01)
      The microtextures of pigeonite in four ureilites, Allan Hills (ALH) 77257, Reckling Peak (RKP) A80239, Yamato (Y-) 791538, and Allan Hills A81101, chosen to span a range of composition and shock level, were investigated by transmission electron microscopy (TEM); two of the samples were also investigated by single crystal X-ray diffraction to determine Fe2+-Mg cation site partitioning.The low-shock and compositionally homogeneous pigeonites in ALHA77257 and RKPA80329 (Wo 6.4 for both, mg 86.3 and 84.3 respectively) display irregularly spaced, shock-induced stacking faults oriented parallel to (100), and largeantiphase domains (50-100 nm). Antiphase domains have no preferential orientation. No evidence of exsolution was observed.The low-shock Y-791538 pigeonite is homogeneous and has higher Ca and mg (Wo 9.4, mg 91.2). TEM investigation showed spinodal decomposition, indicative of incipient exsolution; small antiphase domains were observed (approximately equal to 5 nm). Single crystal refinement yielded R4-sigma = 5.71%, with Fe2+-Mg partitioning coefficient kD = 0.077(8) and Tc = 658(35) degrees C. ALHA81101 has compositionally heterogeneous pyroxenes, with large local variations in Wo and mg (Wo = 4-13, mg = 86-68). No compositional gradients from core to rim of grains were observed, and the heterogeneity is interpreted as related to cation migration during shock. In one relatively Ca-rich region (Wo is approximately equal to 12), TEM analysis showed augite-pigeonite exsolution lamellae, with spacing 145(20) nm.Results for ALHA77257, RKPA80239, and Y-791538 support a model of rapid cooling following breakup of the ureilite parent body. The presence of exsolution lamellae in ALHA81101 can be related to a local shock-induced Ca enrichment and provides no constraint on the late cooling history.
    • Mobile element analysis by secondary ion mass spectrometry (SIMS) of impactite matrix samples from the Yaxcopoil-1 drill core in the Chicxulub impact structure

      Newsom, H. E.; Nelson, M. J.; Shearer, C. K.; Dressler, B. O. (The Meteoritical Society, 2006-01-01)
      The concentrations of the fluid mobile trace elements lithium, beryllium, boron, and barium were measured in samples of the altered matrix of several impactite breccias of the Yaxcopoil-1 drill core using secondary ion mass spectrometry (SIMS) to determine the extent of transport due to aqueous or hydrothermal processes. Three of the elements, Li, Be, and B, have higher concentrations in the upper suevite impact breccias than in the lower impact melt deposits by factors of 3.5, 2.2, and 1.5, respectively. Lithium and B are the most enriched elements up section, and appear to have had the greatest mobility. The similar fractionation of Li and B is consistent with fluid transport and alteration under low-temperature conditions of less than 150 degrees C based on published experimental studies. In contrast to Li, Be, and B, the concentration of Ba in the altered matrix materials decreases upward in the section, and the concentration of Ba in the matrix is an order of magnitude less than the bulk concentrations, likely due to the presence of barite. The origin of the elemental variations with depth may be related to different protolith compositions in the upper versus the lower impactite units. A different protolith in the altered matrix is suggested by the Mg-rich composition of the lower units versus the Al-rich composition of the upper units, which largely correlates with the mobile element variations. The possibility that vertical transport of mobile elements is due to a postimpact hydrothermal system is supported by published data showing that the sediments immediately overlying the impactites are enriched in mobile elements derived from a hydrothermal system. However, the mobile elements in the sediments do not have to originate from the underlying impactites. In conclusion, our data suggests that the impactites at this location did not experience extensive high-temperature hydrothermal processing, and that only limited transport of some elements, including Li, Be, and B, occurred.
    • Modification of impact craters in the northern plains of Mars: Implications for Amazonian climate history

      Kreslavsky, M. A.; Head, J. W. (The Meteoritical Society, 2006-01-01)
      We measured the depth, wall steepness, and ejecta roughness and surveyed the wall and floor morphology of all craters 10-25 km in diameter within the typical Vastitas Borealis Formation in the northern lowlands of Mars north of 52 degrees N. Two of the 130 craters have unusually rough ejecta; they are deep, have steep walls, and are apparently the youngest in the population. Icy mantles filling the local subkilometer-scale topographic lows is the main contribution to ejecta smoothing, which occurs at a time scale on the order of tens of Myr. Wall degradation and crater shallowing generally occur at longer time scales, comparable to the duration of the Amazonian period. Many craters are shallow due to filling of the crater with specific ice-rich material of uncertain origin. We use our collected data to infer the nature of the past climate back through the Amazonian, a period prior to ~10-20 Myr ago, when orbital parameter solutions are chaotic and one must rely on geological data to infer climate conditions. We conclude that moderately high obliquity and wide obliquity variations were probable during the last 40-160 Myr. We tentatively conclude that high obliquity peaks (>40-45 degrees) may have occurred episodically through the last 210-430 Myr. A sharp step in the frequency distribution of wall steepness at 20 degrees may indicate a geologically long period prior to that time where obliquity never exceeded 40-45 degrees.
    • Molecular and isotopic indicators of alteration in CR chondrites

      Pearson, V. K.; Sephton, M. A.; Gilmour, I. (The Meteoritical Society, 2006-01-01)
      The CR group of carbonaceous chondrites may represent some of the most primitive extraterrestrial materials available for analysis. However, in contrast to other chondrite groups, the CR organic fraction is poorly characterized. The carbonaceous chondrite literature shows that relatively anhydrous thermal processing results in a condensed, poorly alkylated, O-poor macromolecular material, while for aqueous processing the converse is true. Such characteristics can be used to discern the alteration histories of the carbonaceous chondrites. We have performed bulk elemental and isotopic analysis and flash pyrolysis on four CR chondrites (Renazzo, Al Rais, Elephant Moraine [EET] 87770, and Yamato [Y-] 790112) to determine the nature of their organic component. Renazzo, Al Rais, and Y-790112 release qualitatively similar pyrolysis products, although there are some variations. Al Rais' macromolecular structure contains substantially higher relative abundances of alkylated and oxidized species and relatively lighter delta-15N, suggesting that it has endured more extensive aqueous processing than the other CR chondrites. Renazzo appears relatively unprocessed, with a low degree of alkylation, a lack of detectable nitrogen-bearing components, and low methylnaphthalene ratio. EET 87770's low abundance of alkylated species suggests its macromolecular structure may be relatively condensed, with condensation potentially assisted by a period of mild thermal alteration.
    • Monte Carlo simulation of GCR neutron capture production of cosmogenic nuclides in stony meteorites and lunar surface

      Kollr, D.; Michel, R.; Masarik, J. (The Meteoritical Society, 2006-01-01)
      A purely physical model based on a Monte Carlo simulation of galactic cosmic ray (GCR) particle interaction with meteoroids is used to investigate neutron interactions down to thermal energies. Experimental and/or evaluated excitation functions are used to calculate neutron capture production rates as a function of the size of the meteoroid and the depth below its surface. Presented are the depth profiles of cosmogenic radionuclides 36Cl, 41Ca, 60Co, 59Ni, and 129I for meteoroid radii from 10 cm up to 500 cm and a 2-pi irradiation. Effects of bulk chemical composition on n-capture processes are studied and discussed for various chondritic and lunar compositions. The mean GCR particle flux over the last 300 ka was determined from the comparison of simulations with measured 41Ca activities in the Apollo 15 drill core. The determined value significantly differs from that obtained using equivalent models of spallation residue production.
    • Morphology and geometry of the distal ramparts of Martian impact craters

      Mouginis-Mark, Peter J.; Baloga, Stephen M. (The Meteoritical Society, 2006-01-01)
      We used Mars Orbiter Laser Altimeter (MOLA), Thermal Emission Imaging System visible light (THEMIS VIS), and Mars Orbiter Camera (MOC) data to identify and characterize the morphology and geometry of the distal ramparts surrounding Martian craters. Such information is valuable for investigating the ejecta emplacement process, as well as searching for spatial variations in ejecta characteristics that may be due to target material properties and/or latitude, altitude, or temporal variations in the climate. We find no systematic trend in rampart height that would indicate regional variations in target properties for 54 ramparts at 37 different craters 5.7-35.9 km in diameter between 52.3 degrees S to 47.6 degrees N. Rampart heights for multi-lobe and single-lobe ejecta are each normally distributed with a common standard deviation, but statistically distinct mean values. Ramparts range in height from 20-180 m, are not symmetric, are typically steeper on their distal sides, and may be as much as ~4 km wide. The ejecta blanket proximal to parent crater from the rampart may be very thin (<5 m). A detailed analysis of two craters, Toconao crater (21 degrees S, 285 degrees E) (28 measurements), and an unnamed crater within Chryse Planitia (28.4 degrees N, 319.6 degrees E) (20 measurements), reveals that ejecta runout distance increases with an increase in height between the crater rim and the rampart, but that rampart height is not correlated with ejecta runout distance or the thickness of the ejecta blanket.
    • Nature of the Martian uplands: Effect on Martian meteorite age distribution and secondary cratering

      Hartmann, William K.; Barlow, Nadine G. (The Meteoritical Society, 2006-01-01)
      Martian meteorites (MMs) have been launched from an estimated 5-9 sites on Mars within the last 20 Myr. Some 80-89% of these launch sites sampled igneous rock formations from only the last 29% of Martian time. We hypothesize that this imbalance arises not merely from poor statistics, but because the launch processes are dominated by two main phenomena: first, much of the older Martian surface is inefficient in launching rocks during impacts, and second, the volumetrically enormous reservoir of original cumulate crust enhances launch probability for 4.5 Gyr old rocks. There are four lines of evidence for the first point, not all of equal strength. First, impact theory implies that MM launch is favored by surface exposures of near-surface coherent rock (10^2 m deep), whereas Noachian surfaces generally should have 10^2 m of loose or weakly cemented regolith with high ice content, reducing efficiency of rock launch. Second, similarly, both Mars Exploration Rovers found sedimentary strata, 1-2 orders of magnitude weaker than Martian igneous rocks, favoring low launch efficiency among some fluvial-derived Hesperian and Noachian rocks. Even if launched, such rocks may be unrecognized as meteorites on Earth. Third, statistics of MM formation age versus cosmic-ray exposure (CRE) age weakly suggest that older surfaces may need larger, deeper craters to launch rocks. Fourth, in direct confirmation, one of us (N. G. B.) has found that older surfaces need larger craters to produce secondary impact crater fields (cf. Barlow and Block 2004). In a survey of 200 craters, the smallest Noachian, Hesperian, and Amazonian craters with prominent fields of secondaries have diameters of ~45 km, ~19 km, and ~10 km, respectively. Because 40% of Mars is Noachian, and 74% is either Noachian or Hesperian, the subsurface geologic characteristics of the older areas probably affect statistics of recognized MMs and production rates of secondary crater populations, and the MM and secondary crater statistics may give us clues to those properties.
    • New observations on shatter cones in the Vredefort impact structure, South Africa, and evaluation of current hypotheses for shatter cone formation

      Wieland, F.; Reimold, W. U.; Gibson, R. L. (The Meteoritical Society, 2006-01-01)
      Shatter cones have been described from many meteorite impact structures and are widelyregarded as a diagnostic macroscopic recognition feature for impact. However, the origin of thismeso- to macroscopic striated fracture phenomenon has not yet been satisfactorily resolved, and thetiming of shatter cone formation in the cratering process still remains enigmatic. Here, previousresults from studies of shatter cones from the Vredefort impact structure and other impact structuresare discussed in the light of new field observations made in the Vredefort Dome. Contrary to earlier claims, Vredefort cone fractures do not show uniform apex orientations at any given outcrop, nor do small cones show a pattern consistent with the previously postulated master cone concept. Simple back-rotation of impact-rotated strata to a horizontal pre-impact position also does not lead to a uniform centripetal-upward orientation of the cone apices. Striation patterns on the cone surfaces are variable, ranging from the typically diverging pattern branching off the cone apex to subparallel-to parallel patterns on almost flat surfaces. Striation angles on shatter cones do not increase with distance from the center of the dome, as alleged in the literature. Instead, a range of striation angles is measured on individual shatter cones from a specific outcrop. New observations on small-scale structures in the collar around the Vredefort Dome confirm the relationship of shatter cones with subparallel sets of curviplanar fractures (so-called multipli-striated joint sets, MSJS). Pervasive, meter-scale tensile fractures cross-cut shatter cones and appear to have formed after the closely spaced MSJ-type fractures. The results of this study indicate that none of the existing hypotheses for the formation of shatter cones are currently able to adequately explain all characteristics of this fracturing phenomenon. Therefore, we favor a combination of aspects of different hypotheses that includes the interaction of elastic waves, as supported by numerical modeling results and which reasonably explains the variety of shatter cone shapes, the range of striation geometries and angles, and the relationship of closely spaced fracture systems with the striated surfaces. In the light of the currently available theoretical basis for the formation of shatter cones, the results of this investigation lead to the conclusion that shatter cones are tensile fractures and might have formed during shock unloading, after the passage of the shock wave through the target rocks.
    • Noble gas space exposure ages of individual interplanetary dust particles

      Kehm, K.; Flynn, G. J.; Sutton, S. R.; Hohenberg, C. M. (The Meteoritical Society, 2006-01-01)
      The He, Ne, and Ar compositions of 32 individual interplanetary dust particles (IDPs) were measured using low-blank laser probe gas extraction. These measurements reveal definitive evidence of space exposure. The Ne and Ar isotopic compositions in the IDPs are primarily a mixture between solar wind (SW) and an isotopically heavier component dubbed fractionated solar (FS), which could be implantation-fractionated solar wind or a distinct component of the solar corpuscular radiation previously identified as solar energetic particles (SEP). Space exposure ages based on the Ar content of individual IDPs are estimated for a subset of the grains that appear to have escaped significant volatile losses during atmosphere entry. Although model-dependent, most of the particles in this subset have ages that are roughly consistent with origin in the asteroid belt. A short (<1000 years) space exposure age is inferred for one particle, which is suggestive of cometary origin. Among the subset of grains that show some evidence for relatively high atmospheric entry heating, two possess elevated 21Ne/22Ne ratios generated by extended exposure to solar and galactic cosmic rays. The inferred cosmic ray exposure ages of these particles exceeds 107 years, which tends to rule out origin in the asteroid belt. A favorable possibility is that these 21Ne‐rich IDPs previously resided on a relatively stable regolith of an Edgeworth‐Kuiper belt or Oort cloud body and were introduced into the inner solar system by cometary activity. These results demonstrate the utility of noble gas measurements in constraining models for the origins of interplanetary dust particles.
    • Noble gases in the Martian meteorite Northwest Africa 2737: A new chassignite signature

      Marty, Bernard; Heber, Veronika S.; Grimberg, Ansgar; Wieler, Rainer; Barrat, Jean-Alix (The Meteoritical Society, 2006-01-01)
      We report noble gas data for the second chassignite, Northwest Africa (NWA) 2737, which was recently found in the Moroccan desert. The cosmic ray exposure (CRE) age based on cosmogenic 3He, 21Ne, and 38Ar around 10-11 Ma is comparable to the CRE ages of Chassigny and the nakhlites and indicates ejection of meteorites belonging to these two families during a discrete event, or a suite of discrete events having occurred in a restricted interval of time. In contrast, U-Th/He and K/Ar ages <0.5 Ga are in the range of radiometric ages of shergottites, despite a Sm-Nd signature comparable tothat of Chassigny and the nakhlites (Misawa et al. 2005). Overall, the noble gas signature of NWA2737 resembles that of shergottites rather than that of Chassigny and the nakhlites: NWA 2737 doesnot contain, in detectable amount, the solar-like xenon found in Chassigny and thought to characterizethe Martian mantle nor apparently fission xenon from 244Pu, which is abundant in Chassigny andsome of the nakhlites. In contrast, NWA 2737 contains Martian atmospheric noble gases trapped inamounts comparable to those found in shergottite impact glasses. The loss of Martian mantle noblegases, together with the trapping of Martian atmospheric gases, could have occurred duringassimilation of Martian surface components, or more likely during shock metamorphism, which isrecorded in the petrology of this meteorite.