• Halite and stable chlorine isotopes in the Zag H3-6 breccia

      Bridges, J. C.; Banks, D. A.; Smith, M.; Grady, M. M. (The Meteoritical Society, 2004-01-01)
      Zag is an H36 chondrite regolith breccia within which we have studied 14 halite grains less than or equal to 3 micrometers. The purity of the associated NaCl-H2O brine is implied by freezing characteristics of fluid inclusions in the halite and EPMA analyses together with a lack of other evaporite-like phases in the Zag H36 component. This is inconsistent with multi-stage evolution of the fluid involving scavenging of cations in the Zag region of the parent body. We suggest that the halite grains are clastic and did not crystallize in situ. Halite and water-soluble extracts from Zag have light Cl isotopic compositions, delta-37Cl = 1.4 to 2.8 ppm. Previously reported bulk carbonaceous chondrite values are approximately delta-37Cl = +3 to +4 ppm. This difference is too great to be the result of fractionation during evaporation, and instead, we suggest that Cl isotopes in chondrites are fractionated between a light reservoir associated with fluids and a heavier reservoir associated with higher temperature phases such as phosphates and silicates. Extraterrestrial carbon released at 600 degrees C from the H34 matrix has delta-13C = -20 ppm, consistent with poorly graphitized material being introduced into the matrix rather than indigenous carbonate derived from a brine. We have also examined 28 other H chondrite falls to ascertain how widespread halite or evaporite-like mineral assemblages are in ordinary chondrites.
    • Harvey Nininger's 1948 attempt to nationalize Meteor Crater

      Plotkin, H.; Clarke, R. S. (The Meteoritical Society, 2008-01-01)
      Harvey Nininger successfully petitioned the American Astronomical Society to pass a motion in support of nationalizing Meteor Crater, Arizona, at its June 1948 meeting. He alleged that the Barringer family, who held title to the crater, was depriving American citizens of its scenic beauty and scientific value. He then reportedly went on to make the unauthorized--and false--claim that the family would be receptive to a fair purchase offer for the crater. The Barringers, who had not been given advance warning of the petition and were not present at the meeting, felt ambushed. They quickly and forcefully rebutted Niningers allegations, made it clear they had no intention of relinquishing their title to the crater, and terminated his exploration rights. What led Nininger to such a curious and self-defeating act? Based on our reading of his voluminous personal correspondence, we conclude that it was rooted primarily in his complex relationship with Frederick Leonard and Lincoln LaPaz, and his desire to establish a national institute for meteoritical research--with them, originally, but after a serious falling out, on his own. Prevented from moving his American Meteorite Museum to the crater rim, Nininger wondered what would happen if the crater was nationalized and made into a public park, with an accompanying tourist center and museum. With characteristic lan, he could picture himself at its head, with a secure salary and adequate space to exhibit his meteorite collection.
    • Heating effects of the matrix of experimentally shocked Murchison CM chondrite: Comparison with micrometeorites

      Tomioka, Naotaka; Tomeoka, Kazushige; Nakamura-Messenger, Keiko; Sekine, Toshimori (The Meteoritical Society, 2007-01-01)
      Micrometeorites have been significantly altered or melted by heating, which has been mainly ascribed to aerodynamic drag during atmospheric entry. However, if a major fraction of micrometeorites are produced by impacts on porous asteroids, they may have experienced shock heating before contact with the Earth's atmosphere (Tomeoka et al. 2003). A transmission electron microscope (TEM) study of the matrix of Murchison CM chondrite experimentally shocked at pressures of 10-49 GPa shows that its mineralogy and texture change dramatically, mainly due to shock heating, with the progressive shock pressures. Tochilinite is completely decomposed to an amorphous material at 10 GPa. Fe-Mg serpentine is partially decomposed and decreases in amount with increasing pressure from 10 to 30 GPa and is completely decomposed at 36 GPa. At 49 GPa, the matrix is extensively melted and consists mostly of aggregates of equigranular grains of Fe-rich olivine and less abundant low-Ca pyroxene embedded in Si-rich glass. The mineralogy and texture of the shocked samples are similar to those of some types of micrometeorites. In particular, the samples shocked at 10 and 21 GPa are similar to the phyllosilicate (serpentine)-rich micrometeorites, and the sample shocked at 49 GPa is similar to the olivine-rich micrometeorites. The shock heating effects also resemble the effects of pulse-heating experiments on the CI and CM chondrite matrices that were conducted to simulate atmospheric entry heating. We suggest that micrometeorites derived from porous asteroids are likely to go through both shock and atmospheric-entry heating processes.
    • Heavily-hydrated lithic clasts in CH chondrites and the related, metal-rich chondrites Queen Alexandra Range 94411 and Hammadah al Hamra 237

      Greshake, A.; Krot, A. N.; Meibom, A.; Weisberg, M. K.; Zolensky, M. E.; Keil, K. (The Meteoritical Society, 2002-01-01)
      Fine-grained, heavily-hydrated lithic clasts in the metal-rich (CB) chondrites Queen Alexandra Range (QUE) 94411 and Hammadah al Hamra 237 and CH chondrites, such as Patuxent Range (PAT) 91546 and Allan Hills (ALH) 85085, are mineralogically similar suggesting genetic relationship between these meteorites. These clasts contain no anhydrous silicates and consist of framboidal and platelet magnetite, prismatic sulfides (pentlandite and pyrrhotite), and Fe-Mn-Mg-bearing Ca-carbonates set in a phyllosilicate-rich matrix. Two types of phyllosilicates were identified: serpentine, with basal spacing of ~0.73 nm, and saponite, with basal spacings of about 1.1-1.2 nm. Chondrules and FeNi-metal grains in CB and CH chondrites are believed to have formed at high temperature (>1300 K) by condensation in a solar nebula region that experienced complete vaporization. The absence of aqueous alteration of chondrules and metal grains in CB and CH chondrites indicates that the clasts experienced hydration in an asteroidal setting prior to incorporation into the CH and CB parent bodies. The hydrated clasts were either incorporated during regolith gardening or accreted together with chondrules and FeNi-metal grains after these high-temperature components had been transported from their hot formation region to a much colder region of the solar nebula.
    • Helium loss from Martian meteorites mainly induced by shock metamorphism: Evidence from new data and a literature compilation

      Schwenzer, S. P.; Fritz, J.; Stöffler, D.; Trieloff, M.; Amini, M.; Greshake, A.; Herrmann, S.; Herwig, K.; Jochum, K. P.; Mohapatra, R. K.; et al. (The Meteoritical Society, 2008-01-01)
      Noble gas data from Martian meteorites have provided key constraints about their origin and evolution, and their parent body. These meteorites have witnessed varying shock metamorphic overprinting (at least 5 to 14 GPa for the nakhlites and up to 45-55 GPa (e.g., the lherzolitic shergottite Allan Hills [ALH] A77005), solar heating, cosmic-ray exposure, and weathering both on Mars and Earth. Influences on the helium budgets of Martian meteorites were evaluated by using a new data set and literature data. Concentrations of 3He, 4He, U, and Th are measured and shock pressures for same sample aliquots of 13 Martian meteorites were determined to asses a possible relationship between shock pressure and helium concentration. Partitioning of 4He into cosmogenic and radiogenic components was performed using the lowest 4He/3He ratio we measured on mineral separates (4He/3He = 4.1, pyroxene of ALHA77005). Our study revealed significant losses of radiogenic 4He. Systematics of cosmogenic 3He and neon led to the conclusion that solar radiation heating during transfer from Mars to Earth and terrestrial weathering can be ruled out as major causes of the observed losses of radiogenic helium in bulk meteorites. For bulk rock we observed a correlation of shock pressure and radiogenic 4He loss, ranging between ~20% for Chassigny and other moderately shocked Martian meteorites up to total loss for meteorites shocked above 40 GPa. A steep increase of loss occurs around 30 GPa, the pressure at which plagioclase transforms to maskelynite. This correlation suggests significant 4He loss induced by shock metamorphism. Noble gas loss in rocks is seen as diffusion due to (1) the temperature increase during shock loading (shock temperature) and (2) the remaining waste heat after adiabatic unloading (post shock temperature). Modeling of 4He diffusion in the main U,Th carrier phase apatite showed that post-shock temperatures of ~300 degrees C are necessary to explain observed losses. This temperature corresponds to the post-shock temperature calculated for bulk rocks shocked at about 40 GPa. From our investigation, data survey, and modeling, we conclude that the shock event during launch of the meteorites is the principal cause for 4He loss.
    • Helium, neon, and argon in meteorites: A data collection

      Schultz, L.; Franke, L. (The Meteoritical Society, 2004-01-01)
      Noble gases have been measured in meteorites for more than 100 years. The last 50 years have been especially fruitful, with concentration and isotopic compositional analysis of He, Ne, Ar, Kr, and Xe making important contributions to meteorite research. Differently trapped noble gas components are the basis for understanding planetary atmospheres and even different stages of stellar evolution. Noble gases are a valuable tool to detect pairing of meteorite specimens or even to prove whether a rock is a meteorite or not. Noble gas data, however, are distributed over a large number of publications. Sometimes, only concentrations are given for selected isotopes or just a simple derivative quantity is published. We have tried to collect all available measurements of He, Ne, and Ar in meteorites. Here, we present the data in a form that will help easily calculate isotopic or elemental ratios for selected measurements. The present compilation contains all data available as of March 2004.
    • Heterogeneous agglutinitic glass and the fusion of the finest fraction (F3) model

      Basu, A.; Wentworth, S. J.; McKay, D. S. (The Meteoritical Society, 2002-01-01)
      Evidence in favor of the model fusion of the finest fraction (F3) for the origin of lunar agglutinitic glass has been accruing. They include (1) theoretical expectations that shock pulses should engulf and melt smaller grains more efficiently than larger grains, (2) experimental results of impact shock, albeit at lower than presumed hypervelocity impacts of micrometeorites on the lunar regolith, and (3) new analyses confirming previous results that average compositions of agglutinitic glass are biased towards that of the finest fraction of lunar soils from which they had formed. We add another reason in support of the F3 model. Finer grains of lunar soils are also much more abundant. Hence, electrostatic forces associated with the rotating terminator region bring the finest grains that are obviously much lighter than courser grains to the surface of the Moon. This further contributes to the preferential melting of the finest fraction upon micrometeoritic impacts. New backscattered electron imaging shows that agglutinitic glass is inhomogeneous at submicron scale. Composition ranges of agglutinitic glass are extreme and deviate from that of the finest fraction, even by more than an order of magnitude for some components. Additionally, we show how an ilmenite grain upon impact would produce TiO2-rich agglutinitic glass in complete disregard to the requirements of fusion of the finest fraction. We propose an addition to the F3 model to accommodate these observations (i.e., that micrometeorite impacts indiscriminately melt the immediate target regardless of grain size or grain composition). We, therefore, suggest that (1) agglutinitic glass is the sum of (a) the melt produced by the fusion of the finest fraction of lunar soils and (b) the microvolume of the indiscriminate target, which melts at high-shock pressures from micrometeoritic impacts, and that (2) because of the small volume of the melt and incorporating cold soil grains, the melt quenched so rapidly that it did not mix and homogenize to represent any preferential composition, for example, that of the finest fraction.
    • Heterogeneous condensation of presolar titanium carbide core-graphite mantle spherules

      Chigai, T.; Yamamoto, T.; Kozasa, T. (The Meteoritical Society, 2002-01-01)
      We investigate heterogeneous nucleation and growth of graphite on precondensed TiC grains in the gas outflows from carbon-rich asymptotic giant branch (AGB) stars employing a newly-derived heterogeneous nucleation rate taking into account of the chemical reactions at condensation. Competition between heterogeneous and homogeneous nucleations and growths of graphite is investigated to reveal the formation conditions of the TiC core-graphite mantle spherules found in the Murchison meteorite. It is shown that no homogeneous graphite grain condenses whenever TiC condenses prior to graphite in the plausible ranges of the stellar parameters. Heterogeneous condensation of graphite occurs on the surfaces of growing TiC grains, and prevents the TiC cores from reaching the sizes realized if all available Ti atoms were incorporated into TiC grains. The physical conditions at the formation sites of the TiC core-graphite mantle spherules observed in the Murchison meteorite are expressed by the relation 0.2 < v0.1 (M5 / zeta)^(-1/2) L4^(1/4) < 0.7, where v0.1 is the gas outflow velocity at the formation site in units of 0.1 km s^(-1), M^(-5), the mass loss rate in 10^(-5) M solar year^(-1), L4 the stellar luminosity in 10^4 L(solar), and M / zeta is the effective mass loss rate taking account of non-spherical symmetry of the gas outflows. The total gas pressures Pc at the formation sites for the effective mass loss rates M / zeta = 10^(-5) - 10^(-3) M solar year-1 correspond to 0.01 < Pc < 0.9 dyn cm-2, implying that the observed TiC core-graphite mantle spherules are formed not only at the superwind stage but also at the earlier stage of low mass loss rates. The constraint on the degrees C/O abundance ratio, 1 < eta which is less than or approximately equal to 1.03, is imposed to reproduce the observed sizes of the TiC cores. The derived upper limit of the degrees C/O ratio is lower than the values estimated from the calculations without taking into account of heterogeneous condensation of graphite, and is close to the lower end of the degrees C/O ratios inferred from the astronomical observations of carbon-rich AGB stars. Brief discussion is given on other types of graphite spherules.
    • High resolution temperature measurements in the borehole Yaxcopoil-1, Mexico

      Wilhelm, H.; Heidinger, P.; Šafanda, J.; Čermák, V.; Burkhardt, H.; Popov, Y. (The Meteoritical Society, 2004-01-01)
      Within the frame of the International Continental Deep Drilling Program (ICDP) and as a part of the Chicxulub Scientific Drilling Project (CSDP), high resolution temperature measurements were performed in the borehole Yaxcopoil-1 (Yax-1). The temperature was logged to the depth of 858 m seven times between March 6-19, 2002, starting 10 days after the hole was shut in and mud circulation ceased. Successive logs revealed only small temperature variations in time and space, indicating a fast temperature recovery to almost undisturbed conditions prior to the first log. From these logs, a mean temperature gradient of ~37 mK/m was determined below the uppermost 250 m. Another temperature log was recorded on May 24, 2003 (15 months after the shut in) to a depth of 895 m. The obtained temperature profile is very similar to the 2002 profile, with an insignificantly higher mean gradient below 250 m that may indicate a long-term return to the pre-drilling temperature. The temperature in the uppermost part of the hole bears signs of considerable influence of a convective contribution to the vertical thermal heat transfer. The depth extent of the convection seems to have deepened from 150 m in March 2002 to 230 m in May 2003. Based on the observed temperature gradient and the rock types encountered in the borehole above 670 m, the conducted heat flow is expected to be in the range 65-80 mW/m2.
    • High spatial resolution ion microprobe measurements refine chronology of carbonate formation in Orgueil

      Hoppe, P.; MacDougall, D.; Lugmair, G. W. (The Meteoritical Society, 2007-01-01)
      Aqueous activity on meteorite parent bodies is indicated by the presence of carbonates. High spatial resolution ion microprobe analyses of nine individual carbonate grains (four dolomites, five breunnerites) from the Orgueil meteorite reveal linear correlations between 53Cr excesses and Mn/Cr ratios in all grains, indicative of in situ decay of radioactive 53Mn (half-life 3.7 million years). The well-defined isochrons appear to have chronological significance. If this is the case, then 53Mn/ 55Mn ratios between 2.1 and 4.7 x 10^(-6) are inferred for the time of carbonate formation and absolute ages of between 4561 and 4565 Ma are calculated (systematic uncertainty of +/- 2 Ma). Dolomites tend to have formed slightly earlier than the breunnerites. Our data imply extensive aqueous activity on the Orgueil parent body over a period of several million years, starting ~3-4 Myr after formation of the solar system, that most likely was the result of impact heating and latent heat from the decay of radioactive 26Al and 60Fe.
    • High-albedo asteroid 434 Hungaria: Spectrum, composition and genetic connections

      Kelley, M. S.; Gaffey, M. J. (The Meteoritical Society, 2002-01-01)
      New data in the wavelength region of approximately 0.4-2.5 micrometers have been obtained for asteroid 434 Hungaria. This is the most complete visible to near-infrared spectrum to date for this object. The near-infrared portion of the spectrum (about 0.8-2.5 micrometers) is smooth, featureless, and agrees well in the overlap region with new visible region data. However, visible region (about 0.45-0.9 micrometers) data appear to exhibit weak, broad spectral absorption features near 0.5, 0.6-0.7, and 1 micrometers. If real, the presence of such features would strongly constrain the compositional determination of Hungaria since it has a relatively high albedo of 46%. Most minerals that exhibit similar absorption features, and are commonly found in meteorites, have a much lower albedo. Asteroid 434 Hungaria has been observed more than six times in these overlapping spectral regions, and it is now possible to assess its mineral composition with some confidence. The dominant phase on this asteroid is an iron-free mineral, probably enstatite. Hungaria may contain secondary phases causing subtle, visible-region absorption features. Alternatively, the surface layer(s) of the asteroid may be contaminated by an absorbing species from an external source.
    • High-calcium pyroxene as an indicator of igneous differentiation in asteroids and meteorites

      Sunshine, Jessica M.; Bus, Schelte J.; McCoy, Timothy J.; Burbine, Thomas H.; Corrigan, Catherine M.; Binzel, Richard P. (The Meteoritical Society, 2004-01-01)
      Our analyses of high quality spectra of several S-type asteroids (17 Thetis, 847 Agnia, 808 Merxia, and members of the Agnia and Merxia families) reveal that they include both low- and highcalcium pyroxene with minor amounts of olivine (<20%). In addition, we find that these asteroids have ratios of high-calcium pyroxene to total pyroxene of >~0.4. High-calcium pyroxene is a spectrally detectable and petrologically important indicator of igneous history and may prove critical in future studies aimed at understanding the history of asteroidal bodies. The silicate mineralogy inferred for Thetis and the Merxia and Agnia family members requires that these asteroids experienced igneous differentiation, producing broadly basaltic surface lithologies. Together with 4 Vesta (and its smaller "Vestoid" family members) and the main-belt asteroid 1489 Magnya, these new asteroids provide strong evidence for igneous differentiation of at least five asteroid parent bodies. Based on this analysis of a small subset of the near-infrared asteroid spectra taken to date with SpeX at the NASA IRTF, we expect that the number of known differentiated asteroids will increase, consistent with the large number of parent bodies inferred from studies of iron meteorites.
    • 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.
    • High-pressure phases in shock-induced melt veins of the Umbarger L6 chondrite: Constraints of shock pressure

      Xie, Z.; Sharp, T. G. (The Meteoritical Society, 2004-01-01)
      We report a previously undocumented set of high-pressure minerals in shock-induced melt veins of the Umbarger L6 chondrite. High-pressure minerals were identified with transmission electron microscopy (TEM) using selected area electron diffraction and energy-dispersive X-ray spectroscopy. Ringwoodite (Fa30), akimotoite (En11Fs89), and augite (En42Wo33Fs25) were found in the silicate matrix of the melt vein, representing the crystallization from a silicate melt during the shock pulse. Ringwoodite (Fa27) and hollandite-structured plagioclase were also found as polycrystalline aggregates in the melt vein, representing solid state transformation or melting with subsequent crystallization of entrained host rock fragments in the vein. In addition, Fe2SiO4-spinel (Fa66-Fa99) and stishovite crystallized from a FeO-SiO2-rich zone in the melt vein, which formed by shock melting of FeO-SiO2-rich material that had been altered and metasomatized before shock. Based on the pressure stabilities of the high-pressure minerals, ringwoodite, akimotoite, and Ca-clinopyroxene, the melt vein crystallized at approximately 18 GPa. The Fe2SiO4-spinel + stishovite assemblage in the FeO-SiO2- rich melts is consistent with crystallization of the melt vein matrix at the pressure up to 18 GPa. The crystallization pressure of ~18 GPa is much lower than the 4590 GPa pressure one would conclude from the S6 shock effects in melt veins (Stöffler et al. 1991) and somewhat less than the 25-30 GPa inferred from S5 shock effects (Schmitt 2000) found in the bulk rock.
    • Hornblende alteration and fluid inclusions in Kärdla impact crater, Estonia: Evidence for impact-induced hydrothermal activity

      Kirsimäe, Kalle; Suuroja, Sten; Kirs, Juho; Kärki, Aulis; Polikarpus, Maile; Puura, Väino; Suuroja, Kalle (The Meteoritical Society, 2002-01-01)
      The well-preserved Kärdla impact crater, on Hiiumaa Island, Estonia, is a 4-km diameter structure formed in a shallow Ordovician sea about 455 Ma ago into a target composed of thin (~150 m) unconsolidated sedimentary layer above a crystalline basement composed of migmatite granites, amphibolites and gneisses. The fractured and crushed amphibolites in the crater area are strongly altered and replaced with secondary chloritic minerals. The most intensive chloritization is found in permeable breccias and heavily shattered basement around and above the central uplift. Alteration is believed to have resulted from convective flow of hydrothermal fluids through the central areas of the crater. Chloritic mineral associations suggest formation temperatures of 100-300 degrees C, in agreement with the most frequent quartz fluid inclusion homogenization temperatures of 150-300 degrees C in allochthonous breccia. The rather low salinity of fluids in Kärdla crater (<13 wt% NaCleq) suggests that the hydrothermal system was recharged either by infiltration of meteoric waters from the cater rim walls raised above sea level after the impact, or by invasion of sea water through the disturbed sedimentary cover and fractured crystalline basement. The well developed hydrothermal system in Kärdla crater shows that the thermal history of the shock heated and uplifted rocks in the central crater area, rather than cooling of impact-melt or suevite sheets, controlled the distribution and intensity of the impact-induced hydrothermal processes.
    • Hosts of hydrogen in Allan Hills 84001: Evidence for hydrous martian salts in the oldest martian meteorite?

      Eiler, J. M.; Kitchen, N.; Leshin, L.; Strausberg, M. (The Meteoritical Society, 2002-01-01)
      The martian meteorite, Allan Hills (ALH) 84001, contains D‐rich hydrogen of plausible martian origin (Leshin et al., 1996). The phase identity of the host(s) of this hydrogen are not well known and could include organic matter (McKay et al., 1996), phlogopite (Brearley, 2000), glass (Mittlefehldt, 1994) and/or other unidentified components of this rock. Previous ion microprobe studies indicate that much of the hydrogen in ALH 84001 as texturally associated with concretions of nominally anhydrous carbonates, glass and oxides (Boctor et al., 1998; Sugiura and Hoshino, 2000). We examined the physical and chemical properties of the host(s) of this hydrogen by stepped pyrolysis of variously pre‐treated subsamples. A continuous‐flow method of water reduction and mass spectrometry (Eiler and Kitchen, 2001) was used to permit detailed study of the small amounts of this hydrogen‐poor sample available for study. We find that the host(s) of D‐rich hydrogen released from ALH 84001 at relatively low temperatures (~500 degrees C) is soluble in orthophosphoric and dilute hydrochloric acids and undergoes near‐complete isotopic exchange with water within hours at temperatures of 200 to 300 degrees C. These characteristics are most consistent with the carrier phase(s) being a hydrous salt (e.g., carbonate, sulfate or halide); the thermal stability of this material is inconsistent with many examples of such minerals (e.g., gypsum) and instead suggests one or more relatively refractory hydrous carbonates (e.g., hydromagnesite). Hydrous salts (particularly hydrous carbonates) are common on the Earth only in evaporite, sabkha, and hydrocryogenic‐weathering environments; we suggest that much (if not all) of the “martian” hydrogen in ALH 84001 was introduced in analogous environments on or near the martian surface rather than through biological activity or hydrothermal alteration of silicates in the crust.
    • 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.
    • Hydrodynamic modelling of cometary particles captured in aerogel and the Earths atmosphere

      Coulson, S. G. (The Meteoritical Society, 2009-01-01)
      The capture of cometary fragments in aerogel by the Stardust mission is analogous to the process of meteoroid deceleration in the Earths atmosphere. We present a simplified model for the formation of the tracks formed in aerogel by hypervelocity impacts of cometary material. Using a hydrodynamic approach to model this class of problem overcomes some of the errors associated with previous semi-analytical models for track formation (Coulson 2009). The hydrodynamic models developed allow the particle velocity, temperature and pressure to be calculated as a function of track length within aerogel. A qualative description of how this model can be extended to the formation of bulbous cavities using the Chapman-Jouquet theory is provided.
    • Hydrogen concentrations on C-class asteroids derived from remote sensing

      Rivkin, A. S.; Davies, J. K.; Johnson, J. R.; Ellison, S. L.; Trilling, D. E.; Brown, R. H.; Lebofsky, L. A. (The Meteoritical Society, 2003-01-01)
      We present spectroscopic observations of 16 asteroids from 1.9-3.6 micrometers collected from the United Kingdom Infrared Telescope (UKIRT) from 1996-2000. Of these 16 asteroids, 11 show some evidence of a 3 micrometer hydrated mineral absorption feature greater than 2-sigma at 2.9 micrometers. Using relations first recognized for carbonaceous chondrite powders by Miyamoto and Zolensky (1994) and Sato et al. (1997), we have determined the hydrogen to silicon ratio for these asteroids and calculated their equivalent water contents, assuming all the hydrogen was in water. The asteroids split into 2 groups, roughly defined as equivalent water contents greater than ~7% (8 asteroids, all with 3 micrometer band depths greater than ~20%) and less than ~3% for the remaining 8 asteroids. This latter group includes some asteroids for which a weak but statistically significant 3 micrometers band of non-zero depth exists. The G-class asteroids in the survey have higher water contents, consistent with CM chondrites. This strengthens the connection between CM chondrites and G asteroids that was proposed by Burbine (1998). We find that the 0.7 micrometer and 3 micrometer band depths are correlated for the population of target objects.
    • Hydrothermal alteration at the Lonar Lake impact structure, India: Implications for impact cratering on Mars

      Hagerty, J. J.; Newsom, H. E. (The Meteoritical Society, 2003-01-01)
      The 50,000 year old, 1.8 km diameter Lonar crater is one of only two known terrestrial craters to be emplaced in basaltic target rock (the 65 million year old Deccan Traps). The composition of the Lonar basalts is similar to martian basaltic meteorites, which establishes Lonar as an excellent analogue for similarly sized craters on the surface of Mars. Samples from cores drilled into the Lonar crater floor show that there are basaltic impact breccias that have been altered by post-impact hydrothermal processes to produce an assemblage of secondary alteration minerals. Microprobe data and X-ray diffraction analyses show that the alteration mineral assemblage consists primarily of saponite, with minor celadonite, and carbonate. Thermodynamic modeling and terrestrial volcanic analogues were used to demonstrate that these clay minerals formed at temperatures between 130 degrees C and 200 degrees C. By comparing the Lonar alteration assemblage with alteration at other terrestrial craters, we conclude that the Lonar crater represents a lower size limit for impact-induced hydrothermal activity. Based on these results, we suggest that similarly sized craters on Mars have the potential to form hydrothermal systems, as long as liquid water was present on or near the martian surface. Furthermore, the Fe-rich alteration minerals produced by post-impact hydrothermal processes could contribute to the minor iron enrichment associated with the formation of the martian soil.