S.R. Gainey^a, E.M. Hausrath^a, J.A. Hurowitz^b, R.E. Milliken^c

^aDepartment of Geoscience, University of Nevada, Las Vegas 4505 S. Maryland Parkway Las Vegas, NV 89154-4010
^bDepartment of Geosciences, Stony Brook University, 255 Earth and Space Sciences Building (ESS), Stony Brook University, Stony Brook, NY 11794-2100
^cGeological Sciences, Brown University, 324 Brook Street Box 1846 Providence, RI 02912

The Fe-rich smectite nontronite M⁺_1.05[Si_6.98Al_1.02][Al_0.29Fe_3.68Mg_0.04]O₂₀(OH)₄ has been detected using orbital data at multiple locations in ancient terrains on Mars, including Mawrth Vallis, Nilli Fossae, north of the Syrtis Major volcanic plateau, Terra Meridiani, and the landing site of the Mars Science Laboratory (MSL), Gale Crater. Given the antiquity of these sites (>3.0Ga), it is likely that nontronite has been exposed to the martian environment for long periods of time and therefore provides an integrated record of processes in near surface environments including pedogenesis and diagenesis. In particular, nontronite detected at Mawrth Vallis, is overlain by montmorillonite and kaolinite, and it has been previously suggested that this mineralogical sequence may be the result of surface weathering. In order to better understand clay mineral weathering on Mars, we measured dissolution rates of nontronite in column reactors at solution pH values of 0.9, 1.7, and 3.0, and two flow rates (0.16 ml/hr and 0.32 ml/hr). Solution chemistry indicates stoichiometric dissolution at pH = 0.9 and non-stoichiometric dissolution at pH = 1.7 and 3.0. Mineral dissolution rates based on elemental release rates at pH = 1.7 and 3.0 of Ca, Si and Fe follow the order interlayer> tetrahedral> octahedral sites, respectively. The behavior of all experiments suggest far from equilibrium conditions, with the exception of the experiment performed at pH 3.0 and flow rate 0.16 ml/h. A pH-dependent dissolution rate law was calculated through Si release from experiments that showed no dependence on saturation (far from equilibrium conditions) under both flow rates and is r = 10^{-12.06 (±0.123)} • 10^{-0.297 (±0.058)•pH} where r has the units mol mineral m^-2s^-1. When compared to dissolution rates from the literature, our results indicate that nontronite dissolution is significantly slower than dissolution of the primary phases present in basalt under acidic conditions, suggesting that once nontronite forms it could remain stable at or near the surface of Mars for extended periods of time. Nontronite dissolution rates are faster than dissolution rates of montmorillonite (Rozalén et al., 2008) and kaolinite (Huertas et al., 1999), suggesting that chemical weathering of a mixed clay deposit would enrich the proportions of montmorillonite and kaolinite through the preferential dissolution of nontronite. VIS-NIR analyses of our reacted products and thermodynamic modeling of our experimental conditions both indicate the precipitation of amorphous silica within columns, and amorphous silica has also been observed in association with phyllosilicates on the martian surface ( ,  and ). Therefore, chemical weathering of strata containing mixtures of montmorillonite, nontronite and kaolinite provides a potential formation mechanism for the mineralogic stratigraphy observed at Mawrth Vallis and other locations on Mars.

Reference
Gainey SR, Hausrath EM, Hurowitz JA and Milliken RE (in press) Nontronite Dissolution Rates and Implications for Mars. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.10.055]
Copyright Elsevier

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Koji Wada¹, Hidekazu Tanaka², Satoshi Okuzumi³, Hiroshi Kobayashi⁴, Toru Suyama⁵, Hiroshi Kimura⁶ and Tetsuo Yamamoto⁷

¹Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, 275-0016 Chiba Japan
²Institute of Low Temperature Science, Hokkaido University, 060-0819 Sapporo, Japan
³Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro-ku, 152-8511 Tokyo, Japan
⁴Department of Physics, Nagoya University, Nagoya, 464-8602 Aichi, Japan
⁵Nagano City Museum, Hachimanpara Historical Park Ojimada-machi, 381-2212 Nagano, Japan
⁶Graduate School of Science, Kobe University, c/o CPS (Center for Planetary Science), Chuo-ku Minatojima Minamimachi 7-1-48, 650-0047 Kobe, Japan
⁷CPS (Center for Planetary Science), Kobe University, Chuo-ku Minatojima Minamimachi 7-1-48, 650-0047 Kobe, Japan

Context. Collisional growth of dust aggregates is an essential process in forming planetesimals in protoplanetary disks, but disruption through high-velocity collisions (disruption barrier) could prohibit the dust growth. Mass transfer through very different-sized collisions has been suggested as a way to circumvent the disruption barrier.
Aims. We examine how the collisional growth efficiency of dust aggregates with different impact parameters depends on the size and the mass ratio of colliding aggregates.
Methods. We used an N-body code to numerically simulate the collisions of different-sized aggregates.
Results. Our results show that high values for the impact parameter are important and that the growth efficiency averaged over the impact parameter does not depend on the aggregate size, although the growth efficiency for nearly head-on collisions increases with size. We also find that the averaged growth efficiency tends to increase with increasing mass ratio of colliding aggregates. However, the critical collision velocity, above which the growth efficiency becomes negative, does not strongly depend on the mass ratio. These results indicate that icy dust can grow through high-velocity offset collisions at several tens of m s^-1, the maximum collision velocity experienced in protoplanetary disks, whereas it is still difficult for silicate dust to grow in protoplanetary disks.

Reference
Wada K, Tanaka H, Okuzumi S, Kobayashi H, Suyama T, Kimura H and Yamamoto T (2013) Growth efficiency of dust aggregates through collisions with high mass ratios. Astronomy & Astrophysics 559:A62.
[doi:10.1051/0004-6361/201322259]
Reproduced with permission © ESO

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Gerd Weckwerth

Institut für Geologie und Mineralogie, Universität Köln, Zülpicher Str. 49b, 50674, Köln, Germany

We currently seek a copyright agreement with Springer to display abstracts of their cosmochemistry related publications.

Reference
Weckwerth G (in press) Instrumental neutron activation analyses of the most Earth-like meteorites. Journal of Radioanalytical and Nuclear Chemistry
[doi:10.1007/s10967-013-2817-z]

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Eric Tonui^a,b,*, Mike Zolensky^b, Takahiro Hiroi^c, Tomoki Nakamura^d, Michael E. Lipschutz^e, Ming-Sheng Wang^e and Kyoko Okudaira^f

^aBP Upstream Research and Technology, 501 Westlake Boulevard, Houston, TX, 77079, USA
^bNASA Johnson Space Center, Astromaterials Research and Exploration Science (ARES), Mail Code KT, Houston, TX, 77058, USA
^cDepartment of Geological Sciences, Brown University, Providence, RI, 02912, USA
^dDepartment of Earth and Planetary Materials Science, Faculty of Science, Tohoku University Aramaki, Aoba, Sendai, Miyagi 980-8578, Japan
^eDepartment of Chemistry, Purdue University, West Lafayette, IN, 47907-2038, USA
^fThe University of Aizu, Ikki-machi, Aizu-Wakamatsu, Fukushima, 965-8580 Japan

We present a comprehensive description of petrologic, chemical and spectroscopic features of thermally metamorphosed CI-like and CM (and CM-like) chondrites. Only two such CI chondrites have so far been discovered i.e. Y-86029 and Y-82162. Thermal metamorphism in these chondrites is apparent in their low contents of H₂O, C and the most thermally labile trace elements, partial dehydration of matrix phyllosilicates and abundance of thermally decomposed Ca-Mg-Fe-Mn carbonates, which apparently resulted from heating of Mg-Fe carbonate precursors.
The CM chondrites exhibit a wide range of aqueous and thermal alteration characteristics. This alteration was almost complete in Y-86720 and Y-86789, which also escaped alternating episodes of oxidation and sulfidization experienced by the others. Thermal metamorphism in the CM chondrites is apparent in loss of thermally labile trace elements and also in partial to almost complete dehydration of matrix phyllosilicates: heating was less uniform in them than in CI chondrites. This dehydration is also evident in strength and shapes of integrated intensities of the 3 μm bands except in PCA 91008, which experienced extensive terrestrial weathering. Tochilinite is absent in all but Y-793321 probably due to heating. Textural evidence for thermal metamorphism is conspicuous in blurring or integration/fusion of chondrules with matrix in the more extensively heated (⩾600 °C) CM chondrites like PCA 91008 and B-7904. TEM and XRD analyses reveal that phyllosilicate transformation to anhydrous phases proceeds via poorly crystalline, highly desiccated and disordered ‘intermediate’ phases in the least and moderately heated (400-600 °C) carbonaceous chondrites like WIS 91600, PCA 91008 and Y-86029. These findings are significant in that they confirm that these phases occur in meteorites as well as terrestrial samples.
Thermal alteration in these meteorites can be used to identify other carbonaceous chondrites that were thermally metamorphosed in their parent bodies. Combining RNAA trace element data for experimentally heated Murchison CM2 samples with petrographic and spectroscopic data, these thermally metamorphosed carbonaceous chondrites can be ordered by severity of open system heating as 400°C⩽Y-793321<WIS91600=EET90043=A881655 <PCA91008<B-7904=Y-86029<Y-82162<Y-86720=Y-86789⩾700°C. Nearly all heated carbonaceous chondrites discovered so far have been found in Antarctica, which is known to have sampled the flux of near-Earth material for much longer than exemplified by current falls.

Reference
Tonui E, Zolensky M, Hiroi T, Nakamura T, Lipschutz ME, Wang M-S and Okudaira K (in press) Petrographic, chemical and spectroscopic evidence for thermal metamorphism in carbonaceous chondrites I: CI and CM chondrites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.10.053]
Copyright Elsevier

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M. Andersen¹, J. Steinacker¹, W.-F. Thi¹, L. Pagani², A. Bacmann¹ and R. Paladini³

¹UJF – Grenoble 1/CNRS – INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), UMR 5274 38041 Grenoble France
²LERMA, UMR 8112 du CNRS, Observatoire de Paris, 61, Av. de l’Observatoire, 75014 Paris, France
³Infrared Processing and Analysis Center, California Institute of Technology, Pasadena CA 91125, USA

Context. The largest grains (0.5−1 μm) in the interstellar size distribution are efficient in scattering near- and mid-infrared radiation. These wavelengths are therefore particularly well suited to probe the still uncertain high-end of the size distribution.
Aims. We investigate the change in appearance of a typical low-mass molecular core from the Ks (2.2 μm) band to the Spitzer IRAC 3.6 and 8 micron bands, and compare with model calculations, which include variations of the grain size distribution.
Methods. We combine Spitzer IRAC and ground-based near-infrared observations to characterize the scattered light observed at the near- and mid-infrared wavelengths from the core L260. Using a spherical symmetric model core, we perform radiative transfer calculations to study the impact of various dust size distributions on the intensity profiles across the core.
Results. The observed scattered light patterns in the Ks and 3.6 μm bands are found to be similar. By comparison with radiative transfer models the two profiles place constraints on the relative abundance of small and large (more than 0.25 μm) dust grains. The scattered light profiles are found to be inconsistent with an interstellar silicate grain distribution extending only to 0.25 μm and large grains are needed to reach the observed fluxes and the flux ratios. The shape of the Ks band surface brightness profile limits the largest grains to 1−1.5 μm.
Conclusions. In addition to observing coreshine in the Spitzer IRAC channels, the combination with ground-based near-infrared observations are suited to constrain the properties of large grains in cores.

Reference
Andersen M, Steinacker J, Thi W-F, Pagani L, Bacmann A and Paladini R (2013) Scattering from dust in molecular clouds: Constraining the dust grain size distribution through near-infrared cloudshine and infrared coreshine. Astronomy & Astrophysics 559:A30.
[doi:10.1051/0004-6361/201322175]
Reproduced with permission © ESO

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M. E. Brown

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

The formation of the largest objects in the Kuiper belt, with measured densities of ~1.5 g cm^-3 and higher, from the coagulation of small bodies, with measured densities below 1 g cm^-3, is difficult to explain without invoking significant porosity in the smallest objects. If such porosity does occur, measured densities should begin to increase at the size at which significant porosity is no longer supported. Among the asteroids, this transition occurs for diameters larger than ~350 km. In the Kuiper belt, no density measurements have been made between ~350 km and ~850 km, the diameter range where porosities might first begin to drop. Objects in this range could provide key tests of the rock fraction of small Kuiper belt objects (KBOs). Here we report the orbital characterization, mass, and density determination of the 2002 UX25 system in the Kuiper belt. For this object, with a diameter of ~650 km, we find a density of 0.82 ± 0.11 g cm^-3, making it the largest solid known object in the solar system with a measured density below that of pure water ice. We argue that the porosity of this object is unlikely to be above ~20%, suggesting a low rock fraction. If the currently measured densities of KBOs are a fair representation of the sample as a whole, creating ~1000 km and larger KBOs with rock mass fractions of 70% and higher from coagulation of small objects with rock fractions as low as those inferred from 2002 UX25 is difficult.

Reference
Brown ME (2013) Three-dimensional, Off-lattice Monte Carlo Kinetics Simulations of Interstellar Grain Chemistry and Ice Structure. The Astrophysical Journal – Letters 778:L34.
[doi:10.1088/2041-8205/778/2/L34]

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Stephan König^a,*, Jean-Pierre Lorand^b, Ambre Luguet^a and D. Graham Pearson^c

^aRheinische Friedrich-Wilhelms-Universität Bonn, Steinmann Institut für Geologie, Mineralogie und Paläontologie, Poppelsdorfer Schloss, 53115 Bonn, Germany
^bLaboratoire de Planétologie et Géodynamique de Nantes, CNRS UMR 6112, Université de Nantes, 44322 Nantes, France
^cUniversity of Alberta, Department of Earth and Atmospheric Sciences, Edmonton, Canada

The chalcophile and highly siderophile elements Se and Te, like the other Highly Siderophile Elements (HSE) in the terrestrial mantle, may constitute powerful key tracers for meteoritic materials that hit the Earth in its latest accretionary stages (“Late Veneer”). Here the Se and Te systematics of mantle-derived peridotites (orogenic peridotites, ophiolites, cratonic peridotite xenoliths) are assessed. Combined with published in-situ analyses of HSE host minerals, whole-rock data are modelled with respect to current petrogenetic models that affect mantle composition, for example partial melting and magmatic refertilisation. We demonstrate that the near-chondritic Se/Te signature (Se_N/Te_N≈9±4; N = CI-chondrite normalised) of “fertile” ophiolitic and orogenic lherzolites cannot be a primitive signature of the Earthʼs mantle. This signature can however be explained by simple refertilisation models. The HSE–Se–Te budget of these fertile rocks can be modelled by mixing various proportions of a residual assemblage of Fe–Ni monosulphide solid solutions (Mss) and/or refractory platinum group minerals (PGMs – Ru–Os–Ir sulphides + Pt–Ir–Os alloys) with a metasomatic assemblage comprising low-temperature Pt–Pd–Te phases and Cu–Ni-rich sulphides. On the other hand, the reported Se and Te ratios in fertile peridotites are not consistent with melt depletion alone. Additions of late-stage metasomatic S–Se–Te–HSE-rich phases render Primitive Upper Mantle (PUM) estimates for Se and Te highly debatable, especially without appropriate consideration of refertilisation and metasomatism. Our results indicate that there is currently no firm evidence for chondritic S–Se–Te signatures in the Primitive Upper Mantle. This conclusion challenges the simplistic perception that near-chondritic Se/Te ratios may readily trace the Late Veneer composition.

Reference
König S, Lorand J-P, Luguet A Pearson DG (2014) A non-primitive origin of near-chondritic S–Se–Te ratios in mantle peridotites; implications for the Earthʼs late accretionary history. Earth and Planetary Science Letters 385:110–121.
[doi:10.1016/j.epsl.2013.10.036]
Copyright Elsevier

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T.-C. Peng¹, E. M. L. Humphreys¹, L. Testi^1,2,3, A. Baudry^4,5, M. Wittkowski¹, M. G. Rawlings⁶, I. de Gregorio-Monsalvo^1,8, W. Vlemmings⁷, L.-A. Nyman⁸, M. D. Gray⁹ and C. de Breuck¹

¹ESO Garching, Karl-Schwarzschild Str. 2 85748 Garching, Germany
²Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
³INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
⁵CNRS, LAB, UMR 5804, 33270 Floirac, France
⁶National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁷Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁸Joint ALMA Observatory (JAO) and European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁹JBCA, Alan Turing Building, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK

Aims. Galactic chemical evolution (GCE) is important for understanding the composition of the present-day interstellar medium (ISM) and of our solar system. In this paper, we aim to track the GCE by using the ²⁹Si/³⁰Si ratios in evolved stars and tentatively relate this to presolar grain composition.
Methods. We used the APEX telescope to detect thermal SiO isotopologue emission toward four oxygen-rich M-type stars. Together with the data retrieved from the Herschelscience archive and from the literature, we were able to obtain the ²⁹Si/³⁰Si ratios for a total of 15 evolved stars inferred from their optically thin ²⁹SiO and ³⁰SiO emission. These stars cover a range of masses and ages, and because they do not significantly alter ²⁹Si/³⁰Si during their lifetimes, they provide excellent probes of the ISM metallicity (or ²⁹Si/³⁰Si ratio) as a function of time.
Results. The ²⁹Si/³⁰Si ratios inferred from the thermal SiO emission tend to be lower toward low-mass oxygen-rich stars (e.g., down to about unity for W Hya), and close to an interstellar or solar value of 1.5 for the higher-mass carbon star IRC+10216 and two red supergiants. There is a tentative correlation between the ²⁹Si/³⁰Si ratios and the mass-loss rates of evolved stars, where we take the mass-loss rate as a proxy for the initial stellar mass or current stellar age. This is consistent with the different abundance ratios found in presolar grains. Before the formation of the Sun, the presolar grains indicate that the bulk of presolar grains already had ²⁹Si/³⁰Si ratios of about 1.5, which is also the ratio we found for the objects younger than the Sun, such as VY CMa and IRC+10216. However, we found that older objects (up to possibly 10 Gyr old) in our sample trace a previous, lower ²⁹Si/³⁰Si value of about 1. Material with this isotopic ratio is present in two subclasses of presolar grains, providing independent evidence of the lower ratio. Therefore, the ²⁹Si/³⁰Si ratio derived from the SiO emission of evolved stars is a useful diagnostic tool for the study of the GCE and presolar grains.

Reference
Peng T-C, Humphreys EML, Testi L, Baudry A, Wittkowski M Rawlings, MG, de Gregorio-Monsalvo I, Vlemmings W, Nyman L-A, Gray MD and de Breuck C (2013) Silicon isotopic abundance toward evolved stars and its application for presolar grains. Astronomy & Astrophysics 559:L8.
[doi:10.1051/0004-6361/201322466]
Reproduced with permission © ESO

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Katherine R. Compaan^1,2, Jay Agarwal¹, Bryson E. Dye¹, Yukio Yamaguchi¹ and Henry F. Schaefer, III¹

¹Center for Computational Quantum Chemistry, University of Georgia, Athens, GA 30602, USA
²Research Focus Area for Chemical Resource Beneficiation, North-West University, Hoffman Street, Potchefstroom 2520, South Africa

Highly reliable molecular properties have been computed for AlCH₂ and AlCH₂⁺ at the CCSD(T)/cc-pwCVQZ level of theory. These simple aluminum species are relevant to the chemistry of the interstellar medium (ISM) and circumstellar medium, and are proposed to form in regions where aluminum and methylene exist in appreciable concentrations because AlCH₂ and AlCH₂⁺ are thermodynamically stable with respect to dissociation. Herein, dissociation energies were computed for both species through extrapolation to the complete basis set limit using focal point analysis. For the neutral species, 86 kcal mol^-1 is required to heterolytically cleave the Al–C single bond, while the cationic species requires 38 kcal mol^-1. To aid in identification within the ISM, the anharmonic (fundamental) frequencies, spectroscopic constants, and vibrationally averaged properties for AlCH₂ and the corresponding cation are also reported.

Reference
Compaan KR, Agarwal J, Dye BE, Yamaguchi Y and Schaefer, III HF (2013) Toward Detection of AlCH2 and AlCH2+ in the Interstellar Medium. The Astrophysical Journal 778:125.
[doi:10.1088/0004-637X/778/2/125]

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Robin T. Garrod

Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853-6801, USA

The first off-lattice Monte Carlo kinetics model of interstellar dust grain surface chemistry is presented. The positions of all surface particles are determined explicitly, according to the local potential minima resulting from the pair-wise interactions of contiguous atoms and molecules, rather than by a pre-defined lattice structure. The model is capable of simulating chemical kinetics on any arbitrary dust grain morphology, as determined by the user-defined positions of each individual dust grain atom. A simple method is devised for the determination of the most likely diffusion pathways and their associated energy barriers for surface species. The model is applied to a small, idealized dust grain, adopting various gas densities and using a small chemical network. Hydrogen and oxygen atoms accrete onto the grain to produce H₂O, H₂, O₂, and H₂O₂. The off-lattice method allows the ice structure to evolve freely; the ice mantle porosity is found to be dependent on the gas density, which controls the accretion rate. A gas density of 2 × 10⁴cm^-3, appropriate for dark interstellar clouds, is found to produce a fairly smooth and non-porous ice mantle. At all densities, H₂ molecules formed on the grains collect within the crevices that divide nodules of ice and within micropores (whose extreme inward curvature produces strong local potential minima). The larger pores produced in the high-density models are not typically filled with H₂. Direct deposition of water molecules onto the grain indicates that amorphous ices formed in this way may be significantly more porous than interstellar ices that are formed by surface chemistry.

Reference
Garrod RT (2013) Three-dimensional, Off-lattice Monte Carlo Kinetics Simulations of Interstellar Grain Chemistry and Ice Structure. The Astrophysical Journal 778:158.
[doi:10.1088/0004-637X/778/2/158]

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