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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