Christoph Loesche¹, Gerhard Wurm¹, Jens Teiser¹, Jon M. Friedrich^2,3, and Addi Bischoff⁴

¹Faculty of Physics, University of Duisburg-Essen, Lotharstr. 1, D-47057 Duisburg, Germany
²Department of Chemistry, Fordham University, Bronx, NY 10458, USA
³Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
⁴Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany

Photophoresis is a physical process that transports particles in optically thin parts of protoplanetary disks, especially at the inner edge and at the optical surface. To model the transport and resulting effects in detail, it is necessary to quantify the strength of photophoresis for different particle classes as a fundamental input. Here, we explore photophoresis for a set of chondrules. The composition and surface morphology of these chondrules were measured by X-ray tomography. Based on the three-dimensional models, heat transfer through illuminated chondrules was calculated. The resulting surface temperature map was then used to calculate the photophoretic strength. We found that irregularities in particle shape and variations in composition induce variations in the photophoretic force. These depend on the orientation of a particle with respect to the light source. The variation of the absolute value of the photophoretic force on average over all chondrules is 4.17%. The deviation between the direction of the photophoretic force and illumination is 30 ± 15. The average photophoretic force can be well approximated and calculated analytically assuming a homogeneous sphere with a volume equivalent mean radius and an effective thermal conductivity. We found an analytic expression for the effective thermal conductivity. The expression depends on the two main phases of a chondrule and decreases with the amount of fine-grained devitrified, plagioclase-normative mesostasis up to factor of three. For the chondrule sample studied (Bjurböle chondrite), we found a dependence of the photophoretic force on chondrule size.

Reference
Loesche C, Wurm G, Teiser J, Friedrich JM and Bischoff A (2013) Photophoretic Strength on Chondrules. 1. Modeling. The Astrophysical Journal 778:2.
[doi:10.1088/0004-637X/778/2/101]

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Jiří Borovička¹, Pavel Spurný¹, Peter Brown^2,3, Paul Wiegert^2,3, Pavel Kalenda⁴, David Clark^2,3 and Lukáš Shrbený¹

¹Astronomical Institute, Academy of Sciences of the Czech Republic, CZ-251 65 Ondřejov, Czech Republic
²Department of Physics and Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada
³Centre for Planetary Science and Exploration, University of Western Ontario, London, Ontario N6A 5B7, Canada
⁴Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, V Holešovičkách 41, CZ-18209 Praha 8, Czech Republic

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

Reference
Borovička J, Spurný P, Brown P, Wiegert P, Kalenda P, Clark D and Shrbený L (2013) The trajectory, structure and origin of the Chelyabinsk asteroidal impactor. Nature 503:235–237.
[doi:10.1038/nature12671]

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Brown et al. (>>10)*
*Find the extensive, full author and affiliation list on the publishers website.

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

Reference
Brown et al. (2013) A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors. Nature 503:238–241.
[doi:10.1038/nature12741]

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M. Minissale¹, E. Congiu¹, G. Manicò², V. Pirronello² and F. Dulieu¹

¹LERMA, UMR8112 du CNRS, de l’Observatoire de Paris et de l’Université de Cergy Pontoise, 5 mail Gay Lussac, 95000 Cergy Pontoise Cedex, France
²Dipartimento di Fisica ed Astronomia, Universitá degli Studi di Catania, via Santa Sofia 64, 95123 Catania, Italy

Context. The formation of carbon dioxide in quiescent regions of molecular clouds has not yet been fully understood, even though CO₂ is one of the most abundant species in interstellar ices.
Aims. CO₂ formation is studied via oxidation of CO molecules on cold surfaces under conditions close to those encountered in quiescent molecular clouds.
Methods. Carbon monoxide and oxygen atoms are codeposited using two differentially pumped beam lines on two different surfaces (amorphous water ice or oxydized graphite) held at given temperatures between 10 and 60 K. The products are probed via mass spectroscopy by using the temperature-programmed desorption technique.
Results. We show that the reaction CO + O can form carbon dioxide in solid phase with an efficiency that depends on the temperature of the surface. The activation barrier for the reaction, based on modelling results, is estimated to be in the range of 780−475 K/k_b. Our model also allows us to distinguish the mechanisms (Eley Rideal or Langmuir-Hinshelwood) at play in different temperature regimes. Our results suggest that competition between CO₂ formation via CO + O and other surface reactions of O is a key factor in the yields of CO₂ obtained experimentally.
Conclusions. CO₂ can be formed by the CO + O reaction on cold surfaces via processes that mimic carbon dioxide formation in the interstellar medium. Astrophysically, the presence of CO₂ in quiescent molecular clouds could be explained by the reaction CO + O occurring on interstellar dust grains.

Reference
Minissale M, Congiu E, Manicò G, Pirronello V and Dulieu F (2013) CO2 formation on interstellar dust grains: a detailed study of the barrier of the CO + O channel. Astronomy & Astrophysics 559:A49.
[doi:10.1051/0004-6361/201321453]
Reproduced with permission © ESO

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Dirk Elbeshausen^1,*, Kai Wünnemann¹, Gareth S. Collins²

¹Museum für Naturkunde, Leibniz-Institute for Research on Evolution and Biodiversity, Berlin, Germany
²Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, London, UK

Elliptical impact craters are rare among the generally symmetric shape of impact structures on planetary surfaces. Nevertheless, a better understanding of the formation of these craters may significantly contribute to our overall understanding of hypervelocity impact cratering. The existence of elliptical craters raises a number of questions: Why do some impacts result in a circular crater whereas others form elliptical shapes? What conditions promote the formation of elliptical craters? How does the formation of elliptical craters differ from those of circular craters? Is the formation process comparable to those of elliptical craters formed at subsonic speeds? How does crater formation work at the transition from circular to elliptical craters? By conducting more than 800 three-dimensional (3-D) hydrocode simulations, we have investigated these questions in a quantitative manner. We show that the threshold angle for elliptical crater generation depends on cratering efficiency. We have analyzed and quantified the influence of projectile size and material strength (cohesion and coefficient of internal friction) independently from each other. We show that elliptical craters are formed by shock-induced excavation, the same process that forms circular craters and reveal that the transition from circular to elliptical craters is characterized by the dominance of two processes: A directed and momentum-controlled energy transfer in the beginning and a subsequent symmetric, nearly instantaneous energy release.

Reference
Elbeshausen D, Wünnemann K and Collins GS (in press) The transition from circular to elliptical impact craters. Journal of Geophysical Research – Planets
[doi:10.1002/2013JE004477]
Published by arrangement with John Wiley & Sons

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Sherry K. Fieber-Beyer^* and Michael J. Gaffey

Department of Space Studies, University Stop 9008, University of North Dakota, 58202

The 3:1 Kirkwood gap asteroids are a mineralogically diverse set of asteroids located in a region that delivers meteoroids into Earth-crossing orbits. Mineralogical characterizations of asteroids in/near the 3:1 Kirkwood Gap can be used as a tool to “map” conditions and processes in the early solar system. The chronological studies of the meteorite types provide a “clock” for the relative timing of those events and processes. By identifying the source asteroids of particular meteorite types, the “map” and “clock” can be combined to provide a much more sophisticated understanding of the history and evolution of the late solar nebula and the early solar system.
A mineralogical assessment of twelve 3:1 Kirkwood Gap asteroids has been carried out using near-infrared spectral data obtained from 2010-2011 combined with visible spectral data (when available) to cover the spectral interval of 0.4 – 2.5 μm. Eight of these asteroids have surfaces with basaltic-type silicate assemblages, indicating at least partial melting within their parent bodies. Although HED-like mineralogies are present these objects exhibit subdued features indicating the presence of an additional phase (e.g., NiFe metal) or process (e.g., space weathering). Four of these asteroids appear to be ordinary chondrite assemblages. Three of these are plausibly linked to the probable H-chondrite parent body, (6) Hebe.

Reference
Fieber-Beyer SK and Gaffey MJ (in press) Near-infrared Spectroscopy of 3:1 Kirkwood Gap Asteroids II: Probable and Plausible Parent Bodies; Primitive and Differentiated. Icarus
[doi:10.1016/j.icarus.2013.11.001]
Copyright Elsevier

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