Fred J. Ciesla^1,*, Thomas M. Davison², Gareth S. Collins² and David P. O’Brien³

¹Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, 60637, USA
²Impact and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, London, UK
³Planetary Science Institute, Tucson, Arizona, USA

Collisions between planetesimals were common during the first approximately 100 Myr of solar system formation. Such collisions have been suggested to be responsible for thermal processing seen in some meteorites, although previous work has demonstrated that such events could not be responsible for the global thermal evolution of a meteorite parent body. At this early epoch in solar system history, however, meteorite parent bodies would have been heated or retained heat from the decay of short-lived radionuclides, most notably ²⁶Al. The postimpact structure of an impacted body is shown here to be a strong function of the internal temperature structure of the target body. We calculate the temperature–time history of all mass in these impacted bodies, accounting for their heating in an onion-shell–structured body prior to the collision event and then allowing for the postimpact thermal evolution as heat from both radioactivities and the impact is diffused through the resulting planetesimal and radiated to space. The thermal histories of materials in these bodies are compared with what they would be in an unimpacted, onion-shell body. We find that while collisions in the early solar system led to the heating of a target body around the point of impact, a greater amount of mass had its cooling rates accelerated as a result of the flow of heated materials to the surface during the cratering event.

Reference
Ciesla FJ, Davison TM, Collins GS and O’Brien DP (in press) Thermal consequences of impacts in the early solar system. Meteoritics & Planetary Science 
[doi:10.1111/maps.12236]
Published by arrangement with John Wiley & Sons

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Alex Meshik^a, Charles Hohenberg^a, Olga Pravdivtseva^a and Donald Burnett^b

^aDepartment of Physics, Washington University, 1 Brookings Drive, Saint Louis, MO 63130, USA
^bGeological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

One of the major goals of the Genesis Mission was to refine our knowledge of the isotopic composition of the heavy noble gases in solar wind and, by inference, the Sun, which represents the initial composition of the solar system. This has now been achieved with permil precision: ³⁶Ar/³⁸Ar = 5.5005 ± 0.0040, ⁸⁶Kr/⁸⁴Kr = .3012 ± .0004, ⁸³Kr/⁸⁴Kr = .2034 ± .0002, ⁸²Kr/⁸⁴Kr = .2054 ± .0002, ⁸⁰Kr/⁸⁴Kr = .0412 ± .0002, ⁷⁸Kr/⁸⁴Kr = .00642 ± .00005, ¹³⁶Xe/¹³²Xe = .3001 ± .0006, ¹³⁴Xe/¹³²Xe = .3691 ± .0007, ¹³¹Xe/¹³²Xe = .8256 ± .0012,¹³⁰Xe/¹³²Xe = .1650 ± .0004, ¹²⁹Xe/¹³²Xe = 1.0405 ± .0010, ¹²⁸Xe/¹³²Xe = .0842 ± .0003, ¹²⁶Xe/¹³²Xe = .00416 ± .00009, and ¹²⁴Xe/¹³²Xe = .00491 ± .00007 (error-weighted averages of all published data). The Kr and Xe ratios measured in the Genesis solar wind collectors generally agree with the less precise values obtained from lunar soils and breccias, which have accumulated solar wind over hundreds of millions of years, suggesting little if any temporal variability of the isotopic composition of solar wind krypton and xenon. The higher precision for the initial composition of the heavy noble gases in the solar system allows (1) to confirm that, exept ¹³⁶Xe and ¹³⁴Xe, the mathematically derived U-Xe is equivalent to Solar Wind Xe and (2) to provide an opportunity for better understanding the relationship between the starting composition and Xe-Q (and Q-Kr), the dominant current “planetary” component, and its host, the mysterious phase-Q.

Reference
Meshik A, Hohenberg C, Pravdivtseva O and Burnett D (in press) Heavy Noble Gases in Solar Wind Delivered by Genesis Mission. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.11.030]
Copyright Elsevier

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M. Fouchard^a, H. Rickman^b,c, Ch. Froeschlé^d, G.B. Valsecchi^e,f

^aLAL-IMCCE, Université de Lille 1, 1 Impasse de l’Observatoire, F-59000 Lille, France
^bPAS Space Research Center, Bartycka 18A, PL-00-716, Warszawa, Poland
^cDept. of Physics & Astronomy, Uppsala Univ., Box 516, SE-75120 Uppsala, Sweden
^dObservatoire de la Côte d’Azur, UMR Lagrange 7293, Bv. de l’Observatoire, B.P. 4229, F-06304 Nice cedex 4, France
^eIAPS, INAF, via Fosso del Cavaliere 100, I-00133 Roma, Italy
^fIFAC-CNR, Via Madonna del Piano 10, I-50019 Sesto Fiorentino (FI), Italy

We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard et al., 2013b), the planetary perturbations experienced each time the comets penetrate to within 50 AU of the Sun. Each simulation involves an initial sample of four million comets and extends over a maximum of 5 Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. In the companion paper we studied in detail how observable comets are injected from the Oort cloud, when account is taken of the planetary perturbations. In the present paper we concentrate on how the cloud may evolve in the long term and also on the production of decoupled comets, which evolve into semi-major axes less than 1 000AU. Concerning the long-term evolution, we find that the largest stellar perturbations that may statistically be expected during the age of the Solar System induce a large scale migration of comets within the cloud. Thus, comets leave the inner parts, but the losses from the outer parts are even larger, so at the end of our simulations the Oort cloud is more centrally condensed than at the beginning. The decoupled comets, which form a source of centaurs and Halley type comets (roughly in the proportions of 70% and 30%, respectively), are mainly produced by planetary perturbations, Jupiter and Saturn being the most efficient. This effect is dependent on synergies with the Galactic tide and stellar encounters, bringing the perihelia of Oort cloud comets into the planetary region. The star-planet synergy has a large contribution due to the strong encounters that produce major comet showers. However, outside these showers a large majority of decouplings may be attributed to the tide-planet synergy.

Reference
Fouchard M, Rickman H, Froeschlé Ch and Valsecchi GB (in press) Planetary perturbations for Oort cloud comets: III. Evolution of the cloud and production of centaurs and Halley type comets. Icarus
[doi:10.1016/j.icarus.2013.11.034]
Copyright Elsevier

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Neyda M. Abreu^1,* and Emma S. Bullock²

¹Earth Science Program, Pennsylvania State University—Du Bois Campus, Du Bois, Pennsylvania, USA
²Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia, USA

We have studied the petrologic characteristics of sulfide-metal lodes, polymineralic Fe-Ni nodules, and opaque assemblages in the CR2 chondrite Graves Nunataks (GRA) 06100, one of the most altered CR chondrites. Unlike low petrologic type CR chondrites, alteration of metal appears to have played a central role in the formation of secondary minerals in GRA 06100. Differences in the mineralogy and chemical compositions of materials in GRA 06100 suggest that it experienced higher temperatures than other CR2 chondrites. Mineralogic features indicative of high temperature include: (1) exsolution of Ni-poor and Ni-rich metal from nebular kamacite; (2) formation of sulfides, oxides, and phosphates; (3) changes in the Co/Ni ratios; and (4) carbidization of Fe-Ni metal. The conspicuous absence of pentlandite may indicate that peak temperatures exceeded 600 °C. Opaques appear to have been affected by the action of aqueous fluids that resulted in the formation of abundant oxides, Fe-rich carbonates, including endmember ankerite, and the sulfide-silicate-phosphate scorzalite. We suggest that these materials formed via impact-driven metamorphism. Mineralogic features indicative of impact metamorphism include (1) the presence of sulfide-metal lodes; (2) the abundance of polymineralic opaque assemblages with mosaic-like textures; and (3) the presence of suessite. Initial shock metamorphism probably resulted in replacement of nebular Fe-Ni metal in chondrules and in matrix by Ni-rich, Co-rich Fe metal, Al-Ti-Cr-rich alloys, and Fe sulfides, while subsequent hydrothermal alteration produced accessory oxides, phosphates, and Fe carbonates. An extensive network of sulfide-metal veins permitted effective exchange of siderophile elements from pre-existing metal nodules with adjacent chondrules and matrix, resulting in unusually high Fe contents in these objects.

Reference
Abreu NM and Bullock ES (in press) Opaque assemblages in CR2 Graves Nunataks (GRA) 06100 as indicators of shock-driven hydrothermal alteration in the CR chondrite parent body. Meteoritics & Planetary Science 
[doi:10.1111/maps.12227]
Published by arrangement with John Wiley & Sons

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D. S. Burnett

Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA

The Genesis Discovery mission returned solar matter in the form of the solar wind with the goal of obtaining precise solar isotopic abundances (for the first time) and greatly improved elemental abundances. Measurements of the light noble gases in regime samples demonstrate that isotopes are fractionated in the solar wind relative to the solar photosphere. Theory is required for correction. Measurement of the solar wind O and N isotopes shows that these are very different from any inner solar system materials. The solar O isotopic composition is consistent with photochemical self-shielding. For unknown reasons, the solar N isotopic composition is much lighter than essentially all other known solar system materials, except the atmosphere of Jupiter. Ne depth profiling on Genesis materials has demonstrated that Ne isotopic variations in lunar samples are due to isotopic fractionation during implantation without appealing to higher energy solar particles. Genesis provides a precise measurement of the isotopic differences of Ar between the solar wind and the terrestrial atmosphere. The Genesis isotopic compositions of Kr and Xe agree with data from lunar ilmenite separates, showing that lunar processes have not affected the ilmenite data and that solar wind composition has not changed on 100 Ma time scales. Relative to Genesis solar wind, ArKrXe in Q (the chondrite noble gas carrier) and the terrestrial atmosphere show relatively large light isotope depletions.

Reference
Burnett DS (in press) The Genesis solar wind sample return mission: Past, present, and future. Meteoritics & Planetary Science
[doi:10.1111/maps.12241]
Published by arrangement with John Wiley & Sons

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Yoshihiro Furukawa Taro Samejima, Hiromoto Nakazawa and Takeshi Kakegawa

Department of Earth Science, Graduate School of Science, Tohoku University

Late heavy bombardment (LHB) of extraterrestrial objects supplied carbon with metals to the prebiotic Earth. The early oceans were the major target of these impacts, followed by interactions among the atmosphere, oceanic water, and meteorite constituent materials under high-temperature and high-pressure conditions. Post-impact reactions of these hypervelocity impacts have the potential to produce reduced volatiles and organic compounds, including amino acids. Therefore, understanding the reactions in post-impact plumes is of great importance for the investigation of prebiotic organic compounds. The composition of post-impact plumes has been investigated with thermochemical calculations. However, experimental evidence is still needed to understand the reactions in dynamic systems of post-impact plumes. The present study investigates the effects of reaction temperature and availability of water on products from iron, nickel, graphite, nitrogen, and water in a dynamic gas flow system to investigate reactions in post-impact plumes. Results of this study indicate the formation of CO, H₂, NH₃, and HCN by hypervelocity oceanic impacts of iron-rich extraterrestrial objects. The formation of methane was limited in the present experiments, suggesting that the quenching rate is an influential factor for methane formation in post-impact plumes. Availability of water vapor in the plume was also an influential factor for the formation of reduced volatiles that controlled the CO formation rate from graphite. These results provide experimental evidence for the formation of reduced volatiles in post-impact plumes, which influenced the formation of pre-biotic organic compounds.

Reference
Furukawa Y, Samejima T, Nakazawa H and Kakegawa T (in press) Experimental investigation of reduced volatile formation by high-temperature interactions among meteorite constituent materials, water, and nitrogen. Icarus
[doi:10.1016/j.icarus.2013.11.033]
Copyright Elsevier

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M. Fouchard^a, H. Rickman^b,c, Ch. Froeschlé^d, G.B. Valsecchi^e,f

^aLAL-IMCCE, Université de Lille 1, 1 Impasse de l’Observatoire, F-59000 Lille, France
^bPAS Space Research Center, Bartycka 18A, PL-00-716, Warszawa, Poland
^cDept. of Physics & Astronomy, Uppsala Univ., Box 516, SE-75120 Uppsala, Sweden
^dObservatoire de la Côte d’Azur, UMR Lagrange 7293, Bv. de l’Observatoire, B.P. 4229, F-06304 Nice cedex 4, France
^eIAPS, INAF, via Fosso del Cavaliere 100, I-00133 Roma, Italy
^fIFAC-CNR, Via Madonna del Piano 10, I-50019 Sesto Fiorentino (FI), Italy

We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard et al. 2013b), the planetary perturbations experienced each time the comets penetrate to within 50 AU of the Sun. Each simulation involves an initial sample of four million comets and extends over a maximum of 5 Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. We concentrate on the production of observable comets, reaching for the first time a perihelion within 5 AU of the Sun. We distinguish between four categories, depending on whether the comet jumps across, or creeps through, the Jupiter-Saturn barrier (perihelion distances between 5 and 15 AU), and whether the orbit leading to the observable perihelion is preceded by a major planetary perturbation or not. For reasons explained in the paper, we call the strongly perturbed comets “Kaib-Quinn comets”.
We thus derive a synthetic picture of the Oort spike, from which we draw two main conclusions regarding the full dynamical model. One is that 2/3 of the observable comets are injected with the aid of a planetary perturbation at the previous perihelion passage, and about half of the observable comets are of the Kaib-Quinn type. The other is that the creepers dominate over the jumpers. Due to this fact, the spike peaks at only 31 000AU, and the majority of new comets have semi-major axes less than this value. The creepers show a clear preference for retrograde orbits as a consequence of the need to avoid untimely, planetary ejection before becoming observable. Thus, the new comets should have a 60/40 preference for retrograde against prograde orbits in apparent conflict with observations. However, both these and other results depend on our model assumptions regarding the initial structure of the Oort cloud, which is isotropic in shape and has a relatively steep energy distribution. We also find that they depend on the details of the past history of external perturbations including GMC encounters, and we provide special discussions of those issues.

Reference
Fouchard M, Rickman H, Froeschlé Ch and Valsecchi GB (in press) Planetary perturbations for Oort cloud comets: II. Implications for the origin of observable comets. Icarus
[doi:10.1016/j.icarus.2013.11.032]
Copyright Elsevier

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P. J. Wozniakiewicz^1,2, J. P. Bradley³, H. A. Ishii³, M. C. Price² and D. E. Brownlee⁴

¹Earth Sciences Department, Mineral and Planetary Science Division, Natural History Museum, Cromwell Road, London SW7 5BD, UK
²School of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NH, UK
³Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
⁴Department of Astronomy, University of Washington, Seattle, WA 98195, USA

Despite their micrometer-scale dimensions and nanogram masses, chondritic porous interplanetary dust particles (CP IDPs) are an important class of extraterrestrial material since their properties are consistent with a cometary origin and they show no evidence of significant post-accretional parent body alteration. Consequently, they can provide information about grain accretion in the comet-forming region of the outer solar nebula. We have previously reported our comparative study of the sizes and size distributions of crystalline silicate and sulfide grains in CP IDPs, in which we found these components exhibit a size–density relationship consistent with having been sorted together prior to accretion. Here we extend our data set and include GEMS (glass with embedded metal and sulfide), the most abundant amorphous silicate phase observed in CP IDPs. We find that while the silicate and sulfide sorting trend previously observed is maintained, the GEMS size data do not exhibit any clear relationship to these crystalline components. Therefore, GEMS do not appear to have been sorted with the silicate and sulfide crystals. The disparate sorting trends observed in GEMS and the crystalline grains in CP IDPs present an interesting challenge for modeling early transport and accretion processes. They may indicate that several sorting mechanisms operated on these CP IDP components, or alternatively, they may simply be a reflection of different source environments.

Reference
Wozniakiewicz PJ, Bradley JP, Ishii HA, Price MC and Brownlee DE (2013) Pre-Accretional Sorting of Grains in the Outer Solar Nebula. The Astrophysical Journal 779:164.
[doi:10.1088/0004-637X/779/2/164]

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Dana E. Anderson¹, Edwin A. Bergin¹, Sébastien Maret² and Valentine Wakelam^3,4

¹Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
²UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble, F-38041, France
³Univ. Bordeaux, LAB, UMR 5804, F-33270, Floirac, France
⁴CNRS, LAB, UMR 5804, F-33270, Floirac, France

We present observations of fine-structure line emission of atomic sulfur, iron, and rotational lines of molecular hydrogen in shocks associated with several Class 0 protostars obtained with the Infrared Spectrograph of the Spitzer Space Telescope. We use these observations to investigate the “missing sulfur problem,” that significantly less sulfur is found in dense regions of the interstellar medium (ISM) than in diffuse regions. For sources where the sulfur fine-structure line emission is co-spatial with the detected molecular hydrogen emission and in the presence of weak iron emission, we derive sulfur and H2 column densities for the associated molecule-dominated C-shocks. We find the S i abundance to be 5%–10% of the cosmic sulfur abundance, indicating that atomic sulfur is a major reservoir of sulfur in shocked gas. This result suggests that in the quiescent dense ISM sulfur is present in some form that is released from grains as atoms, perhaps via sputtering, within the shock.

Reference
Anderson DE, Bergin EA, Maret S and Wakelam V (2013) New Constraints on the Sulfur Reservoir in the Dense Interstellar Medium Provided by Spitzer Observations of S I in Shocked Gas. The Astrophysical Journal 779:141.
[doi:10.1088/0004-637X/779/2/141]

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Zita Martins¹, Mark C. Price², Nir Goldman³, Mark A. Sephton¹ and Mark J. Burchell²

¹Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK
²School of Physical Sciences, University of Kent, Canterbury CT2 7NH, UK
³Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, USA

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

Reference
Martins Z, Price MC, Goldman N, Sephton MA and Burchell MJ (2013) Shock synthesis of amino acids from impacting cometary and icy planet surface analogues. Nature Geoscience 6:1045–1049.
[doi:10.1038/ngeo1930]

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