Devin L. Schrader^1,a, Kazuhide Nagashima^a, Alexander N. Krot^a, Ryan C. Ogliore^a and Eric Hellebrand^b

^aHawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
^bDepartment of Geology and Geophysics, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
¹Present Address: Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, PO Box 37012, MRC 119, Washington, D.C. 20013, USA

To better understand the environment of chondrule formation and constrain the O-isotope composition of the ambient gas in the Renazzo-like carbonaceous (CR) chondrite chondrule-forming region, we studied the mineralogy, petrology, and in situ O-isotope compositions of olivine in 11 barred olivine (BO) chondrules and pyroxene and silica in three type I porphyritic chondrules from the CR chondrites Gao-Guenie (b), Graves Nunataks (GRA) 95229, Pecora Escarpment (PCA) 91082, and Shişr 033. BO chondrules experienced a higher degree of melting than porphyritic chondrules, and therefore, it has been hypothesized that they more accurately recorded the O-isotope composition of the gas in chondrule-forming regions. We studied the O-isotope composition of silica as it has been hypothesized to have formed via direct condensation from the gas.
BO chondrules constitute ~4% of the total CR chondrule population by volume. On a three-isotope oxygen diagram (δ¹⁷O vs. δ¹⁸O), olivine phenocrysts in type I and type II BO chondrules plot along ~ slope-1 line; with the exception of a type II BO chondrule that plots along ~ slope-0.5 line. Olivine phenocrysts in type I and type II BO chondrules have similar but more restricted ranges of Δ¹⁷O values (~ -3.8 to ~ -1.3‰ and ~ -0.8 to ~ +1.4‰, respectively) than those in type I and type II porphyritic chondrules (~ -4.6 to ~ -0.3‰ and ~ -1.8 to ~ +0.9‰, respectively). The observation that olivine grains in type I BO chondrules have similar chemical and O-isotope compositions to those of olivine in their porphyritic counterparts argues against the hypothesis that olivine grains in type I porphyritic chondrules are xenocrysts and represent relict fragments of early formed planetesimals.
The compositional and O-isotope data suggest that BO chondrules experienced more extensive, but incomplete exchange with the ambient gas than porphyritic chondrules. We suggest that CR chondrules formed from relatively ¹⁶O-enriched solids in the presence of relatively ¹⁶O-depleted gaseous H₂O. The O-isotope compositions of chondrule olivine likely result from differences in the O-isotope composition of both the chondrule precursors and the ambient gas during chondrule formation. The inferred O-isotope composition of this gas (Δ¹⁷O ranges from ~ -3‰ to +3‰) is inconsistent with a high abundance of water from the outer Solar System, which has been predicted to be isotopically heavy.

Reference
Schrader DL, Nagashima K, Krot AN, Ogliore RC and Hellebrand E (in press) Variations in the O-isotope composition of gas during the formation of chondrules from the CR chondrites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.01.034]
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Stefan T.M. Peters^a,b, Carsten Münker^a,b,1, Harry Becker^c,2, Toni Schulz^d,3

^aInstitut für Geologie und Mineralogie, Universität zu Köln, Zülpicherstr. 49b, 50674 Cologne, Germany
^bSteinmann-Institut, Poppelsdorfer Schloss, 53115 Bonn, Germany
^cInstitut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin, Germany
^dDepartment of Lithospheric Research, Universität Wien, Althanstrasse 14, A-1090, Vienna, Austria

The decay of the rare nuclide ¹⁸⁴Os by alpha emission to ¹⁸⁰W has been theoretically predicted, but was previously never observed in experiments. Variable excesses of ¹⁸⁰W were recently observed for iron meteorites, but the contribution to these excesses by ¹⁸⁴Os-decay was regarded as insignificant. Here, we present combined ¹⁸⁰W and Os–W concentration data for meteorites and terrestrial rocks, now indicating that the ¹⁸⁰W heterogeneities can be explained by α-decay of ¹⁸⁴Os. A combined ¹⁸⁴Os–¹⁸⁰W isochron for iron meteorites and chondrites yields a decay constant value of λ¹⁸⁴Os(α) of 6.49±1.34×10⁻¹⁴ a⁻¹ (half life 1.12±0.23×10¹³ yr), in good agreement with theoretical estimates. The ¹⁸⁴Os–¹⁸⁰W decay system may constitute a viable tracer and chronometer for important geological processes like core formation, silicate differentiation or late accretion processes. This is illustrated by a measured ¹⁸⁰W-deficit in terrestrial basalts relative to chondrites by 1.16±0.69 parts in 10 000, consistent with core formation ~4.5 Ga ago.

Reference
Peters STM, Münker C, Becker H and Schulz T (2014) Alpha-decay of 184Os revealed by radiogenic 180W in meteorites: Half life determination and viability as geochronometer. Earth and Planetary Science Letters 391:69–76.
[doi:10.1016/j.epsl.2014.01.030]
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Liping Jin1 and Min Li

College of Physics, Jilin University, Changchun, Jilin 130012, China

We show that the diversity of extrasolar planetary systems may be related to the diversity of molecular cloud cores. In previous studies of planet formation, artificial initial conditions of protoplanetary disks or steady state disks, such as the minimum mass nebula model, have often been used so that the influence of cloud core properties on planet formation is not realized. To specifically and quantitatively demonstrate our point, we calculate the dependence of disk properties on cloud core properties and show that the boundary of the giant planet formation region in a disk is a function of cloud core properties with the conventional core accretion model of giant planet formation. The gravitational stability of a disk depends on the properties of its progenitor cloud core. We also compare our calculations with observations of extrasolar planets. From the observational data of cloud cores, our model could infer the range and most frequent values of observed semimajor axes of extrasolar planets. Our calculations suggest that planet formation at the snowline alone could not completely explain the semimajor axis distribution. If the current observations are not biased, our calculations indicate that the planet formation at the snowline is inefficient. We suggest that there will be more observed planets with semimajor axis <9 AU than >9 AU, even with a longer duration of observations, if the planet formation at the snowline is inefficient.

Reference
Jin L and Li M (2014) Diversity of Extrasolar Planets and Diversity of Molecular Cloud Cores. I. Semimajor Axes. The Astrophysical Journal 783:37.
[doi:10.1088/0004-637X/783/1/37]

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S. C. Lowry¹, P. R. Weissman², S. R. Duddy¹, B. Rozitis³, A. Fitzsimmons⁴, S. F. Green³, M. D. Hicks², C. Snodgrass⁵, S. D. Wolters³, S. R. Chesley², J. Pittichová² and P. van Oers⁶

¹Centre for Astrophysics and Planetary Science, School of Physical Sciences (SEPnet), The University of Kent, Canterbury, CT2 7NH, UK
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
³Planetary and Space Sciences, Department of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
⁴Astrophysics Research Centre, Queens University Belfast, Belfast, BT7 1NN, UK
⁵Max Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
⁶Isaac Newton Group of Telescopes, 38700 Santa Cruz de la Palma, Canary Islands, Spain

Context. Near-Earth asteroid (25143) Itokawa was visited by the Hayabusa spacecraft in 2005, resulting in a highly detailed shape and surface topography model. This model has led to several predictions for the expected radiative torques on this asteroid, suggesting that its spin rate should be decelerating.
Aims. To detect changes in rotation rate that may be due to YORP-induced radiative torques, which in turn may be used to investigate the interior structure of the asteroid.
Methods. Through an observational survey spanning 2001 to 2013 we obtained rotational lightcurve data at various times over the last five close Earth-approaches of the asteroid. We applied a polyhedron-shape-modelling technique to assess the spin-state of the asteroid and its long term evolution. We also applied a detailed thermophysical analysis to the shape model determined from the Hayabusa spacecraft.
Results. We have successfully measured an acceleration in Itokawa’s spin rate of dω/dt = (3.54 ± 0.38) × 10^-8 rad day^-2, equivalent to a decrease of its rotation period of ~45 ms year^-1. From the thermophysical analysis we find that the centre-of-mass for Itokawa must be shifted by ~21 m along the long-axis of the asteroid to reconcile the observed YORP strength with theory.
Conclusions. This can be explained if Itokawa is composed of two separate bodies with very different bulk densities of 1750 ± 110 kg m^-3 and 2850 ± 500 kg m^-3, and was formed from the merger of two separate bodies, either in the aftermath of a catastrophic disruption of a larger differentiated body, or from the collapse of a binary system. We therefore demonstrate that an observational measurement of radiative torques, when combined with a detailed shape model, can provide insight into the interior structure of an asteroid. Futhermore, this is the first measurement of density inhomogeneity within an asteroidal body, that reveals significant internal structure variation. A specialised spacecraft is normally required for this.

Reference
Lowry SC, Weissman PR, Duddy SR, Rozitis B, Fitzsimmons A, Green SF, Hicks MD, Snodgrass C, Wolters SD, Chesley SR, Pittichová J and van Oers P (2014) The internal structure of asteroid (25143) Itokawa as revealed by detection of YORP spin-up.  Astronomy & Astrophysics 562:A48.
[doi:10.1051/0004-6361/201322175]
Reproduced with permission © ESO

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J.E. Chambers

Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road, NW Washington DC 20015

In the core accretion model for giant planet formation, a solid core forms by coagulation of dust grains in a protoplanetary disk and then accretes gas from the disk when the core reaches a critical mass. Both stages must be completed in a few million years before the disk gas disperses. The slowest stage of this process may be oligarchic growth in which a giant-planet core grows by sweeping up smaller, asteroid-size planetesimals. Here, we describe new numerical simulations of oligarchic growth using a particle-in-a-box model. The simulations include several processes that can effect oligarchic growth: (i) planetesimal fragmentation due to mutual collisions, (ii) the modified capture rate of planetesimals due to a core’s atmosphere, (iii) drag with the disk gas during encounters with the core (so-called “pebble accretion”), (iv) modification of particle velocities by turbulence and drift caused by gas drag, (v) the presence of a population of mm-to-m size “pebbles” that represent the transition point between disruptive collisions between larger particles, and mergers between dust grains, and (vi) radial drift of small objects due to gas drag. Collisions between planetesimals rapidly generate a population of pebbles. The rate at which a core sweeps up pebbles is controlled by pebble accretion dynamics. Metre-size pebbles lose energy during an encounter with a core due to drag, and settle towards the core, greatly increasing the capture probability during a single encounter. Millimetre-size pebbles are tightly coupled to the gas and most are swept past the core during an encounter rather than being captured. Accretion efficiency per encounter increases with pebble size in this size range. However, radial drift rates also increase with size, so metre-size objects encounter a core on many fewer occasions than mm-size pebbles before they drift out of a region. The net result is that core growth rates vary weakly with pebble size, with the optimal diameter being about 10 cm. The main effect of planetesimal size is to determine the rate of mutual collisions, fragment production and the formation of pebbles. 1-km-diameter planetesimals collide frequently and have low impact strengths, leading to a large surface density of pebbles and rapid core growth via pebble accretion. 100-km-diameter planetesimals produce fewer pebbles, and pebble accretion plays a minor role in this case. The strength of turbulence in the gas determines the scale height of pebbles in the disk, which affects the rate at which they are accreted. For an initial solid surface density of 12 g/cm² at 5 AU, with10-cm diameter pebbles and a disk viscosity parameter α=10^-4, a 10-Earth mass core can form in 3 My for 1–10 km diameter planetesimals. The growth of such a core requires longer than 3 My if planetesimals are 100 km in diameter.

Reference
Chambers JE (in press) Giant Planet Formation with Pebble Accretion. Icarus
[doi:10.1016/j.icarus.2014.01.036]
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Tomoki Nakamura¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Division of Earth and Planetary Materials Science, Laboratory for Early Solar System Evolution, Graduate School of Science, Tohoku University, Aoba, Sendai, Miyagi, Japan

The mineralogy and mineral chemistry of Itokawa dust particles captured during the first and second touchdowns on the MUSES-C Regio were characterized by synchrotron-radiation X-ray diffraction and field-emission electron microprobe analysis. Olivine and low- and high-Ca pyroxene, plagioclase, and merrillite compositions of the first-touchdown particles are similar to those of the second-touchdown particles. The two touchdown sites are separated by approximately 100 meters and therefore the similarity suggests that MUSES-C Regio is covered with dust particles of uniform mineral chemistry of LL chondrites. Quantitative compositional properties of 48 dust particles, including both first- and second-touchdown samples, indicate that dust particles of MUSES-C Regio have experienced prolonged thermal metamorphism, but they are not fully equilibrated in terms of chemical composition. This suggests that MUSES-C particles were heated in a single asteroid at different temperatures. During slow cooling from a peak temperature of approximately 800 °C, chemical compositions of plagioclase and K-feldspar seem to have been modified: Ab and Or contents changed during cooling, but An did not. This compositional modification is reproduced by a numerical simulation that modeled the cooling process of a 50 km sized Itokawa parent asteroid. After cooling, some particles have been heavily impacted and heated, which resulted in heterogeneous distributions of Na and K within plagioclase crystals. Impact-induced chemical modification of plagioclase was verified by a comparison to a shock vein in the Kilabo LL6 ordinary chondrite where Na-K distributions of plagioclase have been disturbed.

Reference
Nakamura T et al. (in press) Mineral chemistry of MUSES-C Regio inferred from analysis of dust particles collected from the first- and second-touchdown sites on asteroid Itokawa. Meteoritics & Planetary Science
[doi:10.1111/maps.12247]
Published by arrangement with John Wiley & Sons

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Bo Reipurth

Astronomy, University of Hawaii, Hilo, Hawaii, USA

No abstract is available for this article.

Reference
Reipurth B (in press) Road-map to the Indian’s treasure—on the Chilean meteorite Vaca Muerta and its early mistake for silver, by Holger Pedersen. Norderstedt, Germany: Books on Demand, 2012, 312 p., paperback (ISBN-13: 9788771144406). Available through German book dealers. Meteoritics & Planetary Science
[doi:10.1111/maps.12253]
Published by arrangement with John Wiley & Sons

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

^aInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany

The Dawn Framing Camera (FC) has imaged the northern hemisphere of the asteroid (4) Vesta at high spatial resolution and coverage. This study represents the first investigation of the overall geology of the northern hemisphere (22°N-90°N, quadrangles Av-1, 2, 3, 4 and 5) using these unique Dawn mission observations. We have compiled a morphologic map and performed crater size-frequency distribution (CSFD) measurements to date the geologic units. The hemisphere is characterized by a heavily cratered surface with a few highly subdued basins up to ~200 km in diameter. The most widespread unit is a plateau (cratered highland unit), similar to, although of lower elevation than the equatorial Vestalia Terra plateau. Large-scale troughs and ridges have regionally affected the surface. Between ~180° and ~270°E, these tectonic features are well developed and related to the south pole Veneneia impact (Saturnalia Fossae trough unit), elsewhere on the hemisphere they are rare and subdued (Saturnalia Fossae cratered unit). In these pre-Rheasilvia units we observed an unexpectedly high frequency of impact craters up to ~10 km in diameter, whose formation could in part be related to the Rheasilvia basin-forming event. The Rheasilvia impact has potentially affected the northern hemisphere also with S-N small-scale lineations, but without covering it with an ejecta blanket. Post-Rheasilvia impact craters are small (<60 km in diameter) and show a wide range of degradation states due to impact gardening and mass wasting processes. Where fresh, they display an ejecta blanket, bright rays and slope movements on walls. In places, crater rims have dark material ejecta and some crater floors are covered by ponded material interpreted as impact melt.

Reference
Ruesch O et al. (in press) Geologic map of the northern hemisphere of Vesta based on Dawn FC images. Icarus
[doi:10.1016/j.icarus.2014.01.035]
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José M. Madiedo^a,b et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aDepartamento de Física Atómica, Molecular y Nuclear. Facultad de Física. Universidad de Sevilla. 41012 Sevilla, Spain
^bFacultad de Ciencias Experimentales, Universidad de Huelva, 21071 Huelva, Spain

Among the most astonishing astronomical phenomena are the extremely bright bolides produced by the entry of large meteoroids into the Earth’s atmosphere. These events are rare and unexpected because current telescopic surveys are still missing meter-sized meteoroids, particularly those of dark nature and presumably cometary origin. In this work we present the analysis of two very bright fireballs of such origin recently observed over Spain. The first of these was recorded on September 25, 2010, while the second one took place on August 23, 2012. With an absolute magnitude of -18 and -17, respectively, these sporadic events fall within the superbolide category. Their atmospheric trajectory is calculated, together with the heliocentric orbit of the parent meteoroids. Other physical properties of these particles are estimated, such as their preatmospheric mass and tensile strength. The emission spectrum recorded for one of these events is also discussed. Our analysis indicates that none of these superbolides was a meteorite-dropping event. From their orbital parameters, a cometary nature for the parent meteoroids is inferred.

Reference
José M. Madiedo et al. (in press) Analysis of two superbolides with a cometary origin observed over the iberian peninsula. Icarus
[doi:10.1016/j.icarus.2014.01.031]
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Amaury Thiabaud^1,2, Ulysse Marboeuf^1,2, Yann Alibert^1,2,3, Nahuel Cabral^1,2, Ingo Leya^1,2 and Klaus Mezger^1,4

¹Center for Space and Habitability, Universität Bern, 3012 Bern, Switzerland
²Physikalisches Institut, Universität Bern, 3012 Bern, Switzerland
³Observatoire de Besançon, 41 avenue de l’Observatoire, 25000 Besançon, France
⁴Institut für Geologie, Universität Bern, 3012 Bern, Switzerland

Context. To date, calculations of planet formation have mainly focused on dynamics, and only a few have considered the chemical composition of refractory elements and compounds in the planetary bodies. While many studies have been concentrating on the chemical composition of volatile compounds (such as H₂O, CO, CO₂) incorporated in planets, only a few have considered the refractory materials as well, although they are of great importance for the formation of rocky planets.
Aims. We computed the abundance of refractory elements in planetary bodies formed in stellar systems with a solar chemical composition by combining models of chemical composition and planet formation. We also considered the formation of refractory organic compounds, which have been ignored in previous studies on this topic.
Methods. We used the commercial software package HSC Chemistry to compute the condensation sequence and chemical composition of refractory minerals incorporated into planets. The problem of refractory organic material is approached with two distinct model calculations: the first considers that the fraction of atoms used in the formation of organic compounds is removed from the system (i.e., organic compounds are formed in the gas phase and are non-reactive); and the second assumes that organic compounds are formed by the reaction between different compounds that had previously condensed from the gas phase.
Results. Results show that refractory material represents more than 50 wt % of the mass of solids accreted by the simulated planets with up to 30 wt % of the total mass composed of refractory organic compounds. Carbide and silicate abundances are consistent with C/O and Mg/Si elemental ratios of 0.5 and 1.02 for the Sun. Less than 1 wt % of carbides are present in the planets, and pyroxene and olivine are formed in similar quantities. The model predicts planets that are similar in composition to those of the solar system. Starting from a common initial nebula composition, it also shows that a wide variety of chemically different planets can form, which means that the differences in planetary compositions are due to differences in the planetary formation process.
Conclusions. We show that a model in which refractory organic material is absent from the system is more compatible with observations. The use of a planet formation model is essential to form a wide diversity of planets in a consistent way.

Reference
Losiak A, Wild EM, Michlmayr L and Koeberl C (in press) From stellar nebula to planets: The refractory components. Astronomy & Astrophysics 562:A27.
[doi:10.1051/0004-6361/201322208]
Reproduced with permission © ESO

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