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]
Copyright Elsevier

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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]
Copyright Elsevier

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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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John P. Bradley^a,b, Hope A. Ishii^a,b, Jeffrey J. Gillis-Davis^b, James Ciston^c, Michael H. Nielsen^d,e, Hans A. Bechtel^f, and Michael C. Martin^f

^aInstitute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, Livermore, CA 94550;
^bHawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI 96822;
^cNational Center for Electron Microscopy,
^dMaterials Science Division, and
^eAdvanced Light Source Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720; and
^fDepartment of Materials Science and Engineering, University of California, Berkeley, CA 94720

The solar wind (SW), composed of predominantly ~1-keV H⁺ ions, produces amorphous rims up to ∼150 nm thick on the surfaces of minerals exposed in space. Silicates with amorphous rims are observed on interplanetary dust particles and on lunar and asteroid soil regolith grains. Implanted H⁺ may react with oxygen in the minerals to form trace amounts of hydroxyl (−OH) and/or water (H₂O). Previous studies have detected hydroxyl in lunar soils, but its chemical state, physical location in the soils, and source(s) are debated. If −OH or H₂O is generated in rims on silicate grains, there are important implications for the origins of water in the solar system and other astrophysical environments. By exploiting the high spatial resolution of transmission electron microscopy and valence electron energy-loss spectroscopy, we detect water sealed in vesicles within amorphous rims produced by SW irradiation of silicate mineral grains on the exterior surfaces of interplanetary dust particles. Our findings establish that water is a byproduct of SW space weathering. We conclude, on the basis of the pervasiveness of the SW and silicate materials, that the production of radiolytic SW water on airless bodies is a ubiquitous process throughout the solar system.

Reference
Bradley JP, Ishii HA, Gillis-Davis JJ, Ciston J, Nielsen MH, Bechtel HA and Martin MC (in press) Detection of solar wind-produced water in irradiated rims on silicate minerals. PNAS 111:1732–1735.
[doi:10.1073/pnas.1320115111]

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Anna Losiak^1,2, Eva Maria Wild³, Leonard Michlmayr³, Christian Koeberl^1,4

¹Department of Lithospheric Research, University of Vienna, Vienna, Austria
²Institute of Geological Sciences, Polish Academy of Sciences, Wrocław, Poland
³VERA Laboratory, Faculty of Physics, Isotope Research and Nuclear Physics, University of Vienna, Vienna, Austria
⁴Natural History Museum, Vienna, Austria

Rocks from drill cores LB-07A (crater fill) and LB-08A (central uplift) into the Bosumtwi impact crater, Ghana, were analyzed for the presence of the cosmogenic radionuclide ¹⁰Be. The aim of the study was to determine the extent to which target rocks of various depths were mixed during the formation of the crater-filling breccia, and also to detect meteoric water infiltration within the impactite layer. ¹⁰Be abundances above background were found in two (out of 24) samples from the LB-07A core, and in none of five samples from the LB-08A core. After excluding other possible explanations for an elevated ¹⁰Be signal, we conclude that it is most probably due to a preimpact origin of those clasts from target rocks close to the surface. Our results suggest that in-crater breccias were well mixed during the impact cratering process. In addition, the lack of a ¹⁰Be signal within the rocks located very close to the lake sediment–impactite boundary suggests that infiltration of meteoric water below the postimpact crater floor was limited. This may suggest that the infiltration of the meteoric water within the crater takes place not through the aerial pore-space, but rather through a localized system of fractures.

Reference
Losiak A, Wild EM, Michlmayr L and Koeberl C (in press) 10Be content in clasts from fallout suevitic breccia in drill cores from the Bosumtwi impact crater, Ghana: Clues to preimpact target distribution Meteoritics & Planetary Science
[doi:10.1111/maps.12256]
Published by arrangement with John Wiley & Sons

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Roger R. Fu, Bradford H. Hager, Anton I. Ermakov, Maria T. Zuber

Department of Earth, Atmospheric, and Planetary Sciences. Massachusetts Institute of Technology. Cambridge, MA

The asteroid Vesta is a differentiated planetesimal from the accretion phase of solar system formation. Although its present-day shape is dominated by a non-hydrostatic fossil equatorial bulge and two large, mostly unrelaxed impact basins, Vesta may have been able to approach hydrostatic equilibrium during a brief early period of intense interior heating. We use a finite element viscoplastic flow model coupled to a 1D conductive cooling model to calculate the expected rate of relaxation throughout Vesta’s early history. We find that, given sufficient non-hydrostaticity, the early elastic lithosphere of Vesta experienced extensive brittle failure due to self-gravity, thereby allowing relaxation to a more hydrostatic figure. Soon after its accretion, Vesta reached a closely hydrostatic figure with <2 km non-hydrostatic topography at degree-2, which, once scaled, is similar to the maximum disequilibrium of the hydrostatic asteroid Ceres. Vesta was able to support the modern observed amplitude of non-hydrostatic topography only >40-200 My after formation, depending on the assumed depth of megaregolith. The Veneneia and Rheasilvia giant impacts, which generated most non-hydrostatic topography, must have therefore occurred >40-200 My after formation. Based on crater retention ages, topography, and relation to known impact generated features, we identify a large region in the northern hemisphere that likely represents relic hydrostatic terrain from early Vesta. The long-wavelength figure of this terrain suggests that, before the two late giant impacts, Vesta had a rotation period of 5.02 hr (6.3% faster than present) while its spin axis was offset by 3.0 from that of the present. The evolution of Vesta’s figure shows that the hydrostaticity of small bodies depends strongly on its age and specific impact history and that a single body may embody both hydrostatic and non-hydrostatic terrains and epochs.

Reference
Fu RR, Hager BH, Ermakov AI and Zuber MT (2014) Efficient early global relaxation of asteroid Vesta. Icarus
[doi:10.1016/j.icarus.2014.01.023]
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Yuki Shiromoto¹, Hajime Susa¹, and Takashi Hosokawa²

¹Department of Physics, Konan University, Kobe 658-8501, Japan
²Department of Physics and Research Center for the Early Universe, The University of Tokyo, Tokyo 113-0033, Japan

The generation process of a magnetic field around a proto-first-star is studied. Utilizing the recent numerical results of proto-first-star formation based on radiation hydrodynamics simulations, we assess the magnetic field strength generated by the radiative force and the Biermann battery effect. We find that a magnetic field of ~10^-9 G is generated on the surface of the accretion disk around the proto-first-star. The field strength on the accretion disk is smaller by two orders of magnitude than the critical value, above which the gravitational fragmentation of the disk is suppressed. Thus, the generated seed magnetic field hardly affect the dynamics of on-site first star formation directly, unless an efficient amplification process is taken into consideration. We also find that the generated magnetic field is continuously blown out from the disk on the outflows to the poles, that are driven by the thermal pressure of photoheated gas. The strength of the diffused magnetic field in low-density regions is ~10^-14-10^-13 G at n_H = 10³ cm^-3, which could play an important role in the next generation star formation, as well as the seeds of the magnetic field in the present-day universe.

Reference
Shiromoto Y, Susa H and Takashi Hosokawa T (in press) Generation of Magnetic Field on the Accretion Disk around a Proto-first-star. The Astrophysical Journal 782:108.
[doi:10.1088/0004-637X/782/2/108]

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John T. Wasson^1,2 and Alan E. Rubin²

¹Institute of Geophysics and Planetary Physics, University of California Los Angeles, Los Angeles, California, USA
²Departments of Earth and Space Sciences and Chemistry and Biochemistry, University of California Los Angeles, Los Angeles, California, USA

In numerous past papers, it was concluded that the fine (<1 μm) matrix immediately adjacent to, and radially symmetric around, chondrules in primitive chondrites consists of compact (low-porosity) rims that were attached in the solar nebula. We present here textural and compositional evidence that no matrix-like (or accretionary) rims around chondrules are present in the well-preserved CR2 chondrite LAP 02342. Fine-grained matrix-rich regions (i.e., candidate “rims”) at the edges of chondrules were studied with an electron-microprobe-based matrix-grid technique; comparison of the “rims” data for matrix regions near these chondrules showed the candidate “rims” to be compositionally heterogeneous, inconsistent with origins as radially symmetric, matrix-like rims formed by gradual accretion. This evidence (together with simulations and laboratory studies indicating that accretionary processes produced highly porous aggregates) strongly suggests that nebular processes did not produce compact matrix-like rims around chondrules in any chondrite group.

Reference
Wasson JT and Rubin AE (in press) Absence of matrix-like chondrule rims in CR2 LAP 02342. Meteoritics & Planetary Science
[doi:10.1111/maps.12237]
Published by arrangement with John Wiley & Sons

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