J. Monteux^a G. Tobie^a, G. Choblet^a and M. Le Feuvre^b

^aLaboratoire de Planétologie et de Géodynamique de Nantes
^bLaboratoire Auscultation et Imagerie, IFSTTAR, Nantes

The apparent moments of inertia of Callisto and Titan inferred from gravity data suggest incomplete differentiation of their interior, commonly attributed to slow and cold accretion. To understand whether such large icy moons can really avoid global melting and subsequent differentiation during their accretion, we have developed a 3D numerical model that characterizes the thermal evolution of a satellite growing by multi-impacts, simulating the satellite growth and thermal evolution for a body radius ranging from 100 to 2000 kilometers. The effects of individual impacts (energy deposition, excavation) are simulated and integrated for impactor sizes ranging from a few kilometers to one hundred kilometers, while for smaller impactors, a simplified approach with successive thin uniform layers spreading all over the satellite is considered. Our simulations show that the accretion rate plays only a minor role and that extending the duration of accretion does not significantly limit the increase of the internal temperature. The mass fraction brought by large impactors plays a more crucial role. Our results indicate that a satellite exceeding 2000 km in radius may accrete without experiencing significant melting only if its accretion is dominated by small impactors (< a few kilometers) and that the conversion of impact energy into heat is unrealistically inefficient (<10-15%). Based on our simulations, if more than 10% of satellite mass was brought by satellitesimals larger than 1 km, global melting for large bodies like Titan or Callisto cannot be avoided.

Reference
Monteux J, Tobie G, Choblet G and Le Feuvre M (in press) Can Large Icy Moons Accrete Undifferentiated? Icarus
[doi:10.1016/j.icarus.2014.04.041]
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Marian Horstmann^a, Munir Humayun^b and Addi Bischoff^a

^aInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany.
^bNational High Magnetic Field Laboratory & Department of Earth, Ocean and Atmospheric Science, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310, USA.

Enstatite (E) chondrites are a group of texturally highly variable meteorites formed under strongly reducing conditions giving rise to unique mineral and chemical characteristics (e.g., high abundances of various sulfides and Si-bearing metal). In particular the abundant metal comprises a range of textures in E chondrites of different petrologic type, but available in situ siderophile trace element data on metal are limited. Nine samples of E chondrites from the recent Almahata Sitta fall [one EH3, two EL3/4, two EL6, two EL impact melt rocks (IMR), two EH IMR] were investigated in this study in addition to St. Mark’s (EH5) and Grein 002 (EL4/5), with a focus on the nature of their metal constituents. Special attention was given to metal-silicate intergrowths (MSSI) that occur in many primitive E chondrites, which have been interpreted as post-accretionary asteroidal impact melts or primitive nebular condensates. This study shows that siderophile trace element systematics in E chondrite metal are independent of petrologic type of the host rock and distinct from condensation signatures. Three basic types of siderophile trace element signatures can be distinguished, indicating crystallization from a melt, thermal equilibration upon metamorphism/complete melting, and exsolution of schreibersite-perryite-sulfide. Textural and mineral-chemical constraints from EL3/4s are used to evaluate previously proposed formation processes of MSSI (impact melting vs. nebular condensation) and elucidate which other formation scenarios are feasible. It is shown that post-accretionary (in situ) impact melting or metallic melt injection forming MSSI on the thin section scale, and nebular condensation, are unlikely formation processes. This leads to the conclusion that MSSIs are pre-accretionary melt objects that were formed during melting processes prior to the accretion of the primitive E chondrites. The same can be concluded for metal nodules in the EH3 chondrite examined. The pre-accretionary origin of MSSIs in E chondrites is consistent with a growing body of evidence for early differentiation followed by impact disruption of early formed planetesimals in all major chondrite types.

Reference
Horstmann M, Humayun M and Bischoff A (in press) Clues to the origin of metal in Almahata Sitta EL and EH chondrites and implications for primitive E chondrite thermal histories. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.041]
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Wolf Uwe Reimold^1,2, Ludovic Ferrière³, Alex Deutsch⁴ and Christian Koeberl^3,5

¹Museum für Naturkunde Berlin, Berlin, Germany
²Humboldt-Universität zu Berlin, Berlin, Germany
³Natural History Museum, Vienna, Austria
⁴Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Muenster, Germany
⁵Department of Lithospheric Research, University of Vienna, Vienna, Austria

This is a letter to the editor with no abstract.

Reference
Reimold WU, Ferrière L, Deutsch A and Koeberl C (in press) Impact controversies: Impact recognition criteria and related issues. Meteoritics & Planetary Science
[doi:10.1111/maps.12284]
Published by arrangement with John Wiley & Sons

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Jafar Arkani-Hamed^a,b and Daniel Boutin^b

^aDepartment of Physics, University of Toronto, Toronto, Ontario, Canada
^bDepartment of Earth and Planetary Sciences, McGill University, Montreal, Quebec, Canada

We investigate the possibility that a strong core dynamo of the Moon has magnetized the lunar crust. The magnetic data from two missions, Lunar Prospector and Kaguya, are used and the magnetic fields of two different features are examined: The isolated small magnetic source bodies with almost no topographic signatures, and the impact craters with diameters larger than 100 km. Five data sets are examined separately for each of the isolated magnetic anomalies: the r, θ, and φ components of the Lunar Prospector data, the rcomponent of a 150-degree spherical harmonic model of the lunar magnetic field, and the r component of the Kaguya data. The r component of the Lunar Prospector data is also used to derive the magnetic field over the impact craters. We conclude that most of the ancient lunar far side crust is heterogeneously magnetized with coherency wavelength about a few hundred km. The paleomagnetic north poles determined from modeling the magnetic field of both features show some clustering whereas the source bodies are widely distributed, suggesting that the magnetizing field may have been a core dynamo field. Paleintensity data suggest that the core field intensity was at least 1 mT at the core mantle boundary. There is also evidence for core field reversals, because further clustering occurs when the south poles of some features are considered.

Reference
Arkani-Hamed J and Boutin D (in press) Analysis of Isolated Magnetic Anomalies and Magnetic Signatures of Impact Craters: Evidence for a Core Dynamo in the Early History of the Moon. Icarus
[doi:10.1016/j.icarus.2014.04.046]
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Christine Floss^a, Corentin Le Guillou^b,c, Adrian Brearley^b

^aLaboratory for Space Sciences and Physics Department, Washington University, One Brookings Drive, St. Louis, MO 63130, USA
^bDepartment of Earth and Planetary Sciences, MCS03-2040, University of New Mexico, Albuquerque, NM 87106, USA
^cInstitut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany

The organic matter in the primitive CR3 chondrites QUE 99177 and MET 00426 exhibits, as in other CR chondrites, N isotopic compositions characterized by large enrichments in ¹⁵N compared to solar. These enrichments are present in the matrices of these two meteorites as localized hotspots associated with C-rich grains, and larger, more diffuse regions with more modest enrichments in ¹⁵N. Occasionally depletions in ¹⁵N are also observed. FIB-TEM analysis of isotopically anomalous as well as isotopically normal C-rich grains from the matrix of MET 00426 shows that both types of grains consist of highly disordered organic matter that exhibits a variety of morphologies. There are no obvious correlations of isotopic composition with morphology, petrographic association or elemental composition. Large diffuse regions with modest ¹⁵N enrichments may be the result of fluid action that redistributed organic matter (and the associated ¹⁵N enrichments) in veins and cracks along grain boundaries. Grain formation likely occurred in a variety of environments (e.g., molecular clouds or the outer regions of the protosolar nebula) via UV photolysis of simpler precursor ices with variable isotopic compositions.

Reference
Floss C, Le Guillou C and Brearley A (in press) Coordinated NanoSIMS and FIB-TEM Analyses of Organic Matter and Associated Matrix Materials in CR3 Chondrites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.023]
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R. Brunetto^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aInstitut d’Astrophysique Spatiale (IAS), CNRS, UMR-8617, Université Paris-Sud, bâtiment 121, F-91405 Orsay Cedex, France

Little is known about carbonaceous asteroids weathering in space as previous studies have struggled to define a general spectral trend among dark surfaces. Here we present experiments on ion irradiation of the Allende meteorite, performed using 40 keV He⁺ and Ar⁺ ions, as a simulation of solar wind irradiation of primitive bodies surfaces. We used different fluences up to 3×10¹⁶ ions/cm², corresponding to short timescales of ∼10³-10⁴ years in the main asteroid belt. Samples were analyzed before and after irradiation using visible to far-IR (0.4 – 50 μm) reflectance spectroscopy, and Raman micro-spectroscopy. Similarly to what observed in previous experiments, results show a reddening and darkening of VIS-NIR reflectance spectra. These spectral variations are however comparable to other spectral variations due to viewing geometry, grain size, and sample preparation, suggesting an explanation for the contradictory space weathering studies of dark asteroids. After irradiation, the infrared bands of the matrix olivine silicates change profile and shift to longer wavelength, possibly as a consequence of a more efficient sputtering effect on Mg than Fe (lighter and more volatile species are preferentially sputtered backwards) and/or preferential amorphisation of Mg-rich olivine. Spectral variations are compatible with the Hapke weathering model. Raman spectroscopy shows that the carbonaceous component is substantially affected by irradiation: different degrees of de-ordering are produced as a function of dose, to finally end with a highly disordered carbon. All observed modifications seem to scale with the nuclear elastic dose.

Reference
Brunetto et al. (in press) Ion irradiation of Allende meteorite probed by visible, IR, and Raman spectroscopies. Icarus
[doi:10.1016/j.icarus.2014.04.047]
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John Moriarty¹, Nikku Madhusudhan^2,3 and Debra Fischer¹

¹Department of Astronomy, Yale University, New Haven, CT 06511, USA
²Departments of Physics and Astronomy, Yale University, New Haven, CT 06511, USA
³Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

The composition of planets is largely determined by the chemical and dynamical evolution of the disk during planetesimal formation and growth. To predict the diversity of exoplanet compositions, previous works modeled planetesimal composition as the equilibrium chemical composition of a protoplanetary disk at a single time. However, planetesimals form over an extended period of time, during which elements sequentially condense out of the gas as the disk cools and are accreted onto planetesimals. To account for the evolution of the disk during planetesimal formation, we couple models of disk chemistry and dynamics with a prescription for planetesimal formation. We then follow the growth of these planetesimals into terrestrial planets with N-body simulations of late-stage planet formation to evaluate the effect of sequential condensation on the bulk composition of planets. We find that our model produces results similar to those of earlier models for disks with C/O ratios close to the solar value (0.54). However, in disks with C/O ratios greater than 0.8, carbon-rich planetesimals form throughout a much larger radial range of the disk. Furthermore, our model produces carbon-rich planetesimals in disks with C/O ratios as low as ~0.65, which is not possible in the static equilibrium chemistry case. These results suggest that (1) there may be a large population of short-period carbon-rich planets around moderately carbon-enhanced stars (0.65 < C/O < 0.8) and (2) carbon-rich planets can form throughout the terrestrial planet region around carbon-rich stars (C/O > 0.8).

Reference
Moriarty J, Madhusudhan N and Fischer D (2014) Chemistry in an Evolving Protoplanetary Disk: Effects on Terrestrial Planet Composition. The Astrophysical Journal 787:81.
[doi:10.1088/0004-637X/787/1/81]

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Paul Glombick^1,†, Douglas R. Schmitt², Wei Xie², Todd Bown^2,3, Ben Hathway¹ and Christopher Banks^1,4

¹Alberta Geological Survey, Alberta Energy Regulator, Edmonton, Alberta, Canada
²Department of Physics, Institute for Geophysical Research, CCIS 4-183, University of Alberta, Edmonton, Alberta, Canada
³OptaSense Canada, Calgary, Alberta, Canada
⁴Schlumberger Information Solutions, Dyce, Aberdeen, AB21 0LQ, UK
^†Alberta, Canada

Geological and geophysical evidence is presented for a newly discovered, probable remnant complex impact structure. The structure, located near Bow City, southern Alberta, has no obvious morphological expression at surface. The geometry of the structure in the shallow subsurface, mapped using downhole geophysical well logs, is a semicircular structural depression approximately 8 km in diameter with a semicircular uplifted central region. Detailed subsurface mapping revealed evidence of localized duplication of stratigraphic section in the central uplift area and omission of strata within the surrounding annular region. Field mapping of outcrop confirmed an inlier of older rocks present within the center of the structure. Evidence of deformation along the eastern margin of the central uplift includes thrust faulting, folding, and steeply dipping bedding. Normal faults were mapped along the northern margin of the annular region. Isopach maps reveal that structural thickening and thinning were accommodated primarily within the Belly River Group. Evidence from legacy 2-D seismic data is consistent with the subsurface mapping and reveals additional insight into the geometry of the structure, including a series of listric normal faults in the annular region and complex faulting within the central uplift. The absence of any ejecta blanket, breccia, suevite, or melt sheet (based on available data) is consistent with the Bow City structure being the remnant of a deeply eroded, complex impact structure. Accordingly, the Bow City structure may provide rare access and insight into zones of deformation remaining beneath an excavated transient crater in stratified siliciclastic target rocks.

Reference
Glombick P, Schmitt DR, Xie W, Bown T, Hathway B and Banks C (in press) The Bow City structure, southern Alberta, Canada: The deep roots of a complex impact structure? Meteoritics & Planetary Science
[doi:10.1111/maps.12296]
Published by arrangement with John Wiley & Sons

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O. Trippella^1,2, M. Busso^1,2, E. Maiorca^2,3, F. Käppeler⁴ and S. Palmerini⁵

¹Department of Physics, University of Perugia, via A. Pascoli, I-06123 Perugia, Italy
²INFN, Section of Perugia, via A. Pascoli, I-06123 Perugia, Italy
³INAF, Observatory of Arcetri, viale E. Fermi 5, I-50125 Florence, Italy
⁴Karlsruhe Institute of Technology, Campus North, Institute of Nuclear Physics, P.O. Box 3640, D-76021 Karlsruhe, Germany
⁵INFN, Laboratori Nazionali del Sud, via Santa Sofia 62, I-95125 Catania, Italy

Slow neutron captures at A  85 are mainly guaranteed by the reaction ¹³C(α,n)¹⁶O in asymptotic giant branch (AGB) stars, requiring proton injections from the envelope. These were so far assumed to involve a small mass ( 10^-3 M☉), but models with rotation suggest that in such tiny layers excessive ¹⁴N hampers s-processing. Furthermore, s-element abundances in galaxies require ¹³C-rich layers substantially extended in mass ( 4 × 10^-3 M_☉). We therefore present new calculations aimed at clarifying those issues and at understanding whether the solar composition helps to constrain the ¹³C “pocket” extension. We show that: (1) mixing “from bottom to top” (as in magnetic buoyancy or other forced mechanisms) can form a ¹³C reservoir substantially larger than assumed so far, covering most of the He-rich layers; (2) on the basis of this idea, stellar models at a fixed metallicity reproduce the main s-component as accurately as before; and (3) they make nuclear contributions from unknown nucleosynthesis processes (LEPP) unnecessary, against common assumptions. These models also avoid problems of mixing at the envelope border and fulfil requirements from C-star luminosities. They yield a large production of nuclei below A = 100, so that ^86,87Sr may be fully synthesized by AGB stars, while ⁸⁸Sr, ⁸⁹Y, and ⁹⁴Zr are contributed more efficiently than before. Finally, we suggest tests suitable for providing a final answer regarding the extension of the ¹³C pocket.

Reference
Trippella O, Busso M, Maiorca E, Käppeler F and Palmerini S (2014) s-Processing in AGB Stars Revisited. I. Does the Main Component Constrain the Neutron Source in the 13C Pocket?. The Astrophysical Journal 787:41.
[doi:10.1088/0004-637X/787/1/41]

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Yoshihiro Hidaka¹, Akira Yamaguchi^2,3 and Mitsuru Ebihara¹

¹Department of Chemistry, Tokyo Metropolitan University, Tokyo, Japan
²National Institute of Polar Research, Tokyo, Japan
³Department of Polar Science, School of Multidisciplinary Science, Graduate University for Advanced Sciences, Tokyo, Japan

We have studied the feldspathic lunar meteorite Dhofar 1428 chemically and petrologically to better understand the evolution of the lunar surface. Dhofar 1428 is a feldspathic regolith breccia derived from the lunar highland. Bulk chemical and mineral compositions of Dhofar 1428 are similar to those of other feldspathic lunar meteorites. We found a few clasts of evolved lithologies, such as K-rich plagioclases and quartz monzogabbro. Dhofar 1428 contains approximately 1 wt% of chondritic materials like CM chondrite on the basis of abundances of platinum group elements (Ru, Rh, Pd, Os, Ir, and Pt).

Reference
Hidaka Y, Yamaguchi A and Ebihara M (in press) Lunar meteorite, Dhofar 1428: Feldspathic breccia containing KREEP and meteoritic components. Meteoritics & Planetary Science
[doi:10.1111/maps.12290]
Published by arrangement with John Wiley & Sons

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