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

^aDLR, Inst. of Planetary Research, Berlin, Germany
^bFreie Universitaet Berlin, Inst. of Geosciences, Berlin, Germany

Deposits of dark material appear on Vesta’s surface as features of relatively low-albedo in the visible wavelength range of Dawn’s camera and spectrometer. Mixed with the regolith and partially excavated by younger impacts, the material is exposed as individual layered outcrops in crater walls or ejecta patches, having been uncovered and broken up by the impact. Dark fans on crater walls and dark deposits on crater floors are the result of gravity-driven mass wasting triggered by steep slopes and impact seismicity. The fact that dark material is mixed with impact ejecta indicates that it has been processed together with the ejected material. Some small craters display continuous dark ejecta similar to lunar dark-halo impact craters, indicating that the impact excavated the material from beneath a higher-albedo surface. The asymmetric distribution of dark material in impact craters and ejecta suggests non-continuous distribution in the local subsurface. Some positive-relief dark edifices appear to be impact-sculpted hills with dark material distributed over the hill slopes. Dark features inside and outside of craters are in some places arranged as linear outcrops along scarps or as dark streaks perpendicular to the local topography. The spectral characteristics of the dark material resemble that of Vesta’s regolith. Dark material is distributed unevenly across Vesta’s surface with clusters of all types of dark material exposures. On a local scale, some craters expose or are associated with dark material, while others in the immediate vicinity do not show evidence for dark material. While the variety of surface exposures of dark material and their different geological correlations with surface features, as well as their uneven distribution, indicate a globally inhomogeneous distribution in the subsurface, the dark material seems to be correlated with the rim and ejecta of the older Veneneia south polar basin structure. The origin of the dark material is still being debated, however, the geological analysis suggests that it is exogenic, from carbon-rich low-velocity impactors, rather than endogenic, from freshly exposed mafic material or melt, exposed or created by impacts.

Reference
Jaumann et al. (in press) The Geological Nature of Dark Material on Vesta and Implicatons for the Subsurface Structure. Icarus
[doi:10.1016/j.icarus.2014.04.035]
Copyright Elsevier

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Tomohiro Ono¹, Hideko Nomura² and Taku Takeuchi²

¹Department of Astronomy, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
²Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8551, Japan

We analytically calculate the marginally stable surface density profile for the rotational instability of protoplanetary disks. The derived profile can be utilized for considering the region in a rotating disk where radial pressure gradient force is comparable to the gravitational force, such as an inner edge, steep gaps or bumps, and an outer region of the disk. In this paper, we particularly focus on the rotational instability in the outer region of disks. We find that a protoplanetary disk with a surface density profile of similarity solution becomes rotationally unstable at a certain radius, depending on its temperature profile and a mass of the central star. If the temperature is relatively low and the mass of the central star is high, disks have rotationally stable similarity profiles. Otherwise, deviation from the similarity profiles of surface density could be observable, using facilities with high sensitivity, such as ALMA.

Reference
Ono T, Nomura H and Takeuchi T (2014) Rotational Instability in the Outer Region of Protoplanetary Disks. The Astrophysical Journal 787:37.
[doi:10.1088/0004-637X/787/1/37]

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