Matthias M. M. Meier^1,2, Birger Schmitz², Carl Alwmark¹, Reto Trappitsch³, Colin Maden⁴, Rainer Wieler⁴

¹Lund University, Department of Geology, Lund, Sweden
²Department of Physics, Lund University, Lund, Sweden
³Department of the Geophysical Sciences, University of Chicago and Chicago Center for Cosmochemistry, Chicago, Illinois, USA
⁴Department of Earth Sciences, ETH Zurich, Zurich, Switzerland

We analyzed He and Ne in chromite grains from the regolith breccia Ghubara (L5), to compare it with He and Ne in sediment-dispersed extraterrestrial chromite (SEC) grains from mid-Ordovician sediments. These SEC grains arrived on Earth as micrometeorites in the aftermath of the L chondrite parent body (LCPB) breakup event, 470 Ma ago. A significant fraction of them show prolonged exposure to galactic cosmic rays for up to several 10 Ma. The majority of the cosmogenic noble gases in these grains were probably acquired in the regolith of the LCPB (Meier et al. ). Ghubara, an L chondritic regolith breccia with an Ar-Ar shock age of 470 Ma, is a sample of that regolith. We find cosmic-ray exposure ages of up to several 10 Ma in some Ghubara chromite grains, confirming for the first time that individual chromite grains with such high exposure ages indeed existed in the LCPB regolith, and that the >10 Ma cosmic-ray exposure ages found in recent micrometeorites are thus not necessarily indicative of an origin in the Kuiper Belt. Some Ghubara chromite grains show much lower concentrations of cosmogenic He and Ne, indicating that the 4π (last-stage) exposure age of the Ghubara meteoroid lasted only 4–6 Ma. This exposure age is considerably shorter than the 15–20 Ma suggested before from bulk analyses, indicating that bulk samples have seen regolith pre-exposure as well. The shorter last-stage exposure age probably links Ghubara to a small peak of ⁴⁰Ar-poor L5 chondrites of the same exposure age. Furthermore, and quite unexpectedly, we find a Ne component similar to presolar Ne-HL in the chromite grains, perhaps indicating that some presolar Ne can be preserved even in meteorites of petrologic type 5.

Reference
Meier MMM, Schmitz B, Alwmark C, Trappitsch R, Maden C and Wieler R (in press) He and Ne in individual chromite grains from the regolith breccia Ghubara (L5): Exploring the history of the L chondrite parent body regolith.. Meteoritics & Planetary Science
[doi:10.1111/maps.12275]
Published by arrangement with John Wiley & Sons

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Gonzalo Tancredi

Departamento de Astronomí a, Facultad de Ciencias, Iguá 4225, 11400. Montevideo, URUGUAY

The classification criterion between asteroids and comets has evolved in recent decades, but the main distinction remains unchanged. Comets present gas and dust ejection from the surface at some point of their orbits, therefore, these objects are considered to be active. On the other hand, asteroids do not show any kind of large scale gas and dust ejection, they are inert. Nevertheless, this classification scheme is impractical when we have more than 500,000 asteroids already discovered. In addition, comets are not active all along their orbits. In order for a comet to display activity at present or in the recent past in the inner region of the Solar System (heliocentric distance <2AU), the cometary orbit must be unstable in the time scale on the order of ten thousands of years; otherwise, the object should have completely consumed its volatile component. Close encounters with the most massive planets is the only mechanism that could produce ”macroscopic” instabilities on a short time scale. The macroscopic changes in the orbital elements can be detected in a numerical integration of the dynamical evolution of the object over a time scale of several thousand years. This procedure to identify asteroids in cometary-like orbits is also impractical because it would require months of computing time. Therefore, a classification scheme based on the orbital elements to identify the border cases between the asteroid and comet populations is urgently required.
We present a criterion to classify asteroids and comets and to find the border case based on the Tisserand’s parameter, the Minimum Orbital Intersection Distance (MOID), and considering some information regarding the aphelion and perihelion distances. Objects in mean-motion are disregarded. After applying a filter to the sample of over half a million asteroids already discovered to select the precise orbits and to the sample of 487 short-period comets, we apply the proposed classification criterion. The resulting sample consists of ∼331 Asteroids in Cometary Orbits (ACOs). The ACOs are further classified in subclasses similar to the cometary classification. There are 436 Jupiter Family Comets and 203 ACOs of the Jupiter Family type. This new criterion is more strict that the criteria used by other authors to identify ACOs; nonetheless, with the new criterion we ensure that the ACOs have a chaotic dynamical evolution similar to the periodic comets. The discovered dormant or extinct comets seems, if they exist at all, to be a small fraction of the active comets.
We also analyse the available photometric data of ACOs to identify possible large brightness variations. Among the sample of ACOs, there is only one object with brightness variations typical of an active comet: 174P/(60558) Echeclus. But this object has already been double classified as asteroid and comet.

Reference
Tancredi G (in press) A criterion to classify asteroids and comets based on the orbital parameters. Icarus
[doi:10.1016/j.icarus.2014.02.013]
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Lauren A. Edgar^a, John P. Grotzinger^b, James F. Bell III^a and Joel A. Hurowitz^c

^aSchool of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
^bDivision of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
^cStony Brook University, Stony Brook, NY, USA

En route to Endeavour crater, the Mars Exploration Rover Opportunity embarked on a short but significant campaign at Santa Maria crater during sols 2450-2551. Santa Maria crater is a relatively young impact crater, approximately 100 m in diameter and 11-17 m deep. Opportunity performed detailed analyses on several ejecta blocks and completed an extensive imaging campaign around the crater. Many of the ejecta blocks are composed of sandstone with abundant wind ripple laminations suggestive of eolian deposition. However, other ejecta blocks are massive, fine-grained, and exhibit a nodular texture. These rocks are interpreted to be the first rocks of a grain size smaller than the Microscopic Imager can resolve, and may represent the first mudstones observed by the rover. Several depositional environments are considered for the origin of the fine-grained rocks, and the observations are best fit by a transient evaporitic lake. If the inferred mudstones were deposited in a lacustrine setting, then surface water may have been present in a broader range of surface environments than previously documented at Meridiani Planum.

Reference
Edgar LA, Grotzinger JP, Bell III JJ and Hurowitz JA (in press) Hypotheses for the origin of fine-grained sedimentary rocks at Santa Maria Crater, Meridiani Planum. Icarus
[doi:10.1016/j.icarus.2014.02.019]
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Kathryn A. Dyl^a,b, James S. Cleverley^b, Phil A. Bland^a, Chris G. Ryan^b, Louise A. Fisher^b and Robert M. Hough^b

^aDepartment of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
^bCSIRO Earth Sciences and Resource Engineering, 26 Dick Perry Avenue, Kensington, Perth, WA 6151, Australia

We present the application of a new synchrotron-based technique for rapid mapping of trace element distributions across large areas of the CV3 meteorites Allende and Vigarano. This technique utilizes the Australian Synchrotron X-ray Fluorescence Microscopy (XFM) beam line with its custom designed and built X-ray detector array called Maia. XFM with Maia allows data to be collected using a 2 μm spot size at very low dwell times (~0.1-0.5 ms), resulting in maps of entire thin sections in ~5 hours. Maia is an energy dispersive detector system with a large collection solid-angle, which allows full spectral acquisition and high sensitivity. Hence, there is no need to constrain the elements of interest a priori.
We collected whole section maps (~2 cm x 1 cm) from 3 thick sections of Allende and a single map (2 cm x 1.5 cm) from a thick section of Vigarano. Our experimental conditions provide data for elements with 20 ⩽ Z ⩽ 40 (K-shell, Ca through Zr) and the L-emissions of Os, Ir, Pt, Au, and Pb. We illustrate the unique capabilities of this technique by presenting observations across myriad length scales, from the centimeter-scale down to the detection of sub-micrometer particles within these objects. Our initial results show the potential of this technique to help decipher spatial and textural variations in trace element chemistry between CAIs, chondrules, matrix, and other chondritic components. We also illustrate how these datasets can be applied to understanding both nebular and parent-body processes within meteorites.

Reference
Dyl KA, Cleverley JS, Bland PA, Ryan CG, Fisher LA and Hough RM (in press) Quantified, whole section trace element mapping of carbonaceous chondrites by Synchrotron X-ray fluorescence microscopy: 1. CV meteorites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.020]
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L.J. Hallis^a,b, H.A. Ishii^c, J.P. Bradley^c and G.J. Taylor^a,b

^aNASA Astrobiology Institute, Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Drive, Honolulu, Hawaii 96822-1839, United States
^bHawai’i Institute of Geophysics and Planetology, Pacific Ocean Science and Technology (POST) Building, University of Hawai’i, 1680 East-West Road, Honolulu, HI 96822, United States
^cInstitute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, L-415, Livermore, CA 94550, United States

The nakhlite group of martian meteorites found in the Antarctic contain varying abundances of both martian and terrestrial secondary alteration phases. The aim of this study was to use transmission electron microscopy (TEM) to compare martian and terrestrial alteration embodied within a single nakhlite martian meteorite find – MIL 090032. Martian alteration veins in MIL 090032 are composed of poorly ordered Fe-smectite phyllosilicate. This poorly-ordered smectite appears to be equivalent to the nanocrystalline phyllosilicate/hydrated amorphous gel phase previously described in the martian alteration veins of other nakhlites. Chemical differences in this nanocrystalline phyllosilicate between different nakhlites imply localised alteration, which occurred close to the martian surface in MIL 090032. Both structurally and compositionally the nakhlite nanocrystalline phyllosilicate shows similarities to the amorphous/poorly ordered phase recently discovered in martian soil by the Mars Curiosity Rover at Rocknest, Gale Crater.
Terrestrially derived alteration phases in MIL 090032 include jarosite and gypsum, amorphous silicates, and Fe-oxides and hydroxides. Similarities between the mineralogy and chemistry of the MIL 090032 terrestrial and martian alteration phases suggest the alteration conditions on Mars were similar to those in the Antarctic. At both sites a small amount of fluid at low temperatures infiltrated the rock and became acidic as a result of the conversion of Fe²⁺ to Fe³⁺ under oxidising conditions.

Reference
Hallis LJ, Ishii HA, Bradley JP and Taylor GJ (in press) Transmission Electron Microscope Analyses of Alteration Phases in Martian Meteorite MIL090032. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.007]
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Tsuyoshi Iizuka^a,b, Yuri Amelin^b, Angela Kaltenbach^c, Piers Koefoed^b and Claudine H. Stirling^c

^aDepartment of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
^bResearch School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
^cCentre for Trace Element Analysis and Department of Chemistry, University of Otago, PO Box 56, Union Place, Dunedin, New Zealand

Ibitira is an unbrecciated, equilibrated vesicular basaltic achondrite that is considered to have originated on a parent body distinct from all other known meteorites. We present the first combined high-precision U and Pb isotopic data for this unique meteorite. The ²³⁸U/²³⁵U value of 137.777 ± 0.013 determined for the whole rock is comparable to values determined for bulk chondrites and other basaltic achondrites. This value results in corrections of -1.1 Ma for Pb-Pb dates calculated using the previously assumed invariant ²³⁸U/²³⁵U value of 137.88. Using the determined ²³⁸U/²³⁵U value, the 7 most radiogenic Pb isotopic analyses for acid-leached pyroxene-rich and whole rock fractions yield an isochron Pb-Pb age of 4556.75 ± 0.57 Ma, in excellent agreement with the results of Mn-Cr chronology which give the ages of 4557.4 ± 2.5 Ma and 4555.9 ± 3.2 Ma using the U-corrected Pb-Pb age of D’Orbigny as a time anchor. Along with the previously proposed thermal history of Ibitira and our closure temperature estimates for Pb diffusion, the Pb-Pb age is interpreted as the timing of the last chemical equilibration and coarse pyroxene exsolution that occurred during high temperature metamorphism. The metamorphism may have been caused by burial of Ibitira lava under successive lava flows and, if so, the Pb-Pb age should post-date the crystallization by a short time interval. The Pb isotopic data for acid leachates suggest partial re-equilibration of Pb between plagioclase and phosphate, perhaps during an impact event at 4.49 Ga, as recorded by K-Ar systematics. The whole rock²³⁸U/²⁰⁴Pb indicates that compared to CI chondrites, Ibitira is less depleted in Pb than in some alkali elements despite a lower condensation temperature of Pb than the alkali elements. The restricted Pb depletion may reflect preferential concentration of metals with high fluid/melt partition coefficients including Pb and Zn as a result of fluid exsolution and migration within the parent magma. We discuss the implications of the U-Pb systematics for the origin and differentiation of the parent body.

Reference
Iizuka T, Amelin Y, Kaltenbach A, Koefoed P and Stirling CH (in press) U-Pb systematics of the unique achondrite Ibitira: Precise age determination and petrogenetic implications. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.017]
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Tetsuo Yamamoto¹, Toshihiko Kadono², and Koji Wada³

¹Center for Planetary Science, Integrated Research Center of Kobe University, Minatojima, Chuo-ku, Kobe 650-0047, Japan
²School of Medicine, University of Occupational and Environmental Health, Yahatanishi-ku, Kitakyushu, Fukuoka 807-8555, Japan
³Planetary Exploration Research Center, Chiba Institute of Technology, Tsudanuma 2-17-1, Chiba 275-0016, Japan

N-body simulations of collisions of dust aggregates in protoplanetary disks performed so far have revealed that silicate aggregates suffer from catastrophic disruption if the collision velocities are higher than about 10 m s^-1, which is much lower than those expected in the disks. This is mainly due to the low surface energy of the quartz used in the simulations. We find a simple relation between the surface energy and melting temperature for various materials including those of astrophysical interest, and show that the surface energy of the quartz used in the previous simulations is much lower than the present estimate. This result may provide a way out of the difficulty of growing silicate dust inside the snowline in disks. We show that silicate dust can evade catastrophic disruption and grow even at high-velocity collisions expected in the disks if one takes the present estimate of the surface energy into account.

Reference
Yamamoto T, Kadono T and Wada K (2014) An examination of collisional growth of silicate dust in protoplanetary disks. The Astrophysical Journal – Letters 784:L36.
[doi:10.1088/2041-8205/783/2/L36]

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A. Chanou¹, G. R. Osinski^1,2, R. A. F. Grieve¹

¹Department of Earth Sciences and the Centre for Planetary Science and Exploration, University of Western Ontario, London, Ontario, Canada
²Department of Physics and Astronomy, University of Western Ontario, London, Ontario, Canada

A semi-automated digital image analysis method is developed for the comparative textural study of impact melt-bearing breccias. This method uses the freeware software ImageJ developed by the National Institute of Health (NIH). Digital image analysis is performed on scans of hand samples (10–15 cm across), based on macroscopic interpretations of the rock components. All image processing and segmentation are done semi-automatically, with the least possible manual intervention. The areal fraction of components is estimated and modal abundances can be deduced, where the physical optical properties (e.g., contrast, color) of the samples allow it. Other parameters that can be measured include, for example, clast size, clast-preferred orientations, average box-counting dimension or fragment shape complexity, and nearest neighbor distances (NnD). This semi-automated method allows the analysis of a larger number of samples in a relatively short time. Textures, granulometry, and shape descriptors are of considerable importance in rock characterization. The methodology is used to determine the variations of the physical characteristics of some examples of fragmental impactites

Reference
Chanou A, Osinski GR and Grieve RAF (in press) A methodology for the semi-automatic digital image analysis of fragmental impactites. Meteoritics & Planetary Science
[doi:10.1111/maps.12267]
Published by arrangement with John Wiley & Sons

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Agata Krzesińska¹, Jörg Fritz²

¹Institute of Geological Sciences, Polish Academy of Sciences INGPAN, Wrocław, Poland
²Museum für Naturkunde, Berlin, Germany

The Pułtusk meteorite is a brecciated H4–5 chondrite cut by darkened cataclastic zones. Within the breccia, relict type IA, IB, and IIA chondrules, and microxenoliths of carbonaceous CM chondrite lithology occur. This is the first description of foreign clasts in the Pułtusk meteorite. The matrix of the xenoliths was identified by usage of microprobe and Raman spectroscopic analyses. Raman spectra show distinct bands related to the presence of slightly ordered carbonaceous matter at approximately 1320 and 1580–1584 cm⁻¹. Bands related to serpentine group minerals are also visible, especially a peak at 692 cm⁻¹ and moreover other weak bands are interpreted as evidence for tochilinite. We decipher the metamorphic and deformational history of the xenoliths. They experienced aqueous alteration before being incorporated into the unaltered and well-equilibrated parent rock of the Pułtusk chondrite. The xenoliths are weakly shocked as indicated by defects in the crystal structure of silicates and carbonates, but hydrated minerals (serpentine and tochilinite) are still present in the matrix. The carbonaceous matter within the clasts’ matrix displays first order D and G Raman bands that suggests it is only slightly ordered as a result of mild thermal processing. Distinct shear bands are present in both the xenoliths and the surrounding rock, which testifies that the xenoliths were affected by a deformational event along with host rock. The host rock was brittly deformed, but the clasts experienced more ductile deformation revealed by semibrittle faulting of minerals, kinking of the tochilinite-cronstedtite matrix, and injections of xenolithic material into the adjacent breccia. We argue that both processes, the high strain-rate shear deformation and the incorporation of the xenoliths into the host Pułtusk breccia, could have been impact-related. The Pułtusk xenoliths are, thus, rather spalled collisional fragments, than trapped fossil micrometeorites.

Reference
Krzesińska A and Fritz J (in press) Weakly shocked and deformed CM microxenoliths in the Pułtusk H chondrite. Meteoritics & Planetary Science
[doi:10.1111/maps.12276]
Published by arrangement with John Wiley & Sons

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F. Marchis¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Carl Sagan Center at the SETI Institute, Mountain View, CA 94043, USA

Asteroids with satellites are natural laboratories to constrain the formation and evolution of our solar system. The binary Trojan asteroid (624) Hektor is the only known Trojan asteroid to possess a small satellite. Based on W. M. Keck adaptive optics observations, we found a unique and stable orbital solution, which is uncommon in comparison to the orbits of other large multiple asteroid systems studied so far. From lightcurve observations recorded since 1957, we showed that because the large Req = 125 km primary may be made of two joint lobes, the moon could be ejecta of the low-velocity encounter, which formed the system. The inferred density of Hektor’s system is comparable to the L5 Trojan doublet (617) Patroclus but due to their difference in physical properties and in reflectance spectra, both captured Trojan asteroids could have a different composition and origin.

Reference
Marchis et al. (2014) The puzzling mutual orbit of the binary trojan asteroid (624) Hektor. The Astrophysical Journal – Letters 784:L37.
[doi:10.1088/2041-8205/783/2/L37]

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