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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Catherine Walsh^1,2, Tom. J. Millar², Hideko Nomura^3,4,5, Eric Herbst^6,7, Susanna Widicus Weaver⁸, Yuri Aikawa⁹, Jacob C. Laas⁸ and Anton I. Vasyunin^10,11

¹Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN, UK
³Department of Astronomy, Graduate School of Science, Kyoto University, 606-8502 Kyoto, Japan
⁴National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588, Japan
⁵Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, 152-8551 Tokyo, Japan
⁶Departments of Physics, Chemistry and Astronomy, The Ohio State University, Columbus OH 43210, USA
⁷Departments of Chemistry, Astronomy, and Physics, University of Virginia, Charlottesville VA 22904, USA
⁸Department of Chemistry, Emory University, Atlanta GA 30322, USA
⁹Department of Earth and Planetary Sciences, Kobe University, 1-1 Rokkodai-cho, Nada, 657-8501 Kobe, Japan
¹⁰Department of Chemistry, University of Virginia, Charlottesville VA 22904, USA
¹¹Visiting Scientist, Ural Federal University, 620075 Ekaterinburg, Russia

Context. Protoplanetary disks are vital objects in star and planet formation, possessing all the material, gas and dust, which may form a planetary system orbiting the new star. Small, simple molecules have traditionally been detected in protoplanetary disks; however, in the ALMA era, we expect the molecular inventory of protoplanetary disks to significantly increase.
Aims. We investigate the synthesis of complex organic molecules (COMs) in protoplanetary disks to put constraints on the achievable chemical complexity and to predict species and transitions which may be observable with ALMA.
Methods. We have coupled a 2D steady-state physical model of a protoplanetary disk around a typical T Tauri star with a large gas-grain chemical network including COMs. We compare the resulting column densities with those derived from observations and perform ray-tracing calculations to predict line spectra. We compare the synthesised line intensities with current observations and determine those COMs which may be observable in nearby objects. We also compare the predicted grain-surface abundances with those derived from cometary comae observations.
Results. We find COMs are efficiently formed in the disk midplane via grain-surface chemical reactions, reaching peak grain-surface fractional abundances ~10^-6–10^-4 that of the H nuclei number density. COMs formed on grain surfaces are returned to the gas phase via non-thermal desorption; however, gas-phase species reach lower fractional abundances than their grain-surface equivalents, ~10^-12–10^-7. Including the irradiation of grain mantle material helps build further complexity in the ice through the replenishment of grain-surface radicals which take part in further grain-surface reactions. There is reasonable agreement with several line transitions of H₂CO observed towards T Tauri star-disk systems. There is poor agreement with HC₃N lines observed towards LkCa 15 and GO Tau and we discuss possible explanations for these discrepancies. The synthesised line intensities for CH₃OH are consistent with upper limits determined towards all sources. Our models suggest CH₃OH should be readily observable in nearby protoplanetary disks with ALMA; however, detection of more complex species may prove challenging, even with ALMA “Full Science” capabilities. Our grain-surface abundances are consistent with those derived from cometary comae observations providing additional evidence for the hypothesis that comets (and other planetesimals) formed via the coagulation of icy grains in the Sun’s natal disk.

Reference
Walsh C, Millar TJ, Nomura H, Herbst E, Weaver SW, Aikawa Y, Laas JC and Vasyunin AI (2014) Complex organic molecules in protoplanetary disks. Astronomy & Astrophysics 563:A33.
[doi:10.1051/0004-6361/201322446]
Reproduced with permission © ESO

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Sandra Pizzarello¹, Laurence A. J. Garvie²

¹Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona, USA
²Center for Meteorite Studies, Arizona State University, Tempe, Arizona, USA

Dicarboxylic acids were searched for in three Sutter’s Mill (SM) fragments (SM2 collected prerain, SM12, and SM41) and found to occur almost exclusively as linear species of 3- to 14-carbon long. Between these, concentrations were low, with measured quantities typically less than 10 nmole g⁻¹ of meteorite and a maximum of 6.8 nmole g⁻¹ of meteorite for suberic acid in SM12. The SM acids’ molecular distribution is consistent with a nonbiological origin and differs from those of CMs, such as Murchison or Murray, and of some stones of the C2-ungrouped Tagish Lake meteorite, where they are abundant and varied. Powder X-ray diffraction of SM12 and SM41 show them to be dominated by clays/amorphous material, with lesser amounts of Fe-sulfides, magnetite, and calcite. Thermal gravimetric (TG) analysis shows mass losses up to 1000 °C of 11.4% (SM12) and 9.4% (SM41). These losses are low compared with other clay-rich carbonaceous chondrites, such as Murchison (14.5%) and Orgueil (21.1%). The TG data are indicative of partially dehydrated clays, in accordance with published work on SM2, for which mineralogical studies suggest asteroidal heating to around 500 °C. In view of these compositional traits and mineralogical features, it is suggested that the dicarboxylic acids observed in the SM fragments we analyzed likely represent a combination of molecular species original to the meteorite as well as secondary products formed during parent-body alteration processes, such as asteroidal heating.

Reference
Pizzarello S and Garvie LAJ (in press) Sutter’s Mill dicarboxylic acids as possible tracers of parent-body alteration processes. Meteoritics & Planetary Science
[doi:10.1111/maps.12264]
Published by arrangement with John Wiley & Sons

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K. H. Joy¹, I. A. Crawford^2,3, G. R. Huss⁴, K. Nagashima⁴ and G. J. Taylor⁴

¹School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK
²Department of Earth and Planetary Sciences, Birkbeck, University of London, Bloomsbury, London, UK
³The Centre for Planetary Sciences at UCL/Birkbeck, London, UK
⁴Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i, USA

Lunar meteorite MacAlpine Hills (MAC) 88105 is a well-studied feldspathic regolith breccia dominated by rock and mineral fragments from the lunar highlands. Thin section MAC 88105,159 contains a small rock fragment, 400 × 350 μm in size, which is compositionally anomalous compared with other MAC 88105 lithic components. The clast is composed of olivine and plagioclase with minor pyroxene and interstitial devitrified glass component. It is magnesian, akin to samples in the lunar High Mg-Suite, and also alkali-rich, akin to samples in the lunar High Alkali Suite. It could represent a small fragment of late-stage interstitial melt from an Mg-Suite parent lithology. However, olivine and pyroxene in the clast have Fe/Mn ratios and minor element concentrations that are different from known types of lunar lithologies. As Fe/Mn ratios are notably indicative of planetary origin, the clast could either (1) have a unique lunar magmatic source, or (2) have a nonlunar origin (i.e., consist of achondritic meteorite debris that survived delivery to the lunar surface). Both hypotheses are considered and discussed.

Reference
Joy KH, Crawford IA, Huss GR, Nagashima K and Taylor GJ (in press) An unusual clast in lunar meteorite MacAlpine Hills 88105: A unique lunar sample or projectile debris? Meteoritics & Planetary Science
[doi:10.1111/maps.12270]
Published by arrangement with John Wiley & Sons

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Jacob B. Simon^1,2,4 and Philip J. Armitage^2,3

¹Department of Space Studies, Southwest Research Institute, Boulder, CO 80302, USA
²JILA, University of Colorado and NIST, 440 UCB, Boulder, CO 80309-0440, USA
³Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309, USA
⁴Sagan Fellow.

We investigate the strength of axisymmetric local pressure maxima (zonal flows) in the outer regions of protoplanetary disks, where ambipolar diffusion reduces turbulent stresses driven by the magnetorotational instability. Using local numerical simulations we show that in the absence of net vertical magnetic fields, the strength of turbulence in the ambipolar dominated region of the disk is low and any zonal flows that are present are weak. For net fields strong enough to yield observed protostellar accretion rates, however, zonal flows with a density amplitude of 10%–20% are formed. These strengths are comparable to those seen in simulations of ideal MHD disk turbulence. We investigate whether these zonal flows are able to reverse the inward radial drift of solids, leading to prolonged and enhanced concentration as a prelude to planetesimal formation. For commonly assumed mean surface density profiles (surface density Σ∝r^-1/2 or steeper) we find that the predicted perturbations to the background disk profile do not correspond to local pressure maxima. This is a consequence of radial width of the simulated zonal flows, which is larger than was assumed in prior analytic models of particle trapping. These larger scale flows would only trap particles for higher amplitude fluctuations than observed. We conclude that zonal flows are likely to be present in the outer regions of protoplanetary disks and are potentially large enough to be observable, but are unlikely to lead to strong particle trapping.

Reference
Jacob B. Simon JB and Armitage PJ (2014) Efficiency of particle trapping in the outer regions of protoplanetary disks. The Astrophysical Journal 784:15.
[doi:10.1088/0004-637X/784/1/15]

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S. Cikota¹, J. L. Ortiz², A. Cikota³, N. Morales² and G. Tancredi⁴

¹Physics Department, University of Split, Nikole Tesle 12, 21000 Split, Croatia
²Instituto de Astrofísica de Andalucía – CSIC, Apt 3004, 18008 Granada, Spain
³Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁴Observatorio Astronómico Los Molinos DICYT-MEC Cno. de los Molinos 5769, 12400 Montevideo, Uruguay

It is well known that some Main Belt asteroids show comet-like features. A representative example is the first known Main Belt comet 133P/(7968) Elst-Pizarro. If the mechanisms causing this activity are too weak to develop visually evident comae or tails, the objects stay unnoticed. We are presenting a novel way to search for active asteroids, based on looking for objects with deviations from their expected brightnesses in a database. Just by using the MPCAT-OBS Observation Archive we have found five new candidate objects that possibly show a type of comet-like activity, and the already known Main Belt comet 133P/(7968) Elst-Pizarro. Four of the new candidates, (315) Constantia, (1026) Ingrid, (3646) Aduatiques, and (24 684) 1990 EU4, show brightness deviations independent of the object’s heliocentric distance, while (35 101) 1991 PL16 shows deviations dependent on its heliocentric distance, which could be an indication of a thermal triggered mechanism. The method could be implemented in future sky survey programmes to detect outbursts on Main Belt objects almost simultaneously with their occurrence.

Reference
Cikota S, Ortiz JL, Cikota A, Morales N and Tancredi G (2014) A photometric search for active Main Belt asteroids. Astronomy & Astrophysics 562:A94.
[doi:10.1051/0004-6361/201321679]
Reproduced with permission © ESO

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Alexandra I. Blinova¹, Thomas J. Zega^2,†, Christopher D. K. Herd¹ and Rhonda M. Stroud²

¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
²Materials Science and Technology Division, Naval Research Laboratory, Washington, District of Columbia, USA
^†Department of of Planetary Sciences, Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA

Four samples (TL5b, TL11h, TL11i, and TL11v) from the pristine collection of the Tagish Lake meteorite, an ungrouped C2 chondrite, were studied to characterize and understand its alteration history using EPMA, XRD, and TEM. We determined that samples TL11h and TL11i have a relatively smaller proportion of amorphous silicate material than sample TL5b, which experienced low-temperature hydrous parent-body alteration conditions to preserve this indigenous material. The data suggest that lithic fragments of TL11i experienced higher degrees of aqueous alteration than the rest of the matrix, based on its low porosity and high abundance of coarse- and fine-grained sheet silicates, suggesting that TL11i was present in an area of the parent body where alteration and brecciation were more extensive. We identified a coronal, “flower”-like, microstructure consisting of a fine-grained serpentine core and coarse-grained saponite-serpentine radial arrays, suggesting varied fluid chemistry and crystallization time scales. We also observed pentlandite with different morphologies: an exsolved morphology formed under nebular conditions; a nonexsolved pentlandite along grain boundaries; a “bulls-eye” sulfide morphology and rims around highly altered chondrules that probably formed by multiple precipitation episodes during low-temperature aqueous alteration (≥100 °C) on the parent body. On the basis of petrologic and mineralogic observations, we conclude that the Tagish Lake parent body initially contained a heterogeneous mixture of anhydrous precursor minerals of nebular and presolar origin. These materials were subjected to secondary, nonpervasive parent-body alteration, and the samples studied herein represent different stages of that hydrous alteration, i.e., TL5b (the least altered) < TL11h < TL11i (the most altered). Sample TL11v encompasses the petrologic characteristics of the other three specimens.

Reference
Blinova, A. I., Zega, T. J., Herd, C. D. K. and Stroud, R. M. (in press) Testing variations within the Tagish Lake meteorite—I: Mineralogy and petrology of pristine samples. Meteoritics & Planetary Science
[doi:10.1111/maps.12271]
Published by arrangement with John Wiley & Sons

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Matthias M.M. Meier^a,b and Rainer Wieler^b

^aLund University, Department of Geology, Sölvegatan 12, SE-22362 Lund, Sweden
^bETH Zürich, Department of Earth Sciences, NW-C84, Clausiusstrasse 25, CH-8092 Zürich, Switzerland

It has been argued that the decay rates of several radioactive nuclides are slightly lower at Earth’s aphelion than at perihelion, and that this effect might depend on heliocentric distance. It might then be expected that nuclear decay rates be considerably lower at larger distances from the sun, e.g., in the asteroid belt at 2–3 AU from where most meteorites originate. If so, ages of meteorites obtained by analyses of radioactive nuclides and their stable daughter isotopes might be in error, since these ages are based on decay rates determined on Earth. Here we evaluate whether the large data base on nuclear cosmochronology offers any hint for discrepancies which might be due to radially variable decay rates. Chlorine-36 (t_1/2 = 301,000 a) is produced in meteorites by interactions with cosmic rays and is the nuclide for which a decay rate dependence from heliocentric distance has been proposed, which, in principle, can be tested with our approach and the current data base. We show that compilations of ³⁶Cl concentrations measured in meteorites offer no support for a spatially variable ³⁶Cl decay rate. For very short-lived cosmic-ray produced radionuclides (half-lives < 10–100 days), the concentration should be different for meteorites hitting the Earth on the incoming vs. outgoing part of their orbit. However, the current data base of very short-lived radionuclides in freshly fallen meteorites is far from sufficient to deduce solid constraints. Constraints on the age of the Earth and the oldest meteorite phases obtained by the U–Pb dating technique give no hints for radially variable decay rates of the α-decaying nuclides ²³⁵U or ²³⁸U. Similarly, some of the oldest phases in meteorites have U–Pb ages whose differences agree almost perfectly with respective age differences obtained with “short-lived” radionuclides present in the early solar system, again indicating no variability of uranium decay rates in different meteorite parent bodies in the asteroid belt. Moreover, the oldest U–Pb ages of meteorites agree with the main-sequence age of the sun derived from helioseismology within the formal ∼1% uncertainty of the latter. Meteorite ages also provide no evidence for a decrease of decay rates with heliocentric distance for nuclides such as ⁸⁷Rb (decay mode β⁻) ⁴⁰K (β⁻ and electron capture), and ¹⁴⁷Sm (α).

Reference
Meier MMM and Wieler R (in press) No evidence for a decrease of nuclear decay rates with increasing heliocentric distance based on radiochronology of meteorites. Astroparticle Physics
[doi:10.1016/j.astropartphys.2014.01.004]
Copyright Elsevier

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