Sachiko Amari^1,*, Jun-ichi Matsuda², Rhonda M. Stroud³, and Matthew F. Chisholm⁴

¹McDonnell Center for the Space Sciences and the Physics Department, Washington University, St. Louis, MO 63130, USA
²Department of Earth and Space Science, Osaka University, Osaka 560-0043, Japan
³Code 6360, Naval Research Laboratory, Washington, DC 20375, USA
⁴Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

The majority of heavy noble gases (Ar, Kr, and Xe) in primitive meteorites are stored in a poorly understood phase called Q. Although Q is thought to be carbonaceous, the full identity of the phase has remained elusive for almost four decades. In order to better characterize phase Q and, in turn, the early solar nebula, we separated carbon-rich fractions from the Saratov (L4) meteorite. We chose this meteorite because Q is most resistant in thermal alteration among carbonaceous noble gas carriers in meteorites and we hoped that, in this highly metamorphosed meteorite, Q would be present but not diamond: these two phases are very difficult to separate from each other. One of the fractions, AJ, has the highest ¹³²Xe concentration of 2.1 × 10^–6 cm³ STP g^–1, exceeding any Q-rich fractions that have yet been analyzed. Transmission electron microscopy studies of the fraction AJ and a less Q-rich fraction AI indicate that they both are primarily porous carbon that consists of domains with short-range graphene orders, with variable packing in three dimensions, but no long-range graphitic order. The relative abundance of Xe and C atoms (6:10⁹) in the separates indicates that individual noble gas atoms are associated with only a minor component of the porous carbon, possibly one or more specific arrangements of the nanoparticulate graphene.

Reference
Amari S, Matsuda J-I, Stroud RM and Chisholm MF (2013) Highly Concentrated Nebular Noble Gases in Porous Nanocarbon Separates from the Saratov (L4) Meteorite. The Astrophysical Journal 778:37.
[doi:10.1088/0004-637X/778/1/37]

Link to Article

S. Richard¹, P. Barge¹ and S. Le Dizès²

¹Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France
²Aix-Marseille Université, CNRS, Centrale Marseille, IRPHE (Institut de Recherche sur les Phénomènes Hors Equilibre), UMR 7342, 49 rue F. Joliot Curie, 13013 Marseille, France

Context. Large-scale persistent vortices could play a key role in the evolution of protoplanetary disks, particularly in the dead zone where no turbulence associated with a magnetic field is expected. These vortices are known to form easily in 2D disks via the Rossby wave or the baroclinic instability. In three dimensions, however, their formation and stability is a complex problem and still a matter of debate.
Aims. We study the formation of vortices by the Rossby wave instability in a stratified inviscid disk and describe their 3D structure, stability, and long-term evolution.
Methods. Numerical simulations were performed using a fully compressible hydrodynamical code based on a second-order finite volume method. We assumed a perfect-gas law and a non-homentropic adiabatic flow.
Results. The Rossby wave instability is found to proceed in 3D in a similar way as in 2D. Vortices produced by the instability look like columns of vorticity in the whole disk thickness; the weak vertical motions are related to the weak inclination of the vortex axis that appears during the development of the RWI. Vortices with aspect ratios higher than 6 are unaffected by the elliptical instability. They relax into a quasi-steady columnar structure that survives hundreds of rotations while slowly migrating inward toward the star at a rate that reduces with the vortex aspect ratio. Vortices with a lower aspect ratio are by contrast affected by the elliptic instability. Short aspect ratio vortices (χ < 4) are completely destroyed in a few orbital periods. Vortices with an intermediate aspect ratio (4 < χ < 6) are partially destroyed by the elliptical instability in a region away from the midplane where the disk stratification is sufficiently strong.
Conclusions. Elongated Rossby vortices can survive many orbital periods in protoplanetary disks in the form of vorticity columns. They could play a significant role in the evolution of the gas and the gathering of solid particles to form planetesimals or planetary cores, a possibility that receives a renewed interest with the recent discovery of a particle trap in the disk of Oph IRS 48.

Reference
Richard S, Barge P and Le Dizès S (2013) Structure, stability, and evolution of 3D Rossby vortices in protoplanetary disks. Astronomy & Astrophysics 559:A30.
[doi:10.1051/0004-6361/201322175]
Reproduced with permission © ESO

Link to Article

Corentin Le Guillou^a,b,* and Adrian Brearley^a

^aDepartment of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA
^bPresent address: Department of geology, mineralogy and geophysics, Ruhr-Universität Bochum, Universität Strasse 150, 44780 Bochum, Germany

In order to investigate the nature of organics at the time of their accretion into chondrite parent bodies, as well as their subsequent evolution with aqueous alteration, we have conducted a study of the morphologies, spatial distribution and relationships between organic particles and the surrounding matrix phases in one of the most pristine carbonaceous chondrites currently known (MET 00426, CR), which is therefore likely to contain the best preserved record of pre-accretion features. Focused ion beam sections were extracted for transmission electron microscope observations from its matrix.
Organic matter (OM) shows a heterogeneous population of grains, most of them smaller than 1 micron. Diverse morphologies are observed, such as compact, rounded aggregates, individual and aggregated nanoglobules, and micron- to nanometer-sized veins. A common feature is the systematic presence of cracks connected to the grains and filled with OM. The surrounding matrix groundmass consists of amorphous iron-rich silicate particles intimately mixed with phyllosilicates, sulphides and occasional tochilinite, with sizes ranging from several hundreds of nanometers to below 10 nm. A close spatial relationship is commonly observed between some of the organic matter particles and alteration phases, such as tochilinite and phyllosilicates. Phyllosilicates sometimes occur intimately intercalated with organic matter at a scale below 10 nm.
We used TEM/EDS techniques to quantify the water concentration in the matrix amorphous silicate material and the phyllosilicates.The water contents of both materials are identical at 10 (± 6) wt.% H₂O and demonstrate, that the amorphous silicate material in this meteorite is hydrated. Therefore, even though these CR3.0 chondrites are the least altered objects from a mineralogical point of view, their matrices contain significant amounts of water in the amorphous silicate. This coupled in situ study of organics and aqueous alteration suggests that a significant population of the OM accreted as a mixture of soluble and insoluble molecules together with water ice grains and that the OM was mobile at the micrometer scale. The spatial distribution of the OM grains can therefore, in part, be attributed to parent body processes. We suggest that as accreted ice melted, hydration of the amorphous silicates and formation of tochilinite and phyllosilicates occurred in the immediate vicinity of the composite water ice / organic matter grains. The water-soluble component of the organics was likely transported and redistributed in the surrounding porosity (cracks, grain boundaries) as water circulated. The textural settings suggest that some of the OM material could have been polymerized during aqueous alteration and transformed into insoluble molecules, perhaps during the last stages of alteration as water was consumed by silicate hydration reactions.

Reference
Le Guillou C and Brearley A (in press) Relationships between organics, water and early stages of aqueous alteration in the pristine CR3.0 chondrite MET 00426. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.10.024]
Copyright Elsevier

Link to Article

A. Seizinger¹, R. Speith² and W. Kley¹

¹Institut für Astronomie and Astrophysik, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
²Physikalisches Institut, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 14, 72076 Tübingen, Germany

Context. Within the sequential accretion scenario of planet formation, planets are built up through a sequence of sticking collisions. The outcome of collisions between porous dust aggregates is very important for the growth from very small dust particles to planetesimals. In this work we determine the necessary material properties of dust aggregates as a function of porosity.
Aims. Continuum models such as SPH that are capable of simulating collisions of macroscopic dust aggregates require a set of material parameters. Some of them, such as the tensile and shear strength, are difficult to obtain from laboratory experiments. The aim of this work is to determine these parameters from ab initio molecular dynamics simulations.
Methods. We simulated the behavior of porous dust aggregates using a detailed micro-physical model of the interaction of spherical grains that includes adhesion forces, rolling, twisting, and sliding. Using different methods of preparing the samples, we studied the strength behavior of our samples with varying porosity and coordination number of the material.
Results. For the tensile strength, we can reproduce data from laboratory experiments very well. For the shear strength, there are no experimental data available. The results from our simulations differ significantly from previous theoretical models, which indicates that the latter might not be sufficient to describe porous dust aggregates.
Conclusions. We have provided the functional behavior of tensile and shear strength of porous dust aggregates as a function of the porosity, which can be directly applied to continuum simulations of these objects in planet formation scenarios.

Reference
Seizinger A, Speith R and Kley W (2013) Tensile and shear strength of porous dust agglomerates. Astronomy & Astrophysics
[doi:10.1051/0004-6361/201322046]
Reproduced with permission © ESO

Link to Article

M.M.M. Meier^a, B. Schmitz^b, A. Lindskog^a, C. Maden^c, R. Wieler^c

^aLund University, Department of Geology, Sölvegatan 12, SE-22362 Lund, Sweden
^bLund University, Department of Physics, SE-22100 Lund, Sweden
^cETH Zurich, Department of Earth Sciences, CH-8092 Zurich, Switzerland

We measured the He and Ne concentrations of 50 individual extraterrestrial chromite grains recovered from mid-Ordovician (lower Darriwilian) sediments from the Lynna River section near St. Petersburg, Russia. High concentrations of solar wind-like He and Ne found in most grains indicate that they were delivered to Earth as micrometeoritic dust, while their abundance, stratigraphic position and major element composition indicate an origin related to the L chondrite parent body (LCPB) break-up event, 470 Ma ago. Compared to sediment-dispersed extraterrestrial chromite (SEC) grains extracted from coeval sediments at other localities, the grains from Lynna River are both highly concentrated and well preserved. As in previous work, in most grains from Lynna River, high concentrations of solar wind-derived He and Ne impede a clear quantification of cosmic-ray produced He and Ne. However, we have found several SEC grains poor in solar wind Ne, showing a resolvable contribution of cosmogenic ²¹Ne. This makes it possible, for the first time, to determine robust cosmic-ray exposure (CRE) ages in these fossil micrometeorites, on the order of a few hundred-thousand years. These ages are similar to the CRE ages measured in chromite grains from cm-sized fossil meteorites recovered from coeval sediments in Sweden. As the CRE ages are shorter than the orbital decay time of grains of this size by Poynting-Robertson drag, this suggests that the grains were delivered to Earth through direct injection into an orbital resonance. We demonstrate how CRE ages of fossil micrometeorites can be used, in principle, to determine sedimentation rates, and to correlate the sediments at Lynna River with the fossil meteorite-bearing sediment layers in Sweden. In some grains with high concentrations of solar wind Ne, we nevertheless find a well-resolved cosmogenic ²¹Ne signal. These grains must have been exposed for up to several 10 Ma in the regolith layer of the pre-break-up L chondrite parent body. This confirms an earlier suggestion that such regolith grains should be abundant in sediments deposited shortly after the break-up of the LCPB asteroid.

Reference
Meier MMM, Schmitz B, Lindskog A, Maden C and Wieler R (in press) Cosmic-ray exposure ages of fossil micrometeorites from mid-Ordovician sediments at Lynna River, Russia. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.10.026]
Copyright Elsevier

Link to Article

E. Gnos^1,*, B. A. Hofmann², M. A. Halawani³, Y. Tarabulsi³, M. Hakeem³, M. Al Shanti³, N. D. Greber^2,4, S. Holm⁵, C. Alwmark⁵, R. C. Greenwood⁶, K. Ramseyer⁴

¹Natural History Museum of Geneva, Geneva 6, Switzerland
²Natural History Museum Bern, Bern, Switzerland
³Saudi Geological Survey, Jeddah, Saudi Arabia
⁴Institute of Geological Sciences, University of Bern, Bern, Switzerland
⁵Department of Geology, Lund University, Lund, Sweden
⁶Planetary and Space Sciences, The Open University, Milton Keynes, UK

The very young Wabar craters formed by impact of an iron meteorite and are known to the scientific community since 1933. We describe field observations made during a visit to the Wabar impact site, provide analytical data on the material collected, and combine these data with poorly known information discovered during the recovery of the largest meteorites. During our visit in March 2008, only two craters (Philby-B and 11 m) were visible; Philby-A was completely covered by sand. Mapping of the ejecta field showed that the outcrops are strongly changing over time. Combining information from different visitors with our own and satellite images, we estimate that the large seif dunes over the impact site migrate by approximately 1.0–2.0 m yr⁻¹ southward. Shock lithification took place even at the smallest, 11 m crater, but planar fractures (PFs) and undecorated planar deformation features (PDFs), as well as coesite and stishovite, have only been found in shock-lithified material from the two larger craters. Shock-lithified dune sand material shows perfectly preserved sedimentary structures including cross-bedding and animal burrows as well as postimpact structures such as open fractures perpendicular to the bedding, slickensides, and radiating striation resembling shatter cones. The composition of all impact melt glasses can be explained as mixtures of aeolian sand and iron meteorite. We observed a partial decoupling of Fe and Ni in the black impact glass, probably due to partitioning of Ni into unoxidized metal droplets. The absence of a Ca-enriched component demonstrates that the craters did not penetrate the bedrock below the sand sheet, which has an estimated thickness of 20–30 m.

Reference
Gnos E, Hofmann BA, Halawani MA, Tarabulsi Y, Hakeem M, Al Shanti M, Greber ND, Holm S, Alwmark C, Greenwood RC and Ramseyer K (in press) The Wabar impact craters, Saudi Arabia, revisited. Meteoritics & Planetary Science
[doi:10.1111/maps.12218]
Published by arrangement with John Wiley & Sons

Link to Article

Indrani Banerjee and Banibrata Mukhopadhyay

Department of Physics, Indian Institute of Science, Bangalore 560 012, India

We investigate nucleosynthesis inside the outflows from gamma-ray burst (GRB) accretion disks formed by the Type II collapsars. In these collapsars, massive stars undergo core collapse to form a proto-neutron star initially, and a mild supernova (SN) explosion is driven. The SN ejecta lack momentum, and subsequently this newly formed neutron star gets transformed to a stellar mass black hole via massive fallback. The hydrodynamics and the nucleosynthesis in these accretion disks have been studied extensively in the past. Several heavy elements are synthesized in the disk, and much of these heavy elements are ejected from the disk via winds and outflows. We study nucleosynthesis in the outflows launched from these disks by using an adiabatic, spherically expanding outflow model, to understand which of these elements thus synthesized in the disk survive in the outflow. While studying this, we find that many new elements like isotopes of titanium, copper, zinc, etc., are present in the outflows. ⁵⁶Ni is abundantly synthesized in most of the cases in the outflow, which implies that the outflows from these disks in a majority of cases will lead to an observable SN explosion. It is mainly present when outflow is considered from the He-rich, ⁵⁶Ni/⁵⁴Fe-rich zones of the disks. However, outflow from the Si-rich zone of the disk remains rich in silicon. Although emission lines of many of these heavy elements have been observed in the X-ray afterglows of several GRBs by Chandra, BeppoSAX, XMM-Newton, etc., Swift seems to have not yet detected these lines.

Reference
Banerjee I and Mukhopadhyay B (2013) Nucleosynthesis in the Outflows Associated with Accretion Disks of Type II Collapsars. The Astrophysical Journal 778:8.
[doi:10.1088/0004-637X/778/1/8]

Link to Article

Mark H. Thiemens

Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093-0356

Stable isotope ratio variations are regulated by physical and chemical laws. These rules depend on a relation with mass differences between isotopes. New classes of isotope variation effects that deviate from mass dependent laws, termed mass independent isotope effects, were discovered in 1983 and have a wide range of applications in basic chemistry and nature. In this special edition, new applications of these effects to physical chemistry, solar system origin models, terrestrial atmospheric and biogenic evolution, polar paleo climatology, snowball earth geology, and present day atmospheric sciences are presented.

Reference
Thiemens MH (2013) Introduction to Chemistry and Applications in Nature of Mass Independent Isotope Effects Special Feature. PNAS 110:17631-17637.
[doi:10.1073/pnas.1312926110]

Link to Article

Subrata Chakraborty^a,*, Teresa L. Jackson^a, Musahid Ahmed^b, and Mark H. Thiemens^a

^aDepartment of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093-0356
^bChemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

Select meteoritic classes possess mass-independent sulfur isotopic compositions in sulfide and organic phases. Photochemistry in the solar nebula has been attributed as a source of these anomalies. Hydrogen sulfide (H₂S) is the most abundant gas-phase species in the solar nebula, and hence, photodissociation of H₂S by solar vacuum UV (VUV) photons (especially by Lyman-α radiation) is a relevant process. Because of experimental difficulties associated with accessing VUV radiation, there is a paucity of data and a lack of theoretical basis to test the hypothesis of a photochemical origin of mass-independent sulfur. Here, we present multiisotopic measurements of elemental sulfur produced during the VUV photolysis of H₂S. Mass-independent sulfur isotopic compositions are observed. The observed isotopic fractionation patterns are wavelength-dependent. VUV photodissociation of H₂S takes place through several predissociative channels, and the measured mass-independent fractionation is most likely a manifestation of these processes. Meteorite sulfur data are discussed in light of the present experiments, and suggestions are made to guide future experiments and models.

Reference
Chakraborty S, Jackson TL, Ahmed M and Thiemens MH (2013) Sulfur isotopic fractionation in vacuum UV photodissociation of hydrogen sulfide and its potential relevance to meteorite analysis. PNAS 110:17650-17655.
[doi:10.1073/pnas.1213150110]

Link to Article

Paula Lindgren^a, Mark C. Price^b, Martin R. Lee^a, Mark J. Burchell^b

^aSchool of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK
^bSchool of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NZ, UK

Calcite twinning in carbonaceous chondrite meteorites can be used to reconstruct the deformation history and the parent body environment during and/or after aqueous alteration, but the shock pressure threshold at which the twins develop is unknown. Accordingly, the aim of this study is to determine the magnitude of shock pressure that is needed to generate calcite twins. This was done by measuring the depths of twinning beneath the resulting craters from experimental impacts in six calcite targets, combined with hydrocode modelling of the peak pressures at the corresponding depths within the targets. Brecciation, fracturing and calcite e-twinning occur below the floors of all the craters and results from the hydrocode modelling show that the twins start to form at shock pressures of ~110 to 480 MPa, which is at least a factor of ten higher than the 10 MPa that is considered to be required to produce calcite twins in low strain rate terrestrial settings. These pressures are equivalent to shock stage S1 as determined by olivine microstructures and consistent with calcite twinning in carbonaceous chondrites being a result of impact gardening in shallow levels of their asteroidal parent bodies.

Reference
Lindgren P, Price MC, Lee MR and Burchell MJ (in press) Constraining the pressure threshold of impact induced calcite twinning: Implications for the deformation history of aqueously altered carbonaceous chondrite parent bodies. Earth and Planetary Science Letters 384:71–80.
[doi:10.1016/j.epsl.2013.10.002]
Copyright Elsevier

Link to Article