Sourav Chatterjee¹ and Jonathan C. Tan²

¹Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
²Departments of Astronomy & Physics, University of Florida, Gainesville, FL 32611, USA

The compact multi-transiting planet systems discovered by Kepler challenge planet formation theories. Formation in situ from disks with radial mass surface density, Σ, profiles similar to the minimum mass solar nebula but boosted in normalization by factors  10 has been suggested. We propose that a more natural way to create these planets in the inner disk is formation sequentially from the inside-out via creation of successive gravitationally unstable rings fed from a continuous stream of small (~cm-m size) “pebbles,” drifting inward via gas drag. Pebbles collect at the pressure maximum associated with the transition from a magnetorotational instability (MRI)-inactive (“dead zone”) region to an inner MRI-active zone. A pebble ring builds up until it either becomes gravitationally unstable to form an ~1 M ⊕ planet directly or induces gradual planet formation via core accretion. The planet may undergo Type I migration into the active region, allowing a new pebble ring and planet to form behind it. Alternatively, if migration is inefficient, the planet may continue to accrete from the disk until it becomes massive enough to isolate itself from the accretion flow. A variety of densities may result depending on the relative importance of residual gas accretion as the planet approaches its isolation mass. The process can repeat with a new pebble ring gathering at the new pressure maximum associated with the retreating dead-zone boundary. Our simple analytical model for this scenario of inside-out planet formation yields planetary masses, relative mass scalings with orbital radius, and minimum orbital separations consistent with those seen by Kepler. It provides an explanation of how massive planets can form with tightly packed and well-aligned system architectures, starting from typical protoplanetary disk properties.

Reference
Chatterjee S and Tan JC (2014) Inside-out Planet Formation. The Astrophysical Journal 780:53.
[doi:10.1088/0004-637X/780/1/53]

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E. Sellentin, J. P. Ramsey, F. Windmark and C. P. Dullemond

Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Überle-Str. 2, Heidelberg 69120, Germany

Context. The growth process of dust particles in protoplanetary disks can be modeled via numerical dust coagulation codes. In this approach, physical effects that dominate the dust growth process often must be implemented in a parameterized form. Due to a lack of these parameterizations, existing studies of dust coagulation have ignored the effects a hydrodynamical gas flow can have on grain growth, even though it is often argued that the flow could significantly contribute either positively or negatively to the growth process.
Aims. We intend to qualitatively describe the factors affecting small particle sweep-up under hydrodynamical effects, followed by a quantification of these effects on the growth of dust particles, such that they can be parameterized and implemented in a dust coagulation code.
Methods. Using a simple model for the flow, we numerically integrate the trajectories of small dust particles in disk gas around a proto-planetesimal, sampling a large parameter space in proto-planetesimal radii, headwind velocities, and dust stopping times.
Results. The gas flow deflects most particles away from the proto-planetesimal, such that its effective collisional cross section, and therefore the mass accretion rate, is reduced. The gas flow however also reduces the impact velocity of small dust particles onto a proto-planetesimal. This can be beneficial for its growth, since large impact velocities are known to lead to erosion. We also demonstrate why such a gas flow does not return collisional debris to the surface of a proto-planetesimal.
Conclusions. We predict that a laminar hydrodynamical flow around a proto-planetesimal will have a significant effect on its growth. However, we cannot easily predict which result, the reduction of the impact velocity or the sweep-up cross section, will be more important. Therefore, we provide parameterizations ready for implementation into a dust coagulation code.

Reference
Sellentin E, Ramsey JP, Windmark F and Dullemond CP (2013) A quantification of hydrodynamical effects on protoplanetary dust growth. Astronomy & Astrophysics 560:A96.
[doi:10.1051/0004-6361/201321587]
Reproduced with permission © ESO

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J.A. Barrat^a, B. Zanda^b, A. Jambon^c, C. Bollinger^d

^aU.B.O.-I.U.E.M., CNRS UMR 66538 (Domaines Océaniques), Place Nicolas Copernic, 29280 Plouzané Cedex, France
^bMuséum National d’Histoire Naturelle, Laboratoire de Minéralogie et de Cosmochimie du Muséum, CNRS UMR7202, 61 rue Buffon, 75005 Paris, France
^cUniversité Pierre et Marie Curie-Paris 6 ISTeP, CNRS UMR 7193, case 110, 4 place Jussieu, 75252 Paris cedex 05, France
^dCNRS UMS 3113, I.U.E.M., Place Nicolas Copernic, 29280 Plouzané Cedex, France

We report on the abundances of a selected set of lithophile trace elements (namely REEs, Y, Rb, Ba, Sr, Zr, Hf, Nb, Th, U) in a comprehensive suite of enstatite chondrites (EC – 13 EH and 11 EL). EH3 and EL3 display only minor deviations from chondritic distributions for these elements. In most metamorphosed EC, a wide range of compositions is observed and suggests a mobility of many of the elements studied during the history of these rocks. For example, EL6 chondrites exhibit light-REE and Nb depletions, negative Eu anomalies, and positive Y anomalies. More important trace element fractionations are observed in metamorphosed EH like St Marks (Rb depletion), LAP 02225 (Rb, Nb, Zr, Eu, light REE depletions) and Galim (b), which displays large Ba, Sr, Eu, Nb and light REE depletions.
Leaching experiments were undertaken to investigate the contributions of sulfides in the whole rock budgets. These phases control not only the REE budget, but also important fractions of the other elements we studied. These fractions strongly depend on the type of the rock (EH or EL, and metamorphic grade). For many elements, the sulfide contributions increase with the metamorphic grades. The trace element abundances of silicate residues are extremely variable. Negative Sm and Yb anomalies are observed in EL3 and EH3 residues, and are certainly the results of early nebular processes. Such anomalies are lacking in residues obtained with most metamorphosed EC, underlining the importance of trace element redistributions during metamorphism. In addition, EL6 residues display distinctive positive Y anomalies that could be potentially ascribed to a less chalcophile behavior than Ho in the conditions that prevailed during EL metamorphism.

Reference
Barrat JA, Zanda B, Jambon A and Bollinger C (in press) The lithophile trace elements in enstatite chondrites. Geochimica et Cosmochimica Acta 98:1966-1971.
[doi:10.1016/j.gca.2013.11.042]
Copyright Elsevier

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Arnold Gucsik¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Department of Geology, University of Johannesburg, Johannesburg, South Africa

Highly forsteritic olivine (Fo: 99.2–99.7) in the Kaba meteorite emits bright cathodoluminescence (CL). CL spectra of red luminescent forsterite grains have two broad emission bands at approximately 630 nm (impurity center of divalent Mn ions) in the red region and above 700 nm (trivalent Cr ions) in the red–IR region. The cores of the grains show CL blue luminescence giving a characteristic broad band emission at 400 nm, also associated with minor red emissions related to Mn and Cr ions. CL color variation of Kaba forsterite is attributed to structural defects. Electron probe microanalyzer (EPMA) analysis shows concentrations of Ca, Al, and Ti in the center of the forsterite grain. The migration of diffusible ions of Mn, Cr, and Fe to the rim of the Kaba meteoritic forsterite was controlled by the hydrothermal alteration at relatively low temperature (estimated at about 250 °C), while Ca and Al ions might still lie in the core. A very unusual phase of FeO (wüstite) was also observed, which may be a terrestrial alteration product of FeNi-metal.

Reference
Gucsik et al. (in press) Cathodoluminescence microscopy and spectroscopy of forsterite from Kaba meteorite: An application to the study of hydrothermal alteration of parent body. Meteoritics & Planetary Science 
[doi:10.1111/maps.12238]
Published by arrangement with John Wiley & Sons

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M. Darby Dyar¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Department of Astronomy, Mount Holyoke College, South Hadley, Massachusetts 01075, U.S.A.

Although Fe-sulfate minerals occur only rarely on Earth as alteration products of sulfidic basalts or in hydrothermal systems, multiple lines of evidence point to the importance of Fe- (and other) sulfate minerals on the surface of Mars. One such martian data set comes from the MIMOS II Mössbauer spectrometers on the Mars Exploration Rovers, which acquired hundreds of spectra from the martian surface at two locations. Interpretation of those spectra has been limited by the lack of a comprehensive set of laboratory analog spectra of the broad range of naturally occurring sulfate minerals. Accordingly, this study reports Mössbauer data of 98 samples representing 47 different sulfate mineral species, all containing six- or higher-coordinated Fe. The resultant Mössbauer parameters are related to the local polyhedral environment around the Fe cation in each mineral to explain variations in spectral characteristics. Results show that the size of the coordination polyhedron is the best predictor of quadrupole splitting, which increases with both octahedral volume and mean bond length. Species within groups of structurally similar minerals are shown to have comparable spectral peaks that generally fall within small ranges. Although coordination polyhedron geometry is not necessarily unique to any particular mineral species or group, Mössbauer data can be used to help constrain mineral identifications from martian spectra. The number of mineral species is large, but the range of crystal structures and hyperfine parameters may be small, so that in many cases, individual minerals cannot be uniquely fingerprinted. Examples would include quenstedtite, coquimbite, kornelite, and lausenite, which have indistinguishable spectra, as do apjohnite, bilinite, dietrichite, and römerite. Overlap of Mössbauer parameters is a particular complication for identification of Fe³⁺-rich phases because the range of Mössbauer parameters for Fe³⁺ in any coordination number is so small. Previous analyses of martian Mössbauer spectra reported the presence of jarosite (Klingelhöfer et al. 2004; Morris et al. 2004) and an unspecific ferric sulfate (Morris et al. 2008). New data presented here indicate that botryogen, metasideronatrite, and slavikite exhibit Mössbauer spectra similar to those attributed to jarosite at Meridiani Planum. Fibroferrite and rhomboclase have parameters similar to those observed at Arad Samra, and copiapite and parabutlerite could be present at Tyrone Mount Darwin and Berkner Island. Unique mineral identifications are generally not possible from Mössbauer data alone, particularly for paramagnetic phases, although combining Mössbauer results with other data sets enables a greater level of confidence in constraining mineralogy. This study provides a new expansive data set for future interpretation of iron sulfates on Mars.

Reference
Dyar et al. (2013) Mössbauer parameters of iron in sulfate minerals. American Mineralogist 98:1943-1965.
[doi:10.2138/am.2013.4604]
Copyright: The Mineralogical Society of America

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Yoram Lithwick

Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA and Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Evanston, IL 60208, USA

When planetesimals begin to grow by coagulation, they first enter an epoch of runaway, during which the biggest bodies grow faster than all the others. The questions of how runaway ends and what comes next have not been answered satisfactorily. We show that runaway is followed by a new stage—the “trans-Hill stage”—that commences when the bodies that dominate viscous stirring (“big bodies”) become trans-Hill, i.e., when their Hill velocity matches the random speed of the small bodies they accrete. Subsequently, the small bodies’ random speed grows in lockstep with the big bodies’ sizes, such that the system remains in the trans-Hill state. Trans-Hill growth is crucial for determining the efficiency of growing big bodies, as well as their growth timescale and size spectrum. Trans-Hill growth has two sub-stages. In the earlier one, which occurs while the stirring bodies remain sufficiently small, the evolution is collisionless, i.e., collisional cooling among all bodies is irrelevant. The efficiency of forming big bodies in this collisionless sub-stage is very low, ~10α  1, where α ~ 0.005(a/AU)^–1 is the ratio between the physical size of a body and its Hill radius. Furthermore, the size spectrum is flat (equal mass per size decade, i.e., q = 4). This collisionless trans-Hill solution explains results from previous coagulation simulations for both the Kuiper Belt and the asteroid belt. The second trans-Hill sub-stage commences once the stirring bodies grow big enough (>α^–1 × the size of the accreted small bodies). After that time, collisional cooling among small bodies controls the evolution. The efficiency of forming big bodies rises and the size spectrum becomes more top heavy. Trans-Hill growth can terminate in one of two ways, depending on the sizes of the small bodies. First, mutual accretion of big bodies can become significant and conglomeration proceeds until half of the total mass is converted into big bodies. This mode of growth may explain the observed size distributions of small bodies in the solar system and is explored in our subsequent work. Second, if the big bodies’ orbits become separated by their Hill radius, oligarchy commences. This mode likely precedes the formation of fully fledged planets.

Reference
Lithwick Y (in press) After Runaway: The Trans-Hill Stage of Planetesimal Growth. The Astrophysical Journal 780:22.
[doi:10.1088/0004-637X/780/1/22]

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F. Fröhlich^1,*, G. Poupeau^1,2, A. Badou¹, F. X. Le Bourdonnec², Y. Sacquin³, S. Dubernet², J. M. Bardintzeff^4,5, M. Véran⁶, D. C. Smith⁷, E. Diemer^†

¹Département de Préhistoire, Muséum National d’Histoire Naturelle, UMR CNRS, Paris, France
²Université de Bordeaux 3, CRP2A – UMR CNRS IRAMAT 5060, Pessac, France
³CEA/Saclay, DSM/Irfu, Gif-sur-Yvette Cedex, France
⁴Laboratoire de Pétrographie-Volcanologie/équipe Planétologie, Université Paris-Sud, UMR CNRS IDES 8148, Orsay Cédex, France
⁵Université de Cergy-Pontoise, IUFM, Cergy-Pontoise, France
⁶Département Histoire de la Terre, Muséum National d’Histoire Naturelle, USM 203, Paris, France
⁷Département Histoire de la Terre, Muséum National d’Histoire Naturelle, UMR CNRS 7202, Paris, France
^† Deceased

Results are presented of new geological observations and laboratory analyses on Libyan Desert Glass (LDG), a unique kind of impact glass found in Egypt, probably 28.5–29.4 million years in age. A new LDG occurrence has been discovered some 50 km southward of the main LDG occurrences in the Great Sand Sea. From Fourier transform infrared (FTIR) analysis, the molecular structure of LDG is refined and significant differences are shown between LDG specimens and other pure silica glasses (fulgurite, industrial fused quartz, and amorphous biogenic silica) that are related to differences in their structures. The slight variations observed here for the mean Si-O-Si angle between the different glasses are attributed to their thermal histories. With regard to the other glasses analyzed, the LDG infrared spectral parameters point to a higher ratio of discontinuities and defects in the tetrahedral (SiO₄) network. The quantitative mineralogical constitutions of sandstones and quartzites from the LDG geological setting were analyzed by FTIR. Cretaceous sandstones have a specific composition (about 90 wt% quartz, 10% dickite), clearly different from the Paleozoic ones (about 90 wt% quartz, but ≥7% kaolinite). It is shown that the reddish silts bearing the LDG are constituted mainly of microquartz enriched with dickite, whose particle size distribution is characteristic of fluvio-lacustrine deposits, probably Oligocene to Miocene in age. The target rocks, most probably quartz sand, resulted from the weathering (loss of the cementing microquartz) of the Cretaceous sandstones from the Gilf Khebir Plateau with deposition in a high-energy environment.

Reference
Fröhlich F, Poupeau G, Badou A, Le Bourdonnec FX, Sacquin Y, Dubernet S, Bardintzeff JM, Véran M, Smith DC and Diemer E (in press) Libyan Desert Glass: New field and Fourier transform infrared data. Meteoritics & Planetary Science 
[doi:10.1111/maps.12223]
Published by arrangement with John Wiley & Sons

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Agnese Fazio^1,*, Massimo D’Orazio¹, Luigi Folco¹, Jérôme Gattacceca², Corinne Sonzogni²

¹Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy
²CNRS-IRD Aix-Marseille Université, Centre Européen de Recherche et d’Enseignement de Géosciences de l’Environnement (CEREGE), UM34, Aix-en-Provence, France

Northwest Africa (NWA) 6583 is a silicate-bearing iron meteorite with Ni = 18 wt%. The oxygen isotope composition of the silicates (∆′¹⁷O = −0.439 ‰) indicates a genetic link with the IAB-complex. Other chemical, mineralogical, and textural features of NWA 6583 are consistent with classification as a new member of the IAB-complex. However, some unique features, e.g., the low Au content (1.13 μg g⁻¹) and the extremely reducing conditions of formation (approximately −3.5 ∆IW), distinguish NWA 6583 from the known IAB-complex irons and extend the properties of this group of meteorites. The chemical and textural features of NWA 6583 can be ascribed to a genesis by impact melting on a parent body of chondritic composition. This model is also consistent with one of the most recent models for the genesis of the IAB-complex. Northwest Africa 6583 provides a further example of the wide lithological and mineralogical variety that impact melting could produce on the surface of a single asteroid, especially if characterized by an important compositional heterogeneity in space and time like a regolith.

Reference
Fazio A, D’Orazio M, Folco L, Gattacceca J and Sonzogni C (in press) The extremely reduced silicate-bearing iron meteorite Northwest Africa 6583: Implications on the variety of the impact melt rocks of the IAB-complex parent body. Meteoritics & Planetary Science 
[doi:10.1111/maps.12231]
Published by arrangement with John Wiley & Sons

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Gabriele Giuli^1,*, Maria Rita Cicconi¹, Sigrid Griet Eeckhout², Christian Koeberl³, Billy P. Glass⁴, Giovanni Pratesi⁵, Mariangela Cestelli-Guidi⁶ and Eleonora Paris¹

¹School of Science and Technology, Geology Division, University of Camerino, Via Gentile III da Varano, 62032, Italy
²European Synchrotron Radiation Facility (ESRF), 6 rue Jules Horowitz, 38043 Grenoble, France
³Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria; and Natural History Museum, Burgring 7, A-1010 Vienna, Austria
⁴Department of Geological Sciences, University of Delaware, Newark, Delaware 19716, U.S.A.
⁵Dipartimento di Scienze della Terra, Università di Firenze, Via G. La Pira 4, 50121, Firenze, Italy
⁶Laboratori Nazionali Frascati, Istituto Nazionale Fisica Nucleare, Via Enrico Fermi, Frascati, Italy

Iron oxidation states and coordination numbers have been determined by micro-X-ray absorption near edge spectroscopy (XANES) on the cores of a large group of microtektites from the Australasian, Ivory Coast, and North American (NA) tektite strewn field. The North American microtektites used in this study have been collected from five sites at different distances from the source crater; most have SiO₂ content between 70 and 80 wt%. Accurate analysis of the pre-edge peak energy position and integrated area allowed determination of Fe³⁺/(Fe²⁺+Fe³⁺) ratios on all samples with an estimated error of ±0.05.
Microtektites from the Australasian and Ivory Coast strewn fields show low values of the Fe³⁺/(Fe²⁺+Fe³⁺) ratios, in fair agreement with tektites from the same strewn field. In contrast, microtektites from the North American strewn fields show a wide range of Fe³⁺/(Fe²⁺+Fe³⁺) ratios from 0.02 to ca. 0.61. Comparison of Fe oxidation state data with chemical composition do not show any relation between Fe³⁺/(Fe²⁺+Fe³⁺) ratios and Na, Ca, or K contents, thus suggesting that the high-Fe oxidation states are not the consequence of sea-water alteration.
The difference between the Fe oxidation state of tektites and microtektites from the North American strewn fields suggests that some factors in the formation of the North American microtektites were different than for the North American tektites and for microtektites in the other strewn fields.
Previous Fe oxidation state data on NA tektites strongly suggest that the wide range in Fe oxidation state we found on NA microtektites is not related to lateral heterogeneity of the target rocks. Despite a correlation between microtektite oxidation state and distance from the source crater, we maintain that Fe oxidation state is not related only to the microtektite droplet flight distance. This is in keeping with the fact that no significant variations in the Fe oxidation state have been found in microtektites from the Australasian strewn field, even for Australasian microtektites recovered in Antarctica. The Fe oxidation state in North American microtektites could be explained by interaction of melt droplets with a H₂O-rich vapor plumes generated during the impact. These data point out that some difference must exist between the thermal histories of microtektites and tektites from the NA strewn field. Moreover, microtektites from the NA strewn field show also distinctively higher oxidation states than those from Ivory Coast or the Australasian strewn fields.

Reference
Giuli G, Cicconi MR, Eeckhout SG, Koeberl C, Glass BP, Pratesi G, Cestelli-Guidi M and Paris E (2013) North American microtektites are more oxidized than tektites. American Mineralogist 98:1930-1937.
[doi:10.2138/am.2013.4505]
Copyright: The Mineralogical Society of America

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Gernot Arp*, Claudia Kolepka, Klaus Simon, Volker Karius, Nicole Nolte, Bent T. Hansen

Georg-August-Universität Göttingen, Geowissenschaftliches Zentrum, Göttingen, Germany

The extent of impact-generated hydrothermal activity in the 24 km sized Ries impact structure has been controversially discussed. To date, mineralogical and isotopic investigations point to a restriction of hydrothermal activity to the impact-melt bearing breccias, specifically the crater-fill suevite. Here, we present new petrographic, geochemical, and isotopic data of postimpact carbonate deposits, which indicate a hydrothermal activity more extended than previously assumed. Specifically, carbonates of the Erbisberg, a spring mound located upon the inner crystalline ring of the crater, show travertine facies types not seen in any of the previously investigated sublacustrine soda lake spring mounds of the Ries basin. In particular, the streamer carbonates, which result from the encrustation of microbial filaments in subaerial spring effluents between 60 and 70 °C, are characteristic of a hydrothermal origin. While much of the primary geochemical and isotopic signatures in the mound carbonates have been obliterated by diagenesis, a postimpact calcite vein from brecciated gneiss of the subsurface crater floor revealed a flat rare earth element pattern with a clear positive Eu anomaly, indicating a hydrothermal fluid convection in the crater basement. Finally, the strontium isotope stratigraphic correlation of the travertine mound with the crater basin succession suggests a hydrothermal activity for about 250,000 yr after the impact, which would be much longer than previously assumed.

Reference
Arp G, Kolepka C, Simon K, Karius V, Nolte N and Hansen BT (in press) New evidence for persistent impact-generated hydrothermal activity in the Miocene Ries impact structure, Germany. Meteoritics & Planetary Science 
[doi:10.1111/maps.12235]
Published by arrangement with John Wiley & Sons

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