Andrei A. Shiryaev^1,2 and and Fabrice Gaillard³

¹Institute of Physical Chemistry and Electrochemistry RAS, Leninsky pr. 31, 119071 Moscow, Russia
²Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry RAS, Staromonetny per. 35, 119017 Moscow, Russia
³CNRS/INSU, Institut des Sciences de la Terre d’Orléans – UMR 6113, Université d’Orléans, Campus Géosciences, 1A rue de la Férollerie, 41071 Orléans cedex 2, France

Grains of natural SiC, moissanite, are encountered in various geological settings. According to thermodynamic calculations and high-pressure experiments, SiC formation requires very reducing conditions, approx. 6–10 orders of magnitude in fO2 more reducing than the present-day mantle redox state. SiC occurrences have motivated several studies but the required extremely reducing conditions remain a major inconsistency that has not been solved. It is shown here that such reducing conditions can be achieved during the ultimate steps of ascent of carbon-saturated melts, when pressure is lower than 100 bars. At these conditions, the redox buffering by carbon can impose fO2 similar to IW-6. Conditions favorable to SiC growth can therefore be reached around carbonaceous grains during the shallow emplacement of silicate melts or kimberlites and do not necessarily imply extremely localized oxygen-depleted regions in the mantle. Such reduced conditions can also explain the presence of elemental Si and ironcarbide inclusions in association with moissanite grains.

Reference
Shiryaev AA and and Gaillard F (2014) Local redox buffering by carbon at low pressures and the formation of moissanite – natural SiC. European Journal of Mineralogy 26:53.
[doi:10.1127/0935-1221/2013/0025-2339]
Copyright: E. Schweizerbart’sche Verlagsbuchhandlung

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Gary Lofgren^a

^a1905 Trail View, Friendswood, Texas 77546, U.S.A.

New data based on a detailed analysis of pyroxene zoning strongly suggests that convection is an important process in lunar magmas. Elardo and Shearer (2014) carefully document irregular oscillatory zoning that is best explained by movement of pyroxene crystals in a convecting magma. Lunar samples that contain such data are rare, but this study should inspire more extensive efforts to further document magmatic processes.

Reference
Lofgren G (2014) New data on lunar magmatic processes. American Mineralogist 99:561.
[doi:10.2138/am.2014.4803]
Copyright: The Mineralogical Society of America

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Valérie Malavergne^a,bet al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aUniversité Paris Est Marne La Vallée, Laboratoire des Géomatériaux et Environnement, Champs-sur-Marne Cedex, 77454, France
^bEcole Normale Supérieure, Laboratoire de Géologie, 24 rue Lhomond, 75005 Paris, France

Mercury is notorious as the most reduced planet with the highest metal/silicate ratio, yet paradoxically data from the MESSENGER spacecraft show that its iron-poor crust is high in sulfur (up to ∼6 wt%, ∼80× Earth crust abundance) present mainly as Ca-rich sulfides on its surface. These particularities are simply impossible on the other terrestrial planets. In order to understand the role played by sulfur during the formation of Mercury, we investigated the phase relationships in Mercurian analogs of enstatite chondrite-like composition experimentally under conditions relevant to differentiation of Mercury (∼1 GPa and 1300–2000 °C). Our results show that Mg-rich and Ca-rich sulfides, which both contain Fe, crystallize successively from reduced silicate melts upon cooling below 1550 °C. As the iron concentration in the reduced silicates stays very low (≪1 wt%), these sulfides represent new host phases for both iron and sulfur in the run products. Extrapolated to Mercury, these results show that Mg-rich sulfide crystallization provides the first viable and fundamental means for retaining iron as well as sulfur in the mantle during differentiation, while sulfides richer in Ca would crystallize at shallower levels. The distribution of iron in the differentiating mantle of Mercury was mainly determined by its partitioning between metal (or troilite) and Mg–Fe–Ca-rich sulfides rather than by its partitioning between metal (or troilite) and silicates. Moreover, the primitive mantle might also be boosted in Fe by a reaction at the core mantle boundary (CMB) between Mg-rich sulfides of the mantle and FeS-rich outer core materials to produce (Fe, Mg)S. The stability of Mg–Fe–Ca-rich sulfides over a large range of depths up to the surface of Mercury would be consistent with sulfur, calcium and iron abundances measured by MESSENGER.

Reference
Malavergne et al. (2014) How Mercury can be the most reduced terrestrial planet and still store iron in its mantle. Earth and Planetary Science Letters 394:186.
[doi:10.1016/j.epsl.2014.03.028]
Copyright Elsevier

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Timothy A. Goudge¹et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Department of Geological Sciences, Brown University, Providence, Rhode Island, USA

We present new observations of pyroclastic deposits on the surface of Mercury from data acquired during the orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. The global analysis of pyroclastic deposits brings the total number of such identified features from 40 to 51. Some 90% of pyroclastic deposits are found within impact craters. The locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits, except some deposits cluster around the margins of smooth plains, similar to the relation between many lunar pyroclastic deposits and lunar maria. A survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval. Measurements of surface reflectance by MESSENGER indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain, with higher reflectance values, redder (i.e., steeper) spectral slopes, and a downturn at wavelengths shorter than ~400 nm (i.e., in the near-ultraviolet region of the spectrum). Three possible causes for these distinctive characteristics include differences in transition metal content, physical properties (e.g., grain size), or degree of space weathering from average surface material on Mercury. The strength of the near-ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths. We suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits.

Reference
Goudge et al. (in press) Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004480]
Published by arrangement with John Wiley & Sons

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H. Kurokawa^a,b, M. Sato^b,c, M. Ushioda^b, T. Matsuyama^b, R. Moriwaki^b, J.M. Dohm^d and T. Usui^b

^aDepartment of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
^bDepartment of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan
^cDepartment of Environmental Changes, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
^dEarth-Life-Science Institute, Tokyo Institute of Technology, 2-12-1-1E-1 Ookayama, Meguro-ku, Tokyo, 152-8550, Japan

Martian surface morphology implies that Mars was once warm enough to maintain persistent liquid water on its surface. While the high D/H ratios (∼6 times the Earth’s ocean water) of the current martian atmosphere suggest that significant water has been lost from the surface during martian history, the timing, processes, and the amount of the water loss have been poorly constrained. Recent technical developments of ion-microprobe analysis of martian meteorites have provided accurate estimation of hydrogen isotope compositions (D/H) of martian water reservoirs at the time when the meteorites formed. Based on the D/H data from the meteorites, this study demonstrates that the water loss during the pre-Noachian (>41–99 m global equivalent layers, GEL) was more significant than in the rest of martian history (>10–53 m GEL). Combining our results with geological and geomorphological evidence for ancient oceans, we propose that undetected subsurface water/ice (≃100–1000 m GEL) should exist, and it exceeds the observable present water inventory (≃20–30 m GEL) on Mars.

Reference
Kurokawa H, Sato M, Ushioda M, Matsuyama T, Moriwaki R, Dohm JM and Usui T (2014) Evolution of water reservoirs on Mars: Constraints from hydrogen isotopes in martian meteorites. Earth and Planetary Science Letters 394:179.
[doi:10.1016/j.epsl.2014.03.027]
Copyright Elsevier

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Mohamad Ali-Dib¹, Olivier Mousis¹, Jean-Marc Petit¹ and Jonathan I. Lunine²

¹Université de Franche-Comté, Institut UTINAM, CNRS/INSU, UMR 6213, Observatoire de Besançon, BP 1615, F-25010 Besançon Cedex, France
²Center for Radiophysics and Space Research, Space Sciences Building, Cornell University, Ithaca, NY 14853, USA

The C to O ratio is a crucial determinant of the chemical properties of planets. The recent observation of WASP 12b, a giant planet with a C/O value larger than that estimated for its host star, poses a conundrum for understanding the origin of this elemental ratio in any given planetary system. In this paper, we propose a mechanism for enhancing the value of C/O in the disk through the transport and distribution of volatiles. We construct a model that computes the abundances of major C- and O-bearing volatiles under the influence of gas drag, sublimation, vapor diffusion, condensation, and coagulation in a multi-iceline 1+1D protoplanetary disk. We find a gradual depletion in water and carbon monoxide vapors inside the water’s iceline, with carbon monoxide depleting slower than water. This effect increases the gaseous C/O and decreases the C/H ratio in this region to values similar to those found in WASP 12b’s day side atmosphere. Giant planets whose envelopes were accreted inside the water’s iceline should then display C/O values larger than those of their parent stars, making them members of the class of so-called carbon-rich planets.

Reference
Ali-Dib M, Mousis O, Jean-Marc Petit J-M and Lunine JI (2014) Carbon-rich Planet Formation in a Solar Composition Disk. The Astrophysical Journal 785:125.
[doi:10.1088/0004-637X/785/2/125]

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E. Vilenius¹ et al. (>10)*

*Find the extensive, full author and affiliation list on the publishers website.

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Giessenbachstr., 85741 Garching, Germany

Context. The Kuiper belt is formed of planetesimals which failed to grow to planets and its dynamical structure has been affected by Neptune. The classical Kuiper belt contains objects both from a low-inclination, presumably primordial, distribution and from a high-inclination dynamically excited population.
Aims. Based on a sample of classical trans-Neptunian objects (TNOs) with observations at thermal wavelengths we determine radiometric sizes, geometric albedos and thermal beaming factors for each object as well as study sample properties of dynamically hot and cold classicals.
Methods. Observations near the thermal peak of TNOs using infrared space telescopes are combined with optical magnitudes using the radiometric technique with near-Earth asteroid thermal model (NEATM). We have determined three-band flux densities fromHerschel/PACS observations at 70.0, 100.0 and 160.0 μm and Spitzer/MIPS at 23.68 and 71.42 μm when available. We use reexamined absolute visual magnitudes from the literature and ground based programs in support of Herschel observations.
Results. We have analysed 18 classical TNOs with previously unpublished data and re-analysed previously published targets with updated data reduction to determine their sizes and geometric albedos as well as beaming factors when data quality allows. We have combined these samples with classical TNOs with radiometric results in the literature for the analysis of sample properties of a total of 44 objects. We find a median geometric albedo for cold classical TNOs of 0.14_-0.07^+0.09 and for dynamically hot classical TNOs, excluding the Haumea family and dwarf planets, 0.085_-0.045^+0.084. We have determined the bulk densities of Borasisi-Pabu (2.1_-1.2^+2.6 g cm^-3), Varda-Ilmarë (1.25_-0.43^+0.40 g cm^-3) and 2001 QC₂₉₈ (1.14_-0.30^+0.34 g cm^-3) as well as updated previous density estimates of four targets. We have determined the slope parameter of the debiased cumulative size distribution of dynamically hot classical TNOs as q = 2.3 ± 0.1 in the diameter range 100 < D < 500 km. For dynamically cold classical TNOs we determineq = 5.1 ± 1.1 in the diameter range 160 < D < 280 km as the cold classical TNOs have a smaller maximum size.

Reference
Vilenius et al. (2014) “TNOs are Cool”: A survey of the trans-Neptunian region – X. Analysis of classical Kuiper belt objects from Herschel and Spitzer observations. Astronomy & Astrophysics 564:A35.
[doi:10.1051/0004-6361/201322416]
Reproduced with permission © ESO

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W. Luecke¹, A. Kontny² and U. Kramar¹

¹Institut für Mineralogie und Geochemie, Karlsruher Institut für Technologie (KIT), Karlsruhe, Germany
²Institut für Angewandte Geowissenschaften, Karlsruher Institut für Technologie (KIT), Karlsruhe, Germany

We combined high-resolution and space-resolved elemental distribution with investigations of magnetic minerals across Fe,Ni-alloy and troilite interfaces for two nonmagmatic (Morasko and Mundrabilla) IAB group iron meteorites and an octahedrite found in 1993 in Coahuila/Mexico (Coahuila II) preliminarily classified on Ir and Au content as IIAB group. The aim of this study was to elucidate the crystallization and thermal history using gradients of the siderophile elements Ni, Co, Ge, and Ga and the chalcophile elements Cr, Cu, and Se with a focus on magnetic minerals. The Morasko and Coahuila II meteorite show a several mm-thick carbon- and phosphorous-rich transition zone between Fe,Ni-alloy and troilite, which is characterized by magnetic cohenite and nonmagnetic or magnetic schreibersite. At Morasko, these phases have a characteristic trace element composition with Mo enriched in cohenite. In both Morasko and Coahuila II, Ni is enriched in schreibersite. The minerals have crystallized from immiscible melts, either by fractional crystallization and C- and P-enrichment in the melt, or by partial melting at temperatures slightly above the eutectic point. During crystallization of Mundrabilla, the field of immiscibility was not reached. Independent of meteorite group and cooling history, the magnetic mineralogy (daubreelite, cohenite and/or schreibersite, magnetite) is very similar to the troilite (and transition zone) for all three investigated iron meteorites. If these minerals can be separated from the metal, they might provide important information about the early solar system magnetic field. Magnetite is interpreted as a partial melting or a terrestrial weathering product of the Fe,Ni-alloy under oxidizing conditions.

Reference
Luecke W, Kontny A and Kramar U (in press) Geochemical zoning and magnetic mineralogy at Fe,Ni-alloy–troilite interfaces of three iron meteorites from Morasko, Coahuila II, and Mundrabilla. Meteoritics & Planetary Science
[doi:10.1111/maps.12288]
Published by arrangement with John Wiley & Sons

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Julien Faure¹, Sébastien Fromang¹ and Henrik Latter²

¹Laboratoire AIM, CEA/DSM – CNRS – Université Paris 7, Irfu/Service d’Astrophysique, CEA-Saclay, 91191 Gif-sur-Yvette, France
²Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WA, UK

Context. In protoplanetary disks, the inner boundary between the turbulent and laminar regions could be a promising site for planet formation, thanks to the trapping of solids at the boundary itself or in vortices generated by the Rossby wave instability. At the interface, the disk thermodynamics and the turbulent dynamics are entwined because of the importance of turbulent dissipation and thermal ionization. Numerical models of the boundary, however, have neglected the thermodynamics, and thus miss a part of the physics.
Aims. The aim of this paper is to numerically investigate the interplay between thermodynamics and dynamics in the inner regions of protoplanetary disks by properly accounting for turbulent heating and the dependence of the resistivity on the local temperature.
Methods. Using the Godunov code RAMSES, we performed a series of 3D global numerical simulations of protoplanetary disks in the cylindrical limit, including turbulent heating and a simple prescription for radiative cooling.
Results. We find that waves excited by the turbulence significantly heat the dead zone, and we subsequently provide a simple theoretical framework for estimating the wave heating and consequent temperature profile. In addition, our simulations reveal that the dead-zone inner edge can propagate outward into the dead zone, before stalling at a critical radius that can be estimated from a mean-field model. The engine driving the propagation is in fact density wave heating close to the interface. A pressure maximum appears at the interface in all simulations, and we note the emergence of the Rossby wave instability in simulations with extended azimuth.
Conclusions. Our simulations illustrate the complex interplay between thermodynamics and turbulent dynamics in the inner regions of protoplanetary disks. They also reveal how important activity at the dead-zone interface can be for the dead-zone thermodynamic structure.

Reference
Faure J, Fromang S and Latter H (2014) Thermodynamics of the dead-zone inner edge in protoplanetary disks. Astronomy & Astrophysics 564:A22.
[doi:10.1051/0004-6361/201321911]
Reproduced with permission © ESO

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Yuri I. Fujii¹, Satoshi Okuzumi^1,2, Takayuki Tanigawa³ and Shu-ichiro Inutsuka¹

¹Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
²Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan
³Institute of Low Temperature Science, Hokkaido University, Sapporo 066-0819, Japan

We examine whether the magnetorotational instability (MRI) can serve as a mechanism of angular momentum transport in circumplanetary disks. For the MRI to operate the ionization degree must be sufficiently high and the magnetic pressure must be sufficiently lower than the gas pressure. We calculate the spatial distribution of the ionization degree and search for the MRI-active region where the two criteria are met. We find that there can be thin active layers at the disk surface depending on the model parameters, however, we find hardly any region which can sustain well-developed MRI turbulence; when the magnetic field is enhanced by MRI turbulence at the disk surface layer, a magnetically dominated atmosphere encroaches on a lower altitude and a region of well-developed MRI turbulence becomes smaller. We conclude that if there are no angular momentum transfer mechanisms other than MRI in gravitationally stable circumplanetary disks, gas is likely to pile up until disks become gravitationally unstable, and massive disks may survive for a long time.

Reference
Fujii YI, Okuzumi S, Tanigawa T and Shu-ichiro Inutsuka S-I (2014) On the Viability of the Magnetorotational Instability in Circumplanetary Disks. The Astrophysical Journal 785:101.
[doi:10.1088/0004-637X/785/2/101]

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