Compound chondrule formation in the shock-wave heating model: Three-dimensional hydrodynamics simulation of the disruption of a partially-molten dust particle

We carried out three-dimensional hydrodynamics simulations of the disruption of a partially-molten dust particle exposed to high-speed gas flow to examine the compound chondrule formation due to mutual collisions between the fragments (fragment-collision model; [Miura, H., Yasuda, S., Nakamoto, T., 2008a. Icarus194, 811-821]).In the shock-wave heating model, which is one of the most plausible models for chondrule formation, the gas friction heats and melts the surface of the cm-sized dust particle (parent particle) and then the strong gas ram pressure causes the disruption of the molten surface layer. The hydrodynamics simulation shows details of the disruptive motion of the molten surface, production of many fragments and their trajectories parting from the parent particle, and mutual collisions among them. In our simulation, we identified 32 isolated fragments extracted from the parent particle. The size distribution of the fragments was similar to that obtained from the aerodynamic experiment in which a liquid layer was attached to a solid core and it was exposed to a gas flow. We detected 12 collisions between the fragments, which may result in the compound chondrule formation. We also analyzed the paths of all the fragments in detail and found the importance of the shadow effect in which a fragment extracted later blocks the gas flow toward a fragment extracted earlier. We examined the collision velocity and impact parameter of each collision and found that 11 collisions should result in coalescence. It means that the ratio of coalescent bodies to single bodies formed in this disruption of a parent particle is R_coa=11/(32-11)=0.52. We concluded that compound chondrule formation can occur just after the disruption of a cm-sized molten dust particle in shock-wave heating.     

Comment on “Evaluation of palaeo-oxygenation of the ocean bottom cross the Permian–Triassic boundary” by Kakuwa (2008): Was the Late Permian deep-superocean really oxic?

Nearly 15 years after the proposal of the superanoxia concept (Isozaki, Y., 1994. Superanoxia across the Permo–Triassic boundary: record in accreted deep-sea pelagic chert in Japan. In: Embry, A.F., Beauchamp, B., Glass, D.J. (Eds.), Pangea: Global Environments and Resources. Memoir, Canadian Society of Petroleum Geologists, 17, pp. 805–812.), it is an appropriate timing to re-evaluate its geological context with the updated dataset. Kakuwa (Kakuwa, Y., 2008. Evaluation of palaeo-oxygenation of the ocean bottom across the Permian–Triassic boundary. Global and Planetary Change 63, 40–56.) lately discussed that the deep-sea anoxia across the Permian–Triassic boundary (P–TB) may have been much shorter than previously proposed, on the basis of ichnofabrics and geochemical data; however, his interpretations of the data do not appear straightforward nor persuading, and thus his claim is likely misled. Here we raise comments to his explanation on the following four issues: 1) invalid application of ichnofabric indices for shallow sea sediments to deep-sea cherts, 2) misinterpretation of Ce anomaly as a redox indicator, 3) improper application of various redox sensitive trace elements, and 4) questionable interpretations of δ³⁴S data of pyrites.     

The role of organic haze in Titan's atmospheric chemistry: II. Effect of heterogeneous reaction to the hydrogen budget and chemical composition of the atmosphere

One of the key components controlling the chemical composition and climatology of Titan's atmosphere is the removal of reactive atomic hydrogen from the atmosphere. A proposed process of the removal of atomic hydrogen is the heterogeneous reaction with organic aerosol. In this study, we investigate the effect of heterogeneous reactions in Titan's atmospheric chemistry using new measurements of the heterogeneous reaction rate [Sekine, Y., Imanaka, H., Matsui, T., Khare, B.N., Bakes, E.L.O., McKay, C.P., Sugita, S., 2008. Icarus 194, 186-200] in a one-dimensional photochemical model. Our results indicate that 60-75% of the atomic hydrogen in the stratosphere and mesosphere are consumed by the heterogeneous reactions. This result implies that the heterogeneous reactions on the aerosol surface may predominantly remove atomic hydrogen in Titan's stratosphere and mesosphere. The results of our calculation also indicate that a low concentration of atomic hydrogen enhances the concentrations of unsaturated complex organics, such as C₄H₂ and phenyl radical, by more than two orders in magnitude around 400 km in altitude. Such an increase in unsaturated species may induce efficient haze production in Titan's mesosphere and upper stratosphere. These results imply a positive feedback mechanism in haze production in Titan's atmosphere. The increase in haze production would affect the chemical composition of the atmosphere, which might induce further haze production. Such a positive feedback could tend to dampen the loss and supply cycles of CH₄ due to an episodic CH₄ release into Titan's atmosphere.     

Fragment-collision model for compound chondrule formation: Estimation of collision probability

We propose a new scenario for compound chondrule formation named as “fragment-collision model,” in the framework of the shock-wave heating model. A molten cm-sized dust particle (parent) is disrupted in the high-velocity gas flow. The extracted fragments (ejectors) are scattered behind the parent and the mutual collisions between them will occur. We modeled the disruption event by analytic considerations in order to estimate the probability of the mutual collisions assuming that all ejectors have the same radius. In the typical case, the molten thin () layer of the parent surface will be stripped by the gas flow. The stripped layer is divided into about 200 molten ejectors (assuming that the radius of ejectors is 300 μm) and then they are blown away by the gas flow in a short period of time (). The stripped layer is leaving from the parent with the velocity of depending on the viscosity, and we assumed that the extracted ejectors have a random velocity Δv of the same order of magnitude. Using above values, we can estimate the number density of ejectors behind the parent as . These ejectors occupy ∼9% of the space behind the parent in volume. Considering that the collision rate (number of collisions per unit time experienced by an ejector) is given by Rcoll=σcollneΔv, where σcoll is the cross-section of collision [e.g., Gooding, J.K., Keil, K., 1981. Meteoritics 16, 17-43], we obtain by substituting above values. Since most collisions occur within the short duration () before the ejectors are blown away, we obtain the collision probability of Pcoll∼0.36, which is the probability of collisions experienced by an ejector in one disruption event. The estimated collision probability is about one order of magnitude larger than the observed fraction of compound chondrules. In addition, the model predictions are qualitatively consistent with other observational data (oxygen isotopic composition, textural types, and size ratios of constituents). Based on these results, we concluded that this new model can be one of the strongest candidates for the compound chondrule formation. It should be noted that all collisions do not necessarily lead to the compound chondrule formation. The formation efficiency and the future works which should be investigated in the forthcoming paper are also discussed.     

Thermal generation of magnetic fields with radiation pressure in rotating stars

It has been suggested by Biermann that in rotating stars the electron partial pressure could generate a toroidal magnetic field of a considerable strength. However, Mestel and Roxburgh have shown recently that the generation of such a toroidal magnetic field could almost completely be suppressed when a weak primodial poloidal magnetic field exists in the star. In this paper it is shown that a toroidal magnetic field of a moderate strength could be generated even in the presence of a primodial poloidal magnetic field, if the effect of radiation pressure is taken into consideration. This considered mechanism is effective for moderately massive stars, and numerical estimate indicates that in A type stars a toroidal magnetic field of the order of a thousand gauss can be generated near the surface within the time scale of the evolution of the star.Visiting Scientist to the High Altitude Observatory on leave of absence from the Department of Astronomy, University of Tokyo, Japan.     

Potosiite and incaite from the Hoei mine,Japan

Summary Potosiite and incaite have been found at the Hoei mine, Oita Prefecture, Japan. Potosiite occurs as tabular crystals (3 cm x 2 cm in size), and is associated with arsenopyrite, pyrrhotite, stannite,jamesonite and pyrite, whereas incaite occurs as polycrystalline aggregates rimming galena or as hair-like masses in a matrix of kutnahorite. X-ray powder data reveal that these minerals are potosiite and incaite, respectively. Electron-microprobe analyses of potosiite and incaite gave the general formula (Pb, Sn)²⁺ _6+xFeSb₂Sn⁴⁺ ₂S_14+x, with x < 2. Potosiite is homogeneous, whereas incaite, in contact with galena, is quite heterogeneous. Reflectance data for potosiite show that its bireflectance is one of the lowest among opaque minerals with a sheet-like structure.

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Potosiit und incait von der Hoei-mine, Japan

Zusammenfassung Potosiit and Incait wurden in Erzen der Hoei-Mine, Oita Präfektur, Japan, nachgewiesen. Potosiit kommt in plattigen Kristallen (3 cm x 2 cm) vor and ist mit Arsenkies, Magnetkies, Zinnkies, Jamesonit and Pyrit assoziiert. Incait kommt in polykristallinen Aggregaten, die Ränder um Bleiglanz bilden, oder als Haar-artige Massen in einer Matrix von Kutnahorite vor. Röntgendiffraktionsdaten zeigen, daß es sich hier um Potosiit and Incait handelt; Mikrosondenanalysen der beiden Minerale geben die allgemeine Formel (Pb, Sn)²⁺ _6+x(FeSb₂ ⁴⁺)₂S_4+x, with x < 2. Potosiit ist homogen, während Incait in Kontakt mit Bleiglanz inhomogen ist. Quantitative Reflexionsdaten für Potosiit zeigen, daß seine Bireflexion zu den niedrigsten bei opaken Mineralen mit Schichtgitter gehört.

     

APPLICATION OF MEM SPECTRAL ESTIMATION TO MU RADAR OBSERVATION

Preliminary results of the wind velocity estimation using the Maximum Entropy Method (MEM) to MUradar observation data sets are presented. The comparison of the results from the periodogram method and theMEM shows that the MEM estimation is reliable, and has higher accuracy, resolution and detectability than theestimation from periodogram method. The high accuracy power spectrum obtained by the MEM is veryuseful to studying the atmospheric turbulence structure. However. the MEM needs the longer computingtime for obtaining the high accuracy spectrum. Particularly, the estimation of MEM will bring serious devia-tion at lower signal-to-noise ratio.     

青海海北高寒草甸五种植被生物量及环境条件比较

分析了高寒草甸不同植被类型植物种类组成、生物量变化规律及其差异。研究表明不同植被类型的分布与土壤湿度和温度有很大的关系。藏篙草草甸、金露梅灌丛草甸、矮篙草草甸、正恢复的矮篙草草甸、小嵩草草甸这5种不同植被类型所对应的土壤湿度依次降低，而所对应的土壤温度依次升高；植物种类数量表现为矮嵩草草甸＞金露梅灌丛草甸＞小篙草草甸＞正恢复的矮篙草草甸＞藏篙草草甸。地上生物量高低依次为小嵩草革甸＞矮嵩草草甸＞金露梅灌丛草甸＞正恢复的矮篙草草甸＞藏篙草草甸；地下生物量则表现出金露梅灌丛革甸＞矮嵩草甸＞小篙草草甸＞正恢复的矮篙草草甸的特征，而其在年内的周转值表现出金露梅灌丛草甸＞正承复的矮嵩草草甸＞小篙草草甸＞矮篙草草甸;土壤有机质的季节变化表现为0—40cn整层土壤有机质含量小嵩草草甸＞金露梅灌丛草甸＞矮嵩草草甸＞正恢复的矮篙草草甸，0—10cm的表层土壤有机质金露梅灌丛草甸＞矮嵩草草甸＞小嵩草草甸＞正恢复的矮篙草草甸。     

Diagenetic control on arsenic partitioning in sediments of the Meghna River delta,Bangladesh

A stratigraphic profile of solid phase As was measured to investigate the diagenetic cycling of arsenic and related elements in fluvial sediments of the Meghna River delta plain. The distributions of Fe, Mn, and Al are typically characterized by surficial solid phase enrichment, and As is distributed down to 36.6 m showing similar alternate layers of maxima and minima with Fe, Mn and TOC, which reflects the diagenetic remobilization and periodical differences in source materials of As. Lithological characteristics and geochemical data suggest that elevated levels of As are found in organic-matter-rich clay and silty sand rather than sand samples, with occasionally enriched As content in iron-oxyhydroxide-coated sand grains. Arsenic demonstrates a positive and significant co-variation with total organic carbon in sediments, which suggests the important role of particulate and colloidal organic matter and biological activity in controlling the distribution of arsenic in the Bengal delta. However, the concentrations of Fe and Mn weakly correlate with As contents, whereas Al contents show no relationship with As. The results of this study suggest that reactive oxides or hydroxides of Fe and Mn, rather than Fe and Mn with other minerals, might control arsenic distribution.