Size and compositional constraints of Ganymede's metallic core for driving an active dynamo

Ganymede has an intrinsic magnetic field which is generally considered to originate from a self-excited dynamo in the metallic core. Driving of the dynamo depends critically on the satellite's thermal state and internal structure. However, the inferred structure based on gravity data alone has a large uncertainty, and this makes the possibility of dynamo activity unclear; variations in core size and composition significantly change the heat capacity and alter the cooling history of the core. The main objectives of this study is to explore the structural conditions for a currently active dynamo in Ganymede using numerical simulations of the thermal history, and to evaluate under which conditions Ganymede can maintain the dynamo activity at present. We have investigated the satellite's thermal history using various core sizes and compositions satisfying the mean density and moment of inertia of Ganymede, and evaluate the temperature and heat flux at the core-mantle boundary (CMB). Based on the following two conditions, we evaluate the possibility of dynamo activity, thereby reducing the uncertainty of the previously inferred interior structure. The first condition is that the temperature at the CMB must exceed the melting point of a metallic core, and the second is that the heat flux through the CMB must exceed the adiabatic temperature gradient. The mantle temperature starts to increase because of the decay of long-lived radiogenic elements in the rocky mantle. After a few Gyr, radiogenic elements are exhausted and temperature starts to decrease. As the rocky mantle cools, the heat flux at the CMB steadily increases. If the temperature and heat flux at the CMB satisfy these conditions simultaneously, we consider the case as capable of driving a dynamo. Finally, we identify the Dynamo Regime, which is the specific range of internal structures capable of driving the dynamo, based on the results of simulations with various structures. If Ganymede's self-sustained magnetic field were maintained by thermal convection, the satellite's metallic core would be relatively large and, in comparison to other terrestrial-type planetary cores, strongly enriched in sulfur. The dynamo activity and the generation of the magnetic field of Ganymede should start from a much later stage, possibly close to the present.     

The degree of Cl-loss for the particulate matter (PM) and fog water sampled at the same air mass at the Hachimantai mountain range in northern Japan

It is well known that sea-salt aerosols in particulate matter (PM) react with acids such as H₂SO₄ and HNO₃ during transportation and thereby lose chloride ions (Cl-loss). The PM and fog were sampled concurrently at different altitudes in the Hachimantai mountain range, northern Japan. The PM and fog sampled at different altitudes had nearly identical properties for the ion components. However, the PM was in a Cl-depleted state (more than 80% of all samples), but the fog water was not in so Cl-depleted state (less than 29%). As a result, it could be explained that this phenomenon caused because the fog droplets took up the gaseous state HCl other than sea-salt PM. After all Cl^- in the fog water recovered and was rather rich compared with the sea-salt or the PM by the uptake of the gaseous state HCl. Moreover, it was found that for PM_coarse(2.5 < D < 10), 86% of the acid (H₂SO₄ and HNO₃) was consumed for Cl-loss reactions and/or for dissolution of Ca and Mg in soil particles.     

Determination of the oxidation state of radiogenic Pb in natural zircon using X-ray absorption near-edge structure

We examined the L_III-edge Pb X-ray absorption near-edge structure (XANES) of three natural zircon samples with different amounts of radiation doses (1.9 × 10¹⁵ to 6.8 × 10¹⁵ α-decay events/mg). The results suggest that the oxidation state of radiogenic Pb in the zircon sample with the highest radiation dose is divalent. The XANES spectra of the two other samples with lower radiation doses suggest that radiogenic Pb(II) is present, and further that some Pb may be tetravalent. This is the first work on the determination of the oxidation state of radiogenic Pb in natural zircon using XANES.     

Present environment of Dam Lake Sambe,southwestern Japan: a geochemical study of bottom sediments

Geochemical analyses of lakebed and core sediments from Lake Sambe on the outskirts of Oda City in Shimane prefecture in southwestern Japan were carried out in order to assess the water quality and the concentration and distribution patterns of sixteen elements. The lake water showed a stratified condition with respect to dissolved O₂, and As, Fe, and Mn concentrations in the bottom layers which increased in the summer. The chemical composition of the sediments, as measured by X-ray fluorescence, included major and trace elements (P, Ca, Sc, Ti, V, Cr, Fe, Ni, Cu, Zn, As, Sr, Zr, Pb, and Th), and total sulfur (TS). Elevated values of As, Zn, V, Fe, P, and TS were present in several layers of the upper cores (from 0 to 5 cm) and other surface sediments. Increases in the abundances of these metals in lake sediments are probably related to the reducing condition of the sediments, fine-grained organic rich sediments, and post-depositional diagenetic remobilization. Moreover, correlations between the concentrations of trace metals and iron in the sediments suggest their adsorption onto Fe (oxy)hydroxides, whereas correlations with sulfur indicate that they were precipitated as Fe-sulfides. The average abundances of As, Pb, Zn, and Cu exceeded the lowest effect level and Interim Sediment Quality Guideline values that the New York State Department of Environmental Conservation and the Canadian Council of Ministers of the Environment determined to have moderate impact on aquatic organisms. In addition, concentrations of As and Zn exceeded the Coastal Ocean Sediment Database threshold value, indicating potentially toxic levels. Therefore, the presence of trace metals in the lake sediments may result in adverse effects on biota health.     

Hortonian overland flow from Japanese forest plantations—an aberration,the real thing,or something in between?

There is a growing opinion that poorly managed plantation forests in Japan are contributing to increased storm runoff and erosion. Here we present evidence to the contrary from runoff plots at two scales (hillslope and 0·5 × 2 m plots) for several forest conditions in the Mie and Nariki catchments. Runoff coefficients from small plots in untended hinoki forests were variable but typically higher than from better managed or deciduous forests during small storms at Nariki; at Mie, runoff during small events was highly variable from all small plots but runoff coefficients were similar for hinoki plots with and without understory vegetation, while the deciduous plot had lower runoff coefficients. Storm runoff was less at the hillslope scale than the plot scale in Mie; these results were more evident at sites with better ground cover. During the largest storms at both sites, differences in runoff due to forest condition were not evident regardless of scale. Dynamic soil moisture tension measurements at Nariki indicated that during a large storm, flow in the upper organic‐rich and root‐permeated soil horizons was 3·2 times higher than measured overland runoff from a small hinoki plot with poor ground cover and 8·3 times higher than runoff from a deciduous forest plot. On the basis of field observations during storms, at least a portion of the monitored ‘Hortonian overland flow’ was actually occurring in this near‐surface ‘biomat’. Therefore our field measurements in both small and large plots potentially included biomat flow in addition to short‐lived Hortonian runoff. Because overland flow decreased with increasing scale, rill erosion did not occur on hillslopes. Additionally, runoff coefficients were not significantly different among cover conditions during large storms; thus, the ‘degraded’ forest conditions appear not to greatly enhance peak flows or erosion potential at larger scales, especially when biomat flow is significant. Copyright © 2007 John Wiley & Sons, Ltd.     

Three-dimensional simulations of molecular cloud fragmentation regulated by magnetic fields and ambipolar diffusion

We employ the first fully three-dimensional simulation to study the role of magnetic fields and ion–neutral friction in regulating gravitationally driven fragmentation of molecular clouds. The cores in an initially subcritical cloud develop gradually over an ambipolar diffusion time while the cores in an initially supercritical cloud develop in a dynamical time. The infalling speeds on to cores are subsonic in the case of an initially subcritical cloud, while an extended (≳0.1 pc) region of supersonic infall exists in the case of an initially supercritical cloud. These results are consistent with previous two-dimensional simulations. We also found that a snapshot of the relation between density (ρ) and the strength of the magnetic field ( B ) at different spatial points of the cloud coincides with the evolutionary track of an individual core. When the density becomes large, both the relations tend to   B ∝ρ^0.5  .     

地震液化条件下重力式码头的变形破坏机理

现场调查发现在神户地震期间重力式码头破坏时都发生了相当大的侧向位移，因此，阐明挡土墙有变形机理对于改善抗震设计具有十分重要的意义。为此，根据相似原理设计了重力式码头的地基模型，进行了一系列的振动台试验。试验结果表明：基底土的强度降低和局部液化是挡土墙变形破坏的主导因素。墙后动土压力的增加为挡土墙的运动提供了条件。在液化条件下重力式码头地基的运动以侧向位移为主。重力作用是地基侧向运动的主要影响因素。减少作用于挡土墙上的动土压力和充分填实基底下的砂土是增加重力式码头抗震稳定性的重要措施。     

理学ZSX系列X荧光光谱仪中文软件开发

介绍了日本理学ZSX系列X荧光光谱仪的特点和新开发的中文版软件的功能。     

The effect of microorganisms on Fe precipitation rates at neutral pH

The effect of microorganisms on Fe precipitation rates at neutral pH in the field was examined. The studied area was a cave having Fe-stalactites composed mainly of ferrihydrite and associated microorganisms. The microorganisms were covered with ferrihydrite. Water associated with stalactites was slightly supersaturated with respect to ferrihydrite, and had a dissolved oxygen concentration of 2 ppm, a pH of 6, and an Fe concentration of approximately 14 ppm. Fe precipitation rates were estimated from decreases in Fe concentrations in water during flowing through the Fe-stalactites. The estimated Fe precipitation rate in the field ranges from 6.8×10⁻⁸ to 4.0×10⁻⁷ mol/l/s. These values are in good agreement with bulk estimates of Fe-stalactite growth rates derived from the length increase (1.3 cm) of one formation over 30 days. The estimated Fe precipitation rates are faster by about four orders of magnitude than expected inorganic precipitation rates. On-site Fe precipitation experiments with sterilized and unsterilized Fe-stalactites and without Fe-stalactites indicate that microorganisms are the controlling factor accelerating Fe precipitation rates at neutral pH. These results suggest that microbially accelerated Fe precipitation rates are more likely related to exopolysaccharides and microbial surface properties than metabolic precipitation mechanisms.     

Determination of Total Sulfur in Thirty One Geochemical Reference Materials Using an Inductively Coupled Plasma-Atomic Emission Spectrometer Fitted with a Semiconductor Photodiode Detector

The total sulfur contents of thirty one geochemical reference materials were determined using an inductively coupled plasma-atomic emission spectrometer fitted with a semiconductor photodiode detector. Most samples were decomposed using an HF-aqua regia digestion. However, a few were incompletely decomposed by simple acid digestion, and these were decomposed by a fusion method. The agreement between the analytical results from this work and published data is generally good.