Strength of the lithosphere of the Galilean satellites

Several approaches have been used to estimate the ice shell thickness on Callisto, Ganymede, and Europa. Here we develop a method for placing a strict lower bound on the thickness of the strong part of the shell (lithosphere) using measurements of topography. The minimal assumptions are that the strength of faults in the brittle lithosphere is controlled by lithostatic pressure according to Byerlee's law and the shell has relatively uniform density and thickness. Under these conditions, the topography of the ice provides a direct measure of the bending moment in the lithosphere. This topographic bending moment must be less than the saturation bending moment of the yield strength envelope derived from Byerlee's law. The model predicts that the topographic amplitude spectrum decreases as the square of the topographic wavelength. This explains why Europa is rugged at shorter wavelengths (∼10 km) but extremely smooth, and perhaps conforming to an equipotential surface, at longer wavelengths (>100 km). Previously compiled data on impact crater depth and diameter [Schenk, P.M., 2002. Nature 417, 419-421] on Europa show good agreement with the spectral decrease predicted by the model and require a lithosphere thicker than 2.5 km. A more realistic model, including a ductile lower lithosphere, requires a thickness greater than 3.5 km. Future measurements of topography in the 10-100 km wavelength band will provide tight constraints on lithospheric strength.     

Size scales over which ordinary chondrites and their parent asteroids are homogeneous in oxidation state and oxygen-isotopic composition

Literature data demonstrate that on a global, asteroid-wide scale (plausibly on the order of 100 km), ordinary chondrites (OC) have heterogeneous oxidation states and O-isotopic compositions (represented, respectively, by the mean olivine Fa and bulk Δ¹⁷O compositions of equilibrated samples). Samples analyzed here include: (a) two H5 chondrite Antarctic finds (ALHA79046 and TIL 82415) that have the same cosmic-ray exposure age (7.6 Ma) and were probably within ∼1 km of each other when they were excavated from the H-chondrite parent body, (b) different individual stones from the Holbrook L/LL6 fall that were probably within ∼1 m of each other when their parent meteoroid penetrated the Earth’s atmosphere, and (c) drill cores from a large slab of the Estacado H6 find located within a few tens of centimeters of each other. Our results indicate that OC are heterogeneous in their bulk oxidation state and O-isotopic composition on 100-km-size scales, but homogeneous on meter-, decimeter- and centimeter-size scales. (On kilometer size scales, oxidation state is heterogeneous, but O isotopes appear to be homogeneous.) The asteroid-wide heterogeneity in oxidation state and O-isotopic composition was inherited from the solar nebula. The homogeneity on small size scales was probably caused in part by fluid-assisted metamorphism and mainly by impact-gardening processes (which are most effective at mixing target materials on scales of ?1 m).     

Rare earth elements (REE) and yttrium in stream waters, stream sediments, and Fe-Mn oxyhydroxides: Fractionation, speciation, and controls over REE + Y patterns in the surface environment

We have collected ∼500 stream waters and associated bed-load sediments over an ∼400 km² region of Eastern Canada and analyzed these samples for Fe, Mn, and the rare earth elements (REE + Y). In addition to analyzing the stream sediments by total digestion (multi-acid dissolution with metaborate fusion), we also leached the sediments with 0.25 M hydroxylamine hydrochloride (in 0.05 M HCl), to determine the REE + Y associated with amorphous Fe- and Mn-oxyhydroxide phases. We are thus able to partition the REE into “dissolved” (<0.45 μm), labile (hydroxylamine) and detrital sediment fractions to investigate REE fractionation, and in particular, with respect to the development of Ce and Eu anomalies in oxygenated surface environments. Surface waters are typically LREE depleted ([La/Sm]_NASC ranges from 0.16 to 5.84, average = 0.604, n = 410; where the REE are normalized to the North America Shale Composite), have strongly negative Ce anomalies ([Ce/Ce^∗]_NASC ranges from 0.02 to 1.25, average = 0.277, n = 354), and commonly have positive Eu anomalies ([Eu/Eu^∗]_NASC ranges from 0.295 to 1.77, average = 0.764, n = 84). In contrast, the total sediment have flatter REE + Y patterns relative to NASC ([La/Sm]_NASC ranges from 0.352 to 1.12, average = 0.778, n = 451) and are slightly middle REE enriched ([Gd/Yb]_NASC ranges from 0.55 to 3.75, average = 1.42). Most total sediments have negative Ce and Eu anomalies ([Ce/Ce^∗]_NASC ranges from 0.097 to 2.12, average = 0.799 and [Eu/Eu^∗]_NASC ranges from 0.39 to 1.43, average = 0.802). The partial extraction sediments are commonly less LREE depleted than the total sediments ([La/Sm]_NASC ranges from 0.24 to 3.31, average = 0.901, n = 4537), more MREE enriched ([Gd/Yb]_NASC ranges from 0.765 to 6.28, average = 1.97) and Ce and Eu anomalies (negative and positive) are more pronounced.The partial extraction recovered, on average ∼20% of the Fe in the total sediment, ∼80% of the Mn, and 21-29% of the REEs (Ce = 19% and Y = 32%). Comparison between REEs in water, partial extraction and total sediment analyses indicates that REEs + Y in the stream sediments have two primary sources, the host lithologies (i.e., mechanical dispersion) and hydromorphically transported (the labile fraction). Furthermore, Eu appears to be more mobile than the other REE, whereas Ce is preferentially removed from solution and accumulates in the stream sediments in a less labile form than the other REEs + Y. Despite poor statistical correlations between the REEs + Y and Mn in either the total sediment or partial extractions, based on apparent distribution coefficients and the pH of the stream waters, we suggest that either sediment organic matter and/or possibly δ-MnO₂/FeOOH are likely the predominant sinks for Ce, and to a lesser extent the other REE, in the stream sediments.     

Church-based social capital, networks and geographical scale: Katrina evacuation, relocation, and recovery in a New Orleans Vietnamese American community

This research examines the role of social capital and networks to explain the evacuation, relocation, and recovery experiences of a Vietnamese American community in New Orleans, Louisiana in the aftermath of Hurricane Katrina. As the single largest community institution, the parish church’s complex bonding and bridging social capital and networks proved particularly critical in part because of its historically based ontological security. The process of evacuation, but especially relocation and recovery, was dependent on deploying co-ethnic social capital and networks at a variety of geographical scales. Beyond the local or community scale, extra-local, regional, and national scales of social capital and networks reproduced a spatially redefined Vietnamese American community. Part of the recovery process included constructing discursive place-based collective-action frames to successfully contest a nearby landfill that in turn engendered social capital and networks crossing ethnic boundaries to include the extra-local African American community. Engaging social capital and networks beyond the local geographical scale cultivated a Vietnamese American community with an emergent post-Katrina cultural and political identity.     

Zur Luftverunreinigung durch turbulente Diffusion

Zusammenfassung Nach einer kurzen Diskussion der theoretischen Grundlagen für die Berechnung der Ausbreitung von schädlichen Beimengungen der Luft auf Grund der Gesetzmäßigkeiten der turbulenten Diffusion wird mit Hilfe der vonPasquill angegebenen empirischen Beziehungen zwischen Streuungsmaßen und Quelldistanz die Formel für die Immissionskonzentration am Boden abgeleitet. An Hand einer Reihe von Beispielen wird das Bodenkonzentrationsfeld relativ zu dem Punkt maximaler Immissionskonzentration berechnet, wobei eine feste Schornsteinhöhe von 40 m und eine für mittlere Emittenten zutreffende Auswurfmenge an schädlichen Gasen vorausgesetzt wird. Die Abbildungen zeigen die starke Modifikation des Konzentrationsfeldes durch die meteorologischen Verhältnisse, je nachdem, ob ein stabiler, indifferenter oder labiler Ausbreitungstyp vorliegt.

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Summary After a short discussion of the theoretical background for the calculation of the spreading of the noxious pollutants of the air based on the laws of turbulent diffusion a formula is derived for the concentration of the immisison on the ground with help of the empirical relations between the dispersion measures and the distance from the source given byPasquill. In some examples the concentration field at the ground relative to the point of maximum immission is calculated, considering a fixed height of the funnel of 40 m and a quantity of noxious gases as usually found with average emittents. The figures show a strong modification of the concentration field by the meteorological situation according to whether the spreading type is stable, indifferent or unstable.

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Résumé On discute tout d'abord brièvement les fondements théoriques permettant de calculer la propagation des pollutions atmosphériques; pour ce faire, on se base sur les lois de la diffusion turbulente. On en tire ensuite une formule de calcul de la concentration des immissions à l'aide des relations empiriques dePasquill, relations existant entre les quantités répandues et la distance de la source. On calcule au moyen d'une série d'exemples le champ des concentrations au sol par rapport au point de concentration maximum; pour ce faire, on admet und chemineé fixe de 40 mètres de hauteur et une quantité de gaz nocifs correspondant à une moyenne de polluants. Les figures montrent la forte modification que subit la répartition des concentrations selon les conditions météorologiques, en d'autres termes si l'on a affaire à un type de propagation stable, indifférent ou instable.

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An investigation of the size frequency resolution of an aerosol by the static method of diffusion

Summary Theoretical considerations are put forward for identifying the Dynamic and Static Methods of Diffusion measurements of aerosol particles. On the basic of this equality the Exhaustion Method of Diffusion developed byPollak andMetnieks for the size-frequency resolution of a heterogeneous aerosol by Dynamic Diffusion Methods should be applicable to Static Methods. This hypothesis is tested using a Pollak-Nolan nucleus counter as decay vessel. It is shown that turbulence in the counter is the most serious obstacle against this type of approach to finding the size components of an aerosol. The time involved, the high humidity in the counter and a hypothetical nucleus-free zone are other major difficulties for relatively large Aitken nuclei. It is established, however, that for particles whose diffusion coefficient is of the order of 10^–3 cm² sec^–1, the proposed method should be quite workable.     

Wave Decay and Its Characteristics in Surf Zone

In accordance with the similarity between breaking waves and hydraulic jumps, the expressions for estimating wave decay and wave energy dissipation in the surf zone are derived based on the fundamental equations of fluid mechanics. Using the numerical solution of cnoidal wave theory, the various kinematic properties of waves in the surf zone, including the relative wave crest height, wave energy, and radiation stress are discussed. The values calculated with the method proposed in this paper are in good agreement with the experimental data gained by other researchers. The present expressions can be used in the studies of sediment transport on gently sloping beaches, especially on muddy beaches.