Photometric,spectral phase and temperature effects on 4 Vesta and HED meteorites: Implications for the Dawn mission

Phase angle and temperature are two important parameters that affect the photometric and spectral behavior of planetary surfaces in telescopic and spacecraft data. We have derived photometric and spectral phase functions for the Asteroid 4 Vesta, the first target of the Dawn mission, using ground-based telescopes operating at visible and near-infrared wavelengths (0.4–2.5 μm). Photometric lightcurve observations of Vesta were conducted on 15 nights at a phase angle range of 3.8–25.7° using duplicates of the seven narrowband Dawn Framing Camera filters (0.4–1.0 μm). Rotationally resolved visible (0.4–0.7 μm) and near-IR spectral observations (0.7–2.5 μm) were obtained on four nights over a similar phase angle range. Our Vesta photometric observations suggest the phase slope is between 0.019 and 0.029 mag/deg. The G parameter ranges from 0.22 to 0.37 consistent with previous results (e.g., Lagerkvist, C.-I., Magnusson, P., Williams, I.P., Buontempo, M.E., Argyle, R.W., Morrison, L.V. [1992]. Astron. Astrophys. Suppl. Ser. 94, 43–71; Piironen, J., Magnusson, P., Lagerkvist, C.-I., Williams, I.P., Buontempo, M.E., Morrison, L.V. [1997]. Astron. Astrophys. Suppl. Ser. 121, 489–497; Hasegawa, S. et al. [2009]. Lunar Planet. Sci. 40. ID 1503) within the uncertainty. We found that in the phase angle range of 0° < α ? 25° for every 10° increase in phase angle Vesta’s visible slope (0.5–0.7 μm) increases 20%, Band I and Band II depths increase 2.35% and 1.5% respectively, and the BAR value increase 0.30. Phase angle spectral measurements of the eucrite Moama in the lab show a decrease in Band I and Band II depths and BAR from the lowest phase angle 13° to 30°, followed by possible small increases up to 90°, and then a dramatic drop between 90° and 120° phase angle. Temperature-induced spectral effects shift the Band I and II centers of the pyroxene bands to longer wavelengths with increasing temperature. We have derived new correction equations using a temperature series (80–400 K) of HED meteorite spectra that will enable interpretation of telescopic and spacecraft spectral data using laboratory calibrations at room temperature (300 K).     

Sedimentation in Lake Biel,an eutrophic,hard-water lake in northwestern Switzerland

A 2-year study of sedimentation in Lake Biel reveals that three major factors can account for the spatial and temporal patterns of tripton sedimentation. Allochthonous inputs of particulate matter mostly derived from the Aare river during spring snowmelt and other high-water periods, provide about 50% of the carbonate and 50% of the siliceous matter collected in sediment traps. Phytoplankton activity in this eutrophic, hard-water lake accounts for the rest of the carbonate and siliceous matter, the latter being mostly diatom frustules. Sedimentation rates are thus highest during periods of maximum river flooding, maximum phytoplankton activity and following the breakdown of the thermocline, at which time trapped particles settle out. Resuspension of sediment is important in at least one basin. Resolution of carbonate appears to be minor.     

A dynamo model with double diffusive convection for Mercury's core

A recent dynamo model for Mercury assumes that the upper part of the planet's fluid core is thermally stably stratified because the temperature gradient at the core–mantle boundary is subadiabatic. Vigorous convection driven by a superadiabatic temperature gradient at the boundary of a growing solid inner core and by the associated release of light constituents takes place in a deep sub-layer and powers a dynamo. These models have been successful at explaining the observed weak global magnetic field at Mercury's surface. They have been based on the concept of codensity, which combines thermal and compositional sources of buoyancy into a single variable by assuming the same diffusivity for both components. Actual diffusivities in planetary cores differ by a large factor. To overcome the limitation of the codensity model, we solve two separate transport equations with different diffusivities in a double diffusive dynamo model for Mercury. When temperature and composition contribute comparable amounts to the buoyancy force, we find significant differences to the codensity model. In the double diffusive case convection penetrates the upper layer with a net stable density stratification in the form of finger convection. Compared to the codensity model, this enhances the poloidal magnetic field in the nominally stable layer and outside the core, where it becomes too strong compared to observation. Intense azimuthal flow in the stable layer generates a strong axisymmetric toroidal field. We find in double diffusive models a surface magnetic field of the observed strength when compositional buoyancy plays an inferior role for driving the dynamo, which is the case when the sulphur concentration in Mercury's core is only a fraction of a percent.     

The GeoForschungsZentrum Potsdam/Groupe de Recherche de Gèodésie Spatiale satellite-only and combined gravity field models: EIGEN-GL04S1 and EIGEN-GL04C

The recent improvements in the Gravity Recovery And Climate Experiment (GRACE) tracking data processing at GeoForschungsZentrum Potsdam (GFZ) and Groupe de Recherche de Géodésie Spatiale (GRGS) Toulouse, the availability of newer surface gravity data sets in the Arctic, Antarctica and North-America, and the availability of a new mean sea surface height model from altimetry processing at GFZ gave rise to the generation of two new global gravity field models. The first, EIGEN-GL04S1, a satellite-only model complete to degree and order 150 in terms of spherical harmonics, was derived by combination of the latest GFZ Potsdam GRACE-only (EIGEN-GRACE04S) and GRGS Toulouse GRACE/LAGEOS (EIGEN-GL04S) mean field solutions. The second, EIGEN-GL04S1 was combined with surface gravity data from altimetry over the oceans and gravimetry over the continents to derive a new high-resolution global gravity field model called EIGEN-GL04C. This model is complete to degree and order 360 and thus resolves geoid and gravity anomalies at half- wavelengths of 55 km at the equator. A degree-dependent combination method has been applied in order to preserve the high accuracy from the GRACE satellite data in the lower frequency band of the geopotential and to form a smooth transition to the high-frequency information coming from the surface data. Compared to pre-CHAMP global high-resolution models, the accuracy was improved at a spatial resolution of 200 km (half-wavelength) by one order of magnitude to 3 cm in terms of geoid heights. The accuracy of this model (i.e. the commission error) at its full spatial resolution is estimated to be 15 cm. The model shows a reduced artificial meridional striping and an increased correlation of EIGEN-GL04C-derived geostrophic meridional currents with World Ocean Atlas 2001 (WOA01) data. These improvements have led to select EIGEN-GL04C for JASON-1 satellite altimeter data reprocessing. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Chloritoid stability in manganese rich low-grade metamorphic rocks,Venn-Stavelot Massif,Ardennes

The Southern Venn-Stavelot Massif is characterized by Ordovician and Devonian rocks very rich in manganese and aluminum, which are attacked by a low grade regional metamorphism. The assemblages 1 (phengite, paragonite, chlorite, chloritoid, garnet, quartz, hematite, rutile) and 2 (phengite, paragonite, chlorite, kaolinite (andalusite, pyrophyllite), garnet, quartz, hematite, rutile) are of basic interest for the formation of chloritoid. As the two rock types are isofaciell and quasi-identical in chemistry except for the iron oxides, there is clear evidence for the influence of on the chloritoid formation at its lower p-T stability limit. This can be shown by a discussion of the phase relations of chloritoid, garnet, kaolinite, chlorite and phengite in respect to the oxidation ratio mol 2 Fe₂O₃x 100/2 Fe₂O₃+ FeO of the host rocks. Especially chloritoid and chlorite change their chemistry in a characteristic way with rising oxidation ratio in getting richer and richer in manganese and magnesium (chloritoid) and magnesium (chlorite). A simultaneous increase in trivalent iron in these phases is supposed. At an oxidation ratio of 85–90 the stability limit of chloritoid is reached. The increasing substitution of manganese and magnesium up to this limit should have a stabilizing effect. In a rough estimate the oxygen partial pressure is supposed to be in the order of 10⁻¹⁰ atm at the stability limit of chloritoid assuming a temperature of metamorphism between 360–400° C. Rocks with oxidation ratios between 90 and 100 are characterized by the presence of kaolinite. If the oxidation ratio is still higher (all iron as Fe³⁺, parts of the manganese in the trivalent state), the rocks belong to assemblage 3 (phengite, paragonite, chlorite, viridine, (kaolinite), (garnet), quartz, hematite, braunite, rutile). Dedicated to Prof. Dr. K. Jasmund at his 60. birthday.     

Transfer and early diagenesis of biogenic silica oxygen isotope signals during settling and sedimentation of diatoms in a temperate freshwater lake (Lake Holzmaar, Germany)

We have investigated the transfer of oxygen isotope signals of diatomaceous silica (δ¹⁸O_diatom) from the epilimnion (0-7 m) through the hypolimnion to the lake bottom (∼20 m) in freshwater Lake Holzmaar, Germany. Sediment-traps were deployed in 2001 at depths of 7 and 16 m to harvest fresh diatoms every 28 days. The 7 m trap collected diatoms from the epilimnion being the main zone of primary production, while the 16 m trap collected material already settled through the hypolimnion. Also a bottom sediment sample was taken containing diatom frustules from approximately the last 25 years. The δ¹⁸O_diatom values of the 7 m trap varied from 29.4‰ in spring/autumn to 26.2‰ in summer according to the temperature dependence of oxygen isotope fractionation and represent the initial isotope signal in this study. Remarkably, despite the short settling distance δ¹⁸O_diatom values of the 7 and the 16 m trap were identical only during spring and autumn seasons while from April to September δ¹⁸O_diatom values of the 16 m trap were roughly ∼1.5‰ enriched in ¹⁸O compared to those of the 7 m trap. Isotopic exchange with the isotopically lighter water of the hypolimnion would shift the δ¹⁸O_diatom value to lower values during settling from 7 to 16 m excluding this process as a cause for the deviation. Dissolution of opal during settling with intact organic coatings of the diatom cells and near neutral pH of the water should only cause a minor enrichment of the 16 m values. Nevertheless, opal from the bottom sediment was found to be 2.5‰ enriched in ¹⁸O compared to the weighted average of the opal from the 7 m trap. Thus, resuspension of bottom material must have contributed to the intermediate δ¹⁸O_diatom signal of the 16 m trap during summer. Dissolution experiments allowed further investigation of the cause for the remarkably enriched δ¹⁸O_diatom value of the bottom sediment. Experiments with different fresh diatomaceous materials show an increase of opaline ¹⁸O at high pH values which is remarkably reduced when organic coatings of the cells still exist or at near neutral pH. In contrast, high pH conditions do not affect the δ¹⁸O_diatom values of sub-fossil and even fossil opal. IR analyses show that the ¹⁸O enrichment of the sedimentary silica is associated with a decrease in Si-OH groups and the formation of Si-O-Si linkages. This indicates a silica dehydroxylation process as cause for the isotopic enrichment of the bottom sediment. Silica dissolution and dehydroxylation clearly induce a maturation process of the diatom oxygen isotope signal presumably following an exponential behaviour with a rapid initial phase of signal alteration. The dynamics of this process is of particular importance for the quantitative interpretation of sedimentary δ¹⁸O_diatom values in terms of palaeothermometry.     

Risk-targeted seismic design maps for mainland France

In this article, the recently proposed approach known as ‘risk targeting’ for the development of national seismic design maps is investigated for mainland France. Risk targeting leads to ground-motion maps that, if used for design purposes, would lead to a uniform level of risk nationally. The Eurocode 8 design loads currently in force for France are used as the basis of this study. Because risk targeting requires various choices on, for example, the level of acceptable risk to be made a priori and these choices are not solely engineering decisions but involve input from decision makers we undertake sensitivity tests to study their influence. It is found that, in contrast to applications of this methodology for US cities, risk targeting does not lead to large modifications with respect to the national seismic hazard map nor to changes in the relative ranking of cities with respect to their design ground motions. This is because the hazard curves for French cities are almost parallel. In addition, we find that using a target annual collapse probability of about 10^?5 for seismically designed buildings and a probability of collapse when subjected to the design PGA of 10^?5 leads to reasonable results. This is again in contrast to US studies that have adopted much higher values for both these probabilities.     

Uranium speciation and stability after reductive immobilization in aquifer sediments

It has generally been assumed that the bioreduction of hexavalent uranium in groundwater systems will result in the precipitation of immobile uraninite (UO₂). In order to explore the form and stability of uranium immobilized under these conditions, we introduced lactate (15 mM for 3 months) into flow-through columns containing sediments derived from a former uranium-processing site at Old Rifle, CO. This resulted in metal-reducing conditions as evidenced by concurrent uranium uptake and iron release. Despite initial augmentation with Shewanella oneidensis, bacteria belonging to the phylum Firmicutes dominated the biostimulated columns. The immobilization of uranium (∼1 mmol U per kg sediment) enabled analysis by X-ray absorption spectroscopy (XAS). Tetravalent uranium associated with these sediments did not have spectroscopic signatures representative of U-U shells or crystalline UO₂. Analysis by microfocused XAS revealed concentrated micrometer regions of solid U(IV) that had spectroscopic signatures consistent with bulk analyses and a poor proximal correlation (μm scale resolution) between U and Fe. A plausible explanation, supported by biogeochemical conditions and spectral interpretations, is uranium association with phosphoryl moieties found in biomass; hence implicating direct enzymatic uranium reduction. After the immobilization phase, two months of in situ exposure to oxic influent did not result in substantial uranium remobilization. Ex situ flow-through experiments demonstrated more rapid uranium mobilization than observed in column oxidation studies and indicated that sediment-associated U(IV) is more mobile than biogenic UO₂. This work suggests that in situ uranium bioimmobilization studies and subsurface modeling parameters should be expanded to account for non-uraninite U(IV) species associated with biomass.     

Can coronal loop transients be driven magnetically?

In this note we investigate the possibility of magnetic driving of loop transients. The action of local magnetic forces to balance gravity in a coasting loop and to confine the loop has been proposed by Mouschovias and Poland (1977). In this paper we use similar configurations but deal with the global field structure and present models which show both the initial phase of large acceleration and the later phase of almost constant velocity. We use very simple one-dimensional models consisting of a ring current which is subjected to gravitational attraction. The velocity curves calculated for these models are in good agreement with the observations. Therefore we conclude that if such ring currents can be produced fast enough in the solar corona, they are capable of driving the loop transients observed in the ATM white light coronagraph.On leave from Max-Planck Institut für Physik und Astrophysik, Föhringer Ring 6, 8 München 40, F.R.G.The National Center for Atmospheric Research is sponsored by the National Science Foundation.