Olivine or impact melt: Nature of the “Orange” material on Vesta from Dawn

NASA’s Dawn mission observed a great variety of colored terrains on asteroid (4) Vesta during its survey with the Framing Camera (FC). Here we present a detailed study of the orange material on Vesta, which was first observed in color ratio images obtained by the FC and presents a red spectral slope. The orange material deposits can be classified into three types: (a) diffuse ejecta deposited by recent medium-size impact craters (such as Oppia), (b) lobate patches with well-defined edges (nicknamed “pumpkin patches”), and (c) ejecta rays from fresh-looking impact craters. The location of the orange diffuse ejecta from Oppia corresponds to the olivine spot nicknamed “Leslie feature” first identified by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157) from ground-based spectral observations. The distribution of the orange material in the FC mosaic is concentrated on the equatorial region and almost exclusively outside the Rheasilvia basin. Our in-depth analysis of the composition of this material uses complementary observations from FC, the visible and infrared spectrometer (VIR), and the Gamma Ray and Neutron Detector (GRaND). Several possible options for the composition of the orange material are investigated including, cumulate eucrite layer exposed during impact, metal delivered by impactor, olivine–orthopyroxene mixture and impact melt. Based on our analysis, the orange material on Vesta is unlikely to be metal or olivine (originally proposed by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157)). Analysis of the elemental composition of Oppia ejecta blanket with GRaND suggests that its orange material has ∼25% cumulate eucrite component in a howarditic mixture, whereas two other craters with orange material in their ejecta, Octavia and Arruntia, show no sign of cumulate eucrites. Morphology and topography of the orange material in Oppia and Octavia ejecta and orange patches suggests an impact melt origin. A majority of the orange patches appear to be related to the formation of the Rheasilvia basin. Combining the interpretations from the topography, geomorphology, color and spectral parameters, and elemental abundances, the most probable analog for the orange material on Vesta is impact melt.     

Carbon isotope effects associated with mixed-acid fermentation of saccharides by Clostridium papyrosolvens

In anoxic environments, microbial fermentation is the first metabolic process in the path of organic matter degradation. Since little is known about carbon isotope fractionation during microbial fermentation, we studied mixed-acid fermentation of different saccharides (glucose, cellobiose, and cellulose) in Clostridium papyrosolvens. The bacterium was grown anaerobically in batch under different growth conditions, both in pure culture and in co-culture with Methanobacterium bryantii utilizing H₂/CO₂ or Methanospirillum hungatei utilizing both H₂/CO₂ and formate. Fermentation products were acetate, lactate, ethanol, formate, H₂, and CO₂ (and CH₄ in methanogenic co-culture), with acetate becoming dominant at low H₂ partial pressures. After complete conversion of the saccharides, acetate was ¹³C-enriched (α_sacc/ac = 0.991-0.997), whereas lactate (α_sacc/lac = 1.001-1.006), ethanol (α_sacc/etoh = 1.007-1.013), and formate (α_sacc/form = 1.007-1.011) were ¹³C-depleted. The total inorganic carbon produced was only slightly enriched in ¹³C, but was more enriched, when formate was produced in large amounts, as ¹²CO₂ was preferentially converted with H₂ to formate. During biomass formation, ¹²C was slightly preferred (α_sacc/biom ≈ 1.002). The observations in batch culture were confirmed in glucose-limited chemostat culture at growth rates of 0.02-0.15 h⁻¹ at both low and high hydrogen partial pressures. Our experiments showed that the carbon flow at metabolic branch points in the fermentation path governed carbon isotope fractionation to the accumulated products. During production of pyruvate, C isotopes were not fractionated when using cellulose, but were fractionated to different extents depending on growth conditions when using cellobiose or glucose. At the first catabolic branch point (pyruvate), the produced lactate was depleted in ¹³C, whereas the alternative product acetyl-CoA was ¹³C enriched. At the second branch point (acetyl-CoA), the ethanol formed was 15.6-18.6‰ depleted in ¹³C compared to the alternative product acetate. At low hydrogen partial pressures, as normally observed under environmental conditions, fermentation of saccharides should mainly result in the production of acetate that is only slightly enriched in ¹³C (<3‰).     

CO<Subscript>2</Subscript> leakage through an abandoned well: problem-oriented benchmarks

The efficiency and sustainability of carbon dioxide (CO₂) storage in deep geological formations crucially depends on the integrity of the overlying cap-rocks. Existing oil and gas wells, which penetrate the formations, are potential leakage pathways. This problem has been discussed in the literature, and a number of investigations using semi-analytical mathematical approaches have been carried out by other authors to quantify leakage rates. The semi-analytical results are based on a number of simplifying assumptions. Thus, it is of great interest to assess the influence of these assumptions. We use a numerical model to compare the results with those of the semi-analytical model. Then we ease the simplifying restrictions and include more complex thermodynamic processes including sub- and supercritical fluid properties of CO₂ and non-isothermal as well as compositional effects. The aim is to set up problem-oriented benchmark examples that allow a comparison of different modeling approaches to the problem of CO₂ leakage.     

Natural Cycles and Transfer of Mercury Through Pacific Coastal Marsh Vegetation Dominated by Spartina foliosa and Salicornia virginica

The potential for marsh plants to be vectors in the transport of mercury species was studied in the natural, mature, tidal China Camp salt marsh on San Pablo Bay. The fluxes of organic matter, mercury (THg), and monomethylmercury (MeHg) were studied in natural stands of Spartina foliosa and Salicornia virginica. Seasonal fluxes from the sediment into aboveground biomass of live plants and subsequent transfer into the dead plant community by mortality were measured. Loss of THg and MeHg from the dead plant community through fragmentation, leaching, and excretion were calculated and were similar to net uptake. Seasonal data were added up to calculate annual mass balances. In S. foliosa, annual net production was 1,757 g DW m^?2, and the annual net uptakes in the aboveground biomass were 305 μg THg m^?2 and 5.720 μg MeHg m^?2. In S. virginica, annual net production was 2,117 g DW m^?2, and the annual net uptakes in aboveground biomass were 99.120 μg THg m^?2 and 1.990 μg MeHg m^?2. Of both plant species studied, S. foliosa had a slightly lower production rate but greater mercury species uptake and loss rates than S. virginica, and, consequently, it is to be expected that S. foliosa matter may affect the local and possibly the regional food web relatively more than S. virginica. However, the actual effects of the input of mercury-species-containing plant-derived particulate matter into the food webs would depend on trophic level, food preference, seasonal cycle of the consumer, total sediment surface area vegetated, location of the vegetation in the marsh landscape, and estuary bay landscape. Since the levels of mercury species in dead plant material greatly exceed those in live plant material (on a dry weight basis), detritivores would ingest greater mercury species concentrations than herbivores, and consumers of S. foliosa would ingest more than consumers of S. virginica. The greatest THg and MeHg losses of both plant species due to mortality and to fragmentation–leaching–excretion occurred in late spring and early autumn, which corresponds to peak MeHg levels observed in sediments of coastal systems of previous studies, suggesting enhanced THg–MeHg export from the marsh to the nearshore sediment.     

On the diffusion of rhodamine B on both sides of a river plume front

Conclusions In spite of poor weather conditions, for which vertical mixing down to the bottom is expected in general, the rhodamine is still trapped between vertical and horizontal gradients 3 days after release. The turbulent flux through the front is so small that a significant transport through the front can probably only be effected by vortices occasionally occurring — as observed in the experiment of October '82.The front-parallel and cross-frontal spreading velocities have the same order of magnitude as in October '82. However, the decrease of rhodamine peak concentration runs much slower: in the first experiment, the spreading was hindered by only two boundaries, the front and the bottom. Now, the rhodamine is embedded in a field of gradients, allowing spreading in one dimension only, i.e. parallel to the isopycnicals. In this stage only processes reducing the gradients can intensify the exchange.To understand the dynamic of oceanic features such as fronts and vortices, it is essential to observe the movement of a marked water mass within the whole system. Again, the reconstructed distributions show clearly the ability of rhodamine to trace processes in the order of magnitude of several miles right down to very small scale processes, as the diffusion of the tracer through the front.