PHYSICAL PROPERTIES OF METEORITES AND INTERPLANETARY DUST PARTICLES: CLUES TO THE PROPERTIES OF THE METEORS AND THEIR PARENT BODIES

Meteorites, generally 1 cm or larger in size that are believed to sample asteroids, and interplanetary dust particles (IDPs), generally 5–50 μm in size that are believed to sample both asteroids and comets, span the size range of the meteors. Thus, the physical properties of the meteorites and the IDPs are likely to constrain the properties of the meteors and their parent bodies. Measurements of the density, porosity, longitudinal and transverse speeds of sound, elastic modulus, and bulk modulus, as well as imaging of the internal structure by Computed Microtomography indicate that unweathered samples of chondritic meteorites are more porous and have lower sound velocities than compact terrestrial rocks. In general, the IDPs are even more porous than the chondritic meteorites. The impact energy per unit target mass required to produce a barely catastrophic disruption (Q ^* _D) for anhydrous ordinary chondrite meteorites is twice that for terrestrial basalt or glass, indicating that collisional disruption of anhydrous meteorites requires more energy than for a compact basalt. These results indicate that most stone meteors are likely to be weak, porous objects, and that the parent bodies of the anhydrous stone meteorites are likely to be more difficult to disrupt than compact terrestrial basalt.     

Naturally acid waters from Copahue volcano,Argentina

Volcanic acid sulfate–chloride brines form through absorption of volcanic vapors in shallow reservoirs of meteoric water. Reaction with surrounding volcanic rocks leads to partial neutralization of the fluids and precipitation of secondary minerals. Chemical data of such acid waters from Copahue volcano, Argentina, covering 8 years of observations, show evidence for changes in composition related to water rock interaction at depth prior to emergence of the fluids at the surface. The chemical composition changed dramatically during the 2000 eruption of Copahue, with enhanced concentrations and fluxes of Mg, Na, Fe and Al, followed in 2001 by rapidly declining concentrations and element fluxes. The subsequent 5 years saw more variable element ratios and strong depletions in K and Al. Most incompatible elements are released from the rock matrix stochiometrically, whereas some elements are enriched through vapor input from the magma (As, Pb, Zn). Most fluids have LREE enrichments relative to the rock matrix, but during periods of new magma intrusion the LREE enrichment decreases as does the magnitude of the negative Eu anomaly in the fluids. These observations are interpreted assuming early dissolution of plagioclase, olivine and volcanic glass that occurs during intrusion of new magma into the hydrothermal system. The high field strength elements are virtually immobile even in these hot acid fluids, with Nb and Ta more so than Hf and Zr. The mobility of U and Th in these fluids is comparable, at variance with Th behavior in neutral fluids. The local rivers and lakes of Copahue are fertilized by volcanic dissolved P, and most surface waters with pH < 3 have high levels of As. The acid fluids from Copahue may be surficial analogs for deep subduction fluids that evolve below zones of arc magma generation as well as for early Mars environments that are thought to have had large acid lakes.     

Identifying compact moving groups based on stellar kinematics data

We investigate the possibility of identifying substructures in velocity space in three well-known stellar streams (the Hyades, the Pleiades, and Ursa Major) using a cluster-analysis method (the maximum-density method). Only compact groups in velocity space with radii no greater than 5 km/s and containing at least 10 members were considered. As an example, we analyzed a sample of 5377 stars from the HIPPARCOS catalog with known radial velocities located within 75 pc of the Sun. We identified a total of 24 groups of stars. Monte Carlo simulations show that most (about 80%) of these groups could be the results of Poisson noise and observational errors. Bona fide identification (at a significance level greater than 90%) of compact moving groups based on kinematic data requires higher accuracy of the stellar spatial velocities (with errors no greater than 1–2 km/s in each component).     

Effects of sediment iron mineral composition on microbially mediated changes in divalent metal speciation: Importance of ferrihydrite

Dissimilatory metal reducing bacteria (DMRB) can influence geochemical processes that affect the speciation and mobility of metallic contaminants within natural environments. Most investigations into the effect of DMRB on sediment geochemistry utilize various synthetic oxides as the Fe^III source (e.g., ferrihydrite, goethite, hematite). These synthetic materials do not represent the mineralogical composition of natural systems, and do not account for the effect of sediment mineral composition on microbially mediated processes. Our experiments with a DMRB (Shewanella putrefaciens 200) and a divalent metal (Zn^II) indicate that, while complexity in sediment mineral composition may not strongly impact the degree of “microbial iron reducibility,” it does alter the geochemical consequences of such microbial activity. The ferrihydrite and clay mineral content are key factors. Microbial reduction of a synthetic blend of goethite and ferrihydrite (VHSA-G) carrying previously adsorbed Zn^II increased both [Zn^II-aq] and the proportion of adsorbed Zn^II that is insoluble in 0.5 M HCl. Microbial reduction of Fe^III in similarly treated iron-bearing clayey sediment (Fe-K-Q) and hematite sand, which contained minimal amounts of ferrihydrite, had no similar effect. Addition of ferrihydrite increased the effect of microbial Fe^III reduction on Zn^II association with a 0.5 M HCl insoluble phase in all sediment treatments, but the effect was inconsequential in the Fe-K-Q. Zinc k-edge X-ray absorption spectroscopy (XAS) data indicate that microbial Fe^III reduction altered Zn^II bonding in fundamentally different ways for VHSA-G and Fe-K-Q. In VHSA-G, ZnO₆ octahedra were present in both sterile and reduced samples; with a slightly increased average Zn-O coordination number and a slightly higher degree of long-range order in the reduced sample. This result may be consistent with enhanced Zn^II substitution within goethite in the microbially reduced sample, though these data do not show the large increase in the degree of Zn-O-metal interactions expected to accompany this change. In Fe-K-Q, microbial Fe^III reduction transforms Zn-O polyhedra from octahedral to tetrahedral coordination and leads to the formation of a ZnCl₂ moiety and an increased degree of multiple scattering. This study indicates that, while many sedimentary iron minerals are easily reduced by DMRB, the effects of microbial Fe^III reduction on trace metal geochemistry are dependent on sediment mineral composition.     

Identification of zones of preferential groundwater tracer transport using a mobile downhole fluorometer

A mobile downhole fluorometer was used to detect zones of preferential groundwater tracer transport into an observation well. Identification of such zones is not possible if individual samples are collected over the wells entire screened interval. Laboratory-based tests using the fluorometer, and a purpose-built apparatus demonstrated that the fluorometer could be used with tracers to characterise well water flow regimes. During field investigations in a porous aquifer, the fluorometer monitored tracer concentrations in an observation well with a 12-m-long screen, 10 m down the hydraulic gradient from a fully penetrating injection well. Test results showed that the tracer occurred in the observation well over a discrete 2.5-m-thick interval. Single-well dilution test and vertical-flow data indicated that water entered the well at additional depths, but no tracer was detected at these levels. A numerical model reproducing dilution test concentration profiles indicated that water entered the well in many of these horizons at comparable velocities to those in the tracer-bearing zone. These data suggest that groundwater flow direction varied with depth in the aquifer under investigation. Moreover, simulations of tracer arrival indicated that the tracer distribution observed in the observation well was derived from a horizon that may be no thicker than 0.5 m.

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Resumen Se utilizó un fluorómetro móvil descendente para detectar zonas de transporte preferencial de trazadores de agua subterránea en un pozo de observación. La identificación de tales zonas no es posible si se colectan muestras individuales en todo el intervalo enmallado del pozo. En base a pruebas de laboratorio utilizando el fluorómetro y un aparato especial construido se demostró que el fluorómetro podría utilizarse con trazadores para caracterizar ambientes de flujo de agua en los pozos. Durante investigaciones de campo en un acuífero poroso, el fluorómetro monitoreó concentraciones de trazadores en un pozo de observación con una malla de 12 m de largo, 10 m abajo del gradiente hidráulico de un pozo de inyección que penetra totalmente el acuífero. Los resultados de las pruebas muestran que el trazador se presentó en el pozo de observación en un intervalo discreto de 2.5 m de espesor. Pruebas de dilución en un solo pozo y datos de flujo vertical indicaron que el agua entró al. pozo en profundidades adicionales, pero que no se detectó el trazador en esos niveles. Un modelo numérico que reproduce los perfiles de concentración de las pruebas de dilución indica que el agua entró al. pozo en muchos de estos horizontes en velocidades comparables a las existentes en la zona portadora de trazadores. Estos datos sugieren que la dirección de flujo de agua subterránea varió con la profundidad en el acuífero bajo investigación. Por otra parte, las simulaciones de llegada del trazador indicaron que la distribución del trazador observada en el pozo de observación se derivó de un horizonte cuyo espesor puede no ser mayor de 0.5 m.

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Résumé Un fluorimètre a été utilisé pour détecter les zones découlement préférentiel dun traceur dans un puits dobservation. Les identifications de telles zones nest pas possible si les échantillons individuels sont collectés sur toute les longueurs des parties crépinées du puits. Les tests de laboratoire utilisant un fluorimètre et un dispositif adéquat ont montré que les fluorimètres pouvaient servir à définir les régimes découlement de leau dans les puits. Durant les investigations de terrain dans un aquifère poreux, le fluorimètre a enregistré les concentrations du traceur dans un puits dobservation avec une crépine de 12 m. de long, 10 m sous le gradient hydraulique dans un puits à pénétration totale. Les résultats des tests ont montré que le traceur apparaissait dans un interval discret de 2.5 m de long. Un test de dilution en puits unique et des données découlement vertical ont indiqué que leau rentrait dans le puits à dautres profondeurs, mais le traceur na pas été détecté à ces niveaux. Un modèle numérique reproduisant le test de dilution et le profil de concentration a indiqué que leau entrait dans le puits à ces niveaux à des vitesses comparables de celle du traceur. Ces données suggèrent que lécoulement des eaux souterraines varie avec la profondeur dans laquifère sous la zone dinvestigation. Dailleurs, les simulations de larrivée du traceur ont montré que la distribution des concentrations du traceur dans le puits ne pouvaient pas être dues à une couche plus fine que 0.5 m.

     

Other Types of Contamination

"Hazardous levels of methane in the subsurface occur at numerous locations across the United States."     

Earthshine: not just for romantics

Peter Thejll, Chris Flynn, Hans Gleisner and Andrew Mattingly explain how classic astronomical techniques, married to the latest technology, can be used to observe earthshine, and learn about Earth's climate, among many other things.     

ACCELERATED EROSION ON THE NORTH CAROLINA COASTAL PLAIN

The Atlantic Coastal Plain of the United States has long been considered to have negligible soil erosion owing to low relief and permeable soils. Several recent studies point to a need for reassessment. Post-settlement erosion on the North Carolina Coastal Plain was estimated at the regional scale based on soil-profile truncation of a representative upland soil (Norfolk series). Apparent post-settlement soil-profile truncation ranges from 2.5 to 30 cm (mean 14.5 cm), implying a minimum regional average rate of 0.55 mm/yr since the region was settled in the 1700s. This amounts to a conservative estimated average rate of 9.3 t/ha/yr, a rate of the same general magnitude as estimated erosion rates in the North Carolina Piedmont, long recognized as a problem erosion area. Results imply accelerated water erosion despite the low slopes and permeable soils, and suggest the possibility of significant eolian erosion in the region. This rapid erosion in an area noted for its geological stability is a testament to the importance of vegetation cover and human agency in landscape evolution in the Coastal Plain. [Key words: accelerated erosion, soil profile, geomorphology, coastal plain, North Carolina.]     

The future of Stardust science

Recent observations indicate that >99% of the small bodies in the solar system reside in its outer reaches—in the Kuiper Belt and Oort Cloud. Kuiper Belt bodies are probably the best‐preserved representatives of the icy planetesimals that dominated the bulk of the solid mass in the early solar system. They likely contain preserved materials inherited from the protosolar cloud, held in cryogenic storage since the formation of the solar system. Despite their importance, they are relatively underrepresented in our extraterrestrial sample collections by many orders of magnitude (~10¹³ by mass) as compared with the asteroids, represented by meteorites, which are composed of materials that have generally been strongly altered by thermal and aqueous processes. We have only begun to scratch the surface in understanding Kuiper Belt objects, but it is already clear that the very limited samples of them that we have in our laboratories hold the promise of dramatically expanding our understanding of the formation of the solar system. Stardust returned the first samples from a known small solar system body, the Jupiter‐family comet 81P/Wild 2, and, in a separate collector, the first solid samples from the local interstellar medium. The first decade of Stardust research resulted in more than 142 peer‐reviewed publications, including 15 papers in Science. Analyses of these amazing samples continue to yield unexpected discoveries and to raise new questions about the history of the early solar system. We identify nine high‐priority scientific objectives for future Stardust analyses that address important unsolved problems in planetary science.