Heterogeneous Fe(II) oxidation and zeta potential     

Andrew Barnes  Devin J. Sapsford  Mathew Dey  Keith P. Williams 《Journal of Geochemical Exploration》2009,100(2-3):192-198

The determination of electrophoretic mobility and zeta potential was used as a diagnostic tool, alongside kinetic experiments, to delineate between three plausible mechanisms for the heterogeneous oxidation of Fe(II) by dissolved oxygen. One of these mechanisms is dependant on the positive surface charge that exists on Fe(III) (oxy)hydroxide surfaces at pH values below the Iso-Electric Point (IEP). However, this mechanism can be disputed as catalysis is observed on Fe(III) (oxy)hydroxide surfaces above the IEP despite a negative zeta potential. As well as an IEP shift an overall reduction of the magnitude of the zeta potential is observed in samples of field Fe(III) (oxy)hydroxide collected from the Taff Merthyr mine water treatment site in South Wales, UK. Low zeta potentials determined in mine water treatment systems will have beneficial effects for particle coagulation and settling in passive mine water treatment systems.  相似文献   

Do ice caves exist on Mars?     

K.E. Williams  Christopher P. McKay  James W. Head 《Icarus》2010,209(2):358-368

We have developed a numerical model for assessing the lifetime of ice deposits in martian caves that are open to the atmosphere. Our model results and sensitivity tests indicate that cave ice would be stable over significant portions of the surface of Mars. Ice caves on Earth commonly occur in lava tubes, and Mars has been significantly resurfaced by volcanic activity during its history, including the two main volcanic provinces, the Tharsis and Elysium rises. These areas, known or suspected of having subsurface caves and related voids are among the most favorable regions for the occurrence of ice stability. The martian ice cave model predicts regions which, if caves occur, would potentially be areas of astrobiological importance as well as possible water sources for future human missions to Mars.  相似文献   

Majoritic garnet: A new approach to pressure estimation of shock events in meteorites and the encapsulation of sub-lithospheric inclusions in diamond   总被引：2，自引：0，他引：2  

Kenneth D. Collerson  Quentin Williams  Soichi Omori  Eiji Ohtani 《Geochimica et cosmochimica acta》2010,74(20):5939-5957

A means for estimating pressures in natural samples based on both the coupled substitution (Na+)^[1+] (Ti + ^[VI]Si)^[4+] = (M)^[2+] (Al + Cr)^[3+], and the classic pyroxene-stoichiometry majorite-substitution into garnet at high-pressure, is derived for garnets with majoritic chemistry. The technique is based on a compilation of experimental data for different bulk compositions. It is compositionally and thermally robust and can be used to estimate pressures experienced by natural materials during formation of majoritic garnet. In addition, it can be used either retrospectively, or in new experimental studies to establish the pressures of crystallization of reaction products, and determine if disequilibrium is recorded by the chemistries of majoritic garnets. Pressures are calculated based on majoritic chemistries in chondritic meteorites and diamond inclusions. Majoritic garnets associated with Mg perovskite in shocked L chondrites (n = 4) yield uniform pressures of 23.8 ± 0.2 GPa that are slightly higher than pressures recorded by majoritic garnet in shock-derived melt veins in L chondrites (22.4 ± 0.6 GPa; n = 5). Similar pressures are also exhibited by shock-derived majoritic garnets in H chondrites (22.2 ± 1.1 GPa; n = 3). Diamond inclusions with eclogitic and peridotitic majoritic garnet chemistries exhibit mean pressures of 10.7 ± 2.7 GPa (n = 30) and 8.3 ± 1.6 GPa (n = 15) respectively, consistent with a sub-lithospheric origin. However, pressures defined by majoritic diamond inclusions from Jagersfontein (22.3 ± 0.8 GPa and 16.9 ± 1 GPa), Monastery (15.7 ± 7 GPa) and Kankan (15.5 ± 0.2 GPa) show that these inclusions originated from the mantle transition zone. Thus, this new single-phase method for pressure estimation has unmatched potential to map the depth of formation of garnets with majoritic chemistries that occur as diamond inclusions in all parageneses except those that include Ca silicate perovskite. The derived pressures confirm the sub-lithospheric origin of eclogitic majoritic diamond inclusions, and thus provide a more comprehensive picture of the important role of storage of oceanic lithosphere in the transition zone.  相似文献   

Changes to the Editorial Board: meeting the challenges of a changing landscape in international science     

Bernd Lehmann  Patrick J. Williams 《Mineralium Deposita》2010,45(1):1-1

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Meteor showers associated with 2003EH1     

P. B. Babadzhanov  I. P. Williams  G. I. Kokhirova 《Monthly notices of the Royal Astronomical Society》2008,386(4):2271-2277

Using the Everhart radau19 numerical integration method, the orbital evolution of the near-Earth asteroid 2003EH1 is investigated. This asteroid belongs to the Amor group and is moving on a comet-like orbit. The integrations are performed over one cycle of variation of the perihelion argument ω. Over such a cycle, the orbit intersect that of the Earth at eight different values of ω. The orbital parameters are different at each of these intersections and so a meteoroid stream surrounding such an orbit can produce eight different meteor showers, one at each crossing. The geocentric radiants and velocities of the eight theoretical meteor showers associated with these crossing points are determined. Using published data, observed meteor showers are identified with each of the theoretically predicted showers. The character of the orbit and the existence of observed meteor showers associated with 2003EH1 confirm the supposition that this object is an extinct comet.  相似文献   

The Palomar‐Quest digital synoptic sky survey     

S.G. Djorgovski  C. Baltay  A.A. Mahabal  A.J. Drake  R. Williams  D. Rabinowitz  M.J. Graham  C. Donalek  E. Glikman  A. Bauer  R. Scalzo  N. Ellman  J. Jerke 《Astronomische Nachrichten》2008,329(3):263-265

We describe briefly the Palomar‐Quest (PQ) digital synoptic sky survey, including its parameters, data processing, status, and plans. Exploration of the time domain is now the central scientific and technological focus of the survey. To this end, we have developed a real‐time pipeline for detection of transient sources.We describe some of the early results, and lessons learned which may be useful for other, similar projects, and time‐domain astronomy in general. Finally, we discuss some issues and challenges posed by the real‐time analysis and scientific exploitation of massive data streams from modern synoptic sky surveys. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

A submillimeter view of protoplanetary dust disks     

Sean M. Andrews  Jonathan P. Williams 《Astrophysics and Space Science》2008,313(1-3):119-122

We present some results from our submillimeter single-dish and aperture synthesis imaging surveys of protoplanetary disks using the JCMT, CSO, and Submillimeter Array (SMA) on Mauna Kea, Hawaii. Employing a simple disk model, we simultaneously fit the spectral energy distributions and spatially resolved submillimeter continuum emission from our SMA survey to constrain disk structure properties, including surface density profiles and sizes. The typical disk structure we infer is consistent with a fiducial accretion disk model with a viscosity parameter α≈0.01. Combined with a large, multiwavelength single-dish survey of similar disks, we show how these observations provide evidence for significant grain growth and rapid evolution in the outer regions of disks, perhaps due to an internal photoevaporation process. In addition, we discuss SMA observations of the disks in the Orion Trapezium (proplyds) in the context of disk evolution in a more extreme environment.  相似文献   

Sub-kilometer fans in Mojave Crater, Mars     

Rebecca M.E. Williams  Michael C. Malin 《Icarus》2008,198(2):365-383

In the Xanthe Terra region of Mars, two forms of flow fields are observed on the walls of Mojave Crater, a fresh impact site with a maximum age of Late Hesperian. Flow fields with steep, lobate margins are consistent with emplacement of a highly viscous medium. The focus of this report is on fan-shaped landforms that share many morphologic attributes in common with terrestrial alluvial fans, including a semi-conical form, branching tributary networks, distributary channels and incised channels. Collectively, these sub-kilometer-scale landforms have attributes consistent with overland flow of fluids and formation of fans by water and gravity-driven alluvial sedimentation. Superposition and cross-cutting relationships indicate that fan formation occurred in multiple phases that may have been a single event or multiple, temporally distinct episodes. Many aspects of the fan formation are ill-constrained, including the amount and source of fluid as well as the duration of fan formation and modification. Fans are concentrated on the crater walls and the ejecta blanket shows minimal evidence of fluvial erosion. Similar fan-shaped landforms to those in Mojave Crater are extremely rare on Mars. The localization of fans to Mojave Crater implies that the impact event played a role in the formation of these sub-kilometer fans. This is the first geologic evidence on Mars that tentatively supports a link between impact crater events and the liberation of water for surface runoff.  相似文献   

Recent geological and hydrological activity on Mars: The Tharsis/Elysium corridor     

James M. Dohm  Robert C. Anderson  Nadine G. Barlow  Hirdy Miyamoto  Ashley G. Davies  G. Jeffrey Taylor  Victor R. Baker  William V. Boynton  John Keller  Kris Kerry  Daniel Janes  Alberto G. Fairn  Dirk Schulze-Makuch  Mihaela Glamoclija  Lucia Marinangeli  Gian G. Ori  Robert G. Strom  Jean-Pierre Williams  Justin C. Ferris  J.A.P. Rodríguez  Miguel A. de Pablo  Suniti Karunatillake 《Planetary and Space Science》2008,56(7):985-1013

The paradigm of an ancient warm, wet, and dynamically active Mars, which transitioned into a cold, dry, and internally dead planet, has persisted up until recently despite published Viking-based geologic maps that indicate geologic and hydrologic activity extending into the Late Amazonian epoch. This paradigm is shifting to a water-enriched planet, which may still exhibit internal activity, based on a collection of geologic, hydrologic, topographic, chemical, and elemental evidences obtained by the Viking, Mars Global Surveyor (MGS), Mars Odyssey (MO), Mars Exploration Rovers (MER), and Mars Express (MEx) missions. The evidence includes: (1) stratigraphically young rock materials such as pristine lava flows with few, if any, superposed impact craters; (2) tectonic features that cut stratigraphically young materials; (3) features with possible aqueous origin such as structurally controlled channels that dissect stratigraphically young materials and anastomosing-patterned slope streaks on hillslopes; (4) spatially varying elemental abundances for such elements as hydrogen (H) and chlorine (Cl) recorded in rock materials up to 0.33 m depth; and (5) regions of elevated atmospheric methane. This evidence is pronounced in parts of Tharsis, Elysium, and the region that straddles the two volcanic provinces, collectively referred to here as the Tharsis/Elysium corridor. Based in part on field investigations of Solfatara Crater, Italy, recommended as a suitable terrestrial analog, the Tharsis/Elysium corridor should be considered a prime target for Mars Reconnaissance Orbiter (MRO) investigations and future science-driven exploration to investigate whether Mars is internally and hydrologically active at the present time, and whether the persistence of this activity has resulted in biologic activity.  相似文献   

Is the sky falling? Searching for stellar streams in the local Milky Way disc in the CORAVEL and RAVE surveys     

G. M. Seabroke  G. Gilmore  A. Siebert  O. Bienaymé  J. Binney  J. Bland-Hawthorn  R. Campbell  K. C. Freeman  B. Gibson  E. K. Grebel  A. Helmi  U. Munari  J. F. Navarro  Q. A. Parker  A. Siviero  M. Steinmetz  F. G. Watson  R. F. G. Wyse  T. Zwitter  J. Peñarrubia  M. C. Smith  M. Williams 《Monthly notices of the Royal Astronomical Society》2008,384(1):11-32

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