The rotation of Janus and Epimetheus     

Matthew S. Tiscareno  Peter C. Thomas 《Icarus》2009,204(1):254-261

Epimetheus, a small moon of Saturn, has a rotational libration (an oscillation about synchronous rotation) of 5.9°±1.2°, placing Epimetheus in the company of Earth’s Moon and Mars’ Phobos as the only natural satellites for which forced rotational libration has been detected. The forced libration is caused by the satellite’s slightly eccentric orbit and non-spherical shape.Detection of a moon’s forced libration allows us to probe its interior by comparing the measured amplitude to that predicted by a shape model assuming constant density. A discrepancy between the two would indicate internal density asymmetries. For Epimetheus, the uncertainties in the shape model are large enough to account for the measured libration amplitude. For Janus, on the other hand, although we cannot rule out synchronous rotation, a permanent offset of several degrees between Janus’ minimum moment of inertia (long axis) and the equilibrium sub-Saturn point may indicate that Janus does have modest internal density asymmetries.The rotation states of Janus and Epimetheus experience a perturbation every 4 years, as the two moons “swap” orbits. The sudden change in the orbital periods produces a free libration about synchronous rotation that is subsequently damped by internal friction. We calculate that this free libration is small in amplitude (<0.1°) and decays quickly (a few weeks, at most), and is thus below the current limits for detection using Cassini images.  相似文献   

Grooves on small saturnian satellites and other objects: Characteristics and significance     

Sarah J. Morrison  Peter C. Thomas  Matthew S. Tiscareno  Joseph A. Burns  Joseph Veverka 《Icarus》2009,204(1):262-270

High-resolution images from the Cassini Imaging Science Subsystem (ISS) show parallel sets of grooves on Epimetheus and Pandora. Grooves have previously been observed on other satellites and asteroids, including Phobos, Gaspra, Ida, Eros, and minor occurrences on Phoebe. Sets of parallel grooves are so far observed only on satellites known or likely to be subject to significant tidal stresses, such as forced librations. Grooves on asteroids and on satellites not subject to significant forced librations occur in more globally disorganized patterns that may reflect impacts, varying internal structures, or even thermal stresses. The patterns and individual morphologies of grooves on the tidally-affected satellites suggest fracturing in weak materials due to tidal stresses and forced librations.  相似文献   

Tidal disruption of globular clusters in dwarf galaxies with triaxial dark matter haloes     

Jorge Peñarrubia  Matthew G. Walker  Gerard Gilmore 《Monthly notices of the Royal Astronomical Society》2009,399(3):1275-1292

We use N -body simulations to study the tidal evolution of globular clusters (GCs) in dwarf spheroidal (dSph) galaxies. Our models adopt a cosmologically motivated scenario in which the dSph is approximated by a static Navarro, Frenk & White halo with a triaxial shape. We apply our models to five GCs spanning three orders of magnitude in stellar density and two in mass, chosen to represent the properties exhibited by the five GCs of the Fornax dSph. We show that only the object representing Fornax's least dense GC (F1) can be fully disrupted by Fornax's internal tidal field – the four denser clusters survive even if their orbits decay to the centre of Fornax. For a large set of orbits and projection angles, we examine the spatial and velocity distribution of stellar debris deposited during the complete disruption of an F1-like GC. Our simulations show that such debris appears as shells, isolated clumps and elongated overdensities at low surface brightness (≥26 mag arcsec⁻²), reminiscent of substructure observed in several Milky Way dSphs. Such features arise from the triaxiality of the galaxy potential and do not dissolve in time. The kinematics of the debris depends strongly on the progenitor's orbit. Debris associated with box and resonant orbits does not display stream motions and may appear 'colder'/'hotter' than the dSph's field population if the viewing angle is perpendicular/parallel to the progenitor's orbital plane. In contrast, debris associated with loop orbits shows a rotational velocity that may be detectable out to a few kpc from the galaxy centre. Chemical tagging that can distinguish GC debris from field stars may reveal whether the merger of GCs contributed to the formation of multiple stellar components observed in dSphs.  相似文献   

Iodine-rich waters involved in supergene enrichment of the Mantos de la Luna argentiferous copper deposit,Atacama Desert,Chile     

Martin Reich  Carlos Palacios  Macarena Alvear  Eion M. Cameron  Matthew I. Leybourne  Artur Deditius 《Mineralium Deposita》2009,44(6):719-722

Recent studies have suggested the involvement of highly saline deep formation waters that modified preexisting Cu assemblages to form atacamite during supergene oxidation of Cu deposits in the Atacama region. In this report, we document the occurrence of (Ag–I) inclusions hosted by supergene chalcocite from Mantos de la Luna, an argentiferous Upper Jurassic stratabound Cu deposit in the Coastal Range of northern Chile. The presence of this unusual mineral assemblage indicates that iodargyrite precipitated from reducing iodine-rich waters, suggesting that the fluids involved in supergene enrichment of Cu deposits in the Coastal Range were more complex than previously thought. This suggests the prevalence of hyperarid conditions during the latest stages of supergene enrichment of the Mantos de la Luna Cu deposit in the Atacama region, supporting the notion that supergene enrichment processes in hyperarid areas are dynamic in nature and do not exclusively require the presence of meteoric water.  相似文献   

Channel response to a dam-removal sediment pulse captured at high-temporal resolution using routine gage data     

Matthew J. Cashman  Allen C. Gellis  Eric Boyd  Mathias J. Collins  Scott W. Anderson  Brett D. McFarland  Ashley M. Ryan 《地球表面变化过程与地形》2021,46(6):1145-1159

In this study, we captured how a river channel responds to a sediment pulse originating from a dam removal using multiple lines of evidence derived from streamflow gages along the Patapsco River, Maryland, USA. Gages captured characteristics of the sediment pulse, including travel times of its leading edge (~7.8 km yr⁻¹) and peak (~2.6 km yr⁻¹) and suggest both translation and increasing dispersion. The pulse also changed local hydraulics and energy conditions, increasing flow velocities and Froude number, due to bed fining, homogenization and/or slope adjustment. Immediately downstream of the dam, recovery to pre-pulse conditions occurred within the year, but farther downstream recovery was slower, with the tail of the sediment pulse working through the lower river by the end of the study 7 years later. The patterns and timing of channel change associated with the sediment pulse were not driven by large flow or suspended sediment-transporting events, with change mostly occurring during lower flows. This suggests pulse mobility was controlled by process-factors largely independent of high flow. In contrast, persistent changes occurred to out-of-channel flooding dynamics. Stage associated with flooding increased during the arrival of the sediment pulse, 1 to 2 years after dam removal, suggesting persistent sediment deposition at the channel margins and nearby floodplain. This resulted in National Weather Service-indicated flood stages being attained by 3–43% smaller discharges compared to earlier in the study period. This study captured a two-signal response from the sediment pulse: (1) short- to medium-term (weeks to months) translation and dispersion within the channel, resulting in aggradation and recovery of bed elevations and changing local hydraulics; and (2) dispersion and persistent longer-term (years) effects of sediment deposition on overbank surfaces. This study further demonstrated the utility of US Geological Survey gage data to quantify geomorphic change, increase temporal resolution, and provide insights into trajectories of change over varying spatial and temporal scales.  相似文献   

Structural complexity influences the ecosystem engineering effects of in-stream large wood     

Matthew J. Cashman  Gemma L. Harvey  Geraldene Wharton 《地球表面变化过程与地形》2021,46(10):2079-2091

Large wood (LW) is an ecosystem engineer and keystone structure in river ecosystems, influencing a range of hydromorphological and ecological processes and contributing to habitat heterogeneity and ecosystem condition. LW is increasingly being used in catchment restoration, but restored LW jams have been observed to differ in physical structure to naturally occurring jams, with potential implications for restoration outcomes. This article examines the structural complexity and ecosystem engineering effects of LW jams at four sites with varying management intensity incorporating natural and restored wood. Our results reveal: (i) structural complexity and volume of jams was highest in the site with natural jams and low intensity riparian management, and lowest in the suburban site with simple restored jams; and (ii) that structural complexity influences the ecosystem engineering role of LW, with more complex jams generating the greatest effects on flow hydraulics (flow concentration, into bed flows) and sediment characteristics (D₅₀, organic content, fine sediment retention) and the simplest flow deflector-style restored jams having the least pronounced effects. We present a conceptual model describing a continuum of increasing jam structural complexity and associated hydromorphological effects that can be used as a basis for positioning and evaluating other sites along the management intensity spectrum to help inform restoration design and best practice.  相似文献   

A Comparative Plume Study of DRO,GRO, Benzene,and MTBE: Implications for Risk Management     

Kirk O'Reilly  Matthew A. Lahvis  George E. DeVaull  Andrew M. Deines 《Ground Water Monitoring & Remediation》2021,41(3):58-64

Groundwater remediation and no-further action decision making at petroleum underground storage tank (UST) sites has largely been based on an understanding of plume length, plume stability, and attenuation rates for key hydrocarbon constituents. Regulatory guidance to support and guide such decisions is based in part on plume studies involving individual hydrocarbon constituents, namely benzene and methyl tert-butyl ether (MTBE). Questions remain regarding whether current guidance is applicable to chemical mixtures such as gasoline range organics (GRO), diesel range organics (DRO), and oxygen containing organic compounds (OCOCs) resulting from hydrocarbon biodegradation. To help address this concern, data from California's GeoTracker database were used to estimate maximum plume lengths, plume stability, and attenuation rates of DRO (which can be used as an analytical surrogate for OCOCs) and GRO relative to benzene and MTBE. The distributions of maximum plume lengths were similar for the four constituents with medians ranging from 27 to 32 m. The fraction of monitoring wells with a decreasing concentration trend ranged from 19% for DRO to 40% for MTBE, while fewer than 7% of the wells had an increasing concentration trend for any of the constituents. Median attenuation rates ranged from 0.10% day⁻¹ for DRO to 0.17% day⁻¹ for MTBE. The results suggest attenuation based risk management is appropriate for DRO and GRO plumes at most petroleum UST sites.  相似文献   

Improving Risk-Based Screening at Vapor Intrusion Sites in California     

Matthew A. Lahvis  Robert A. Ettinger 《Ground Water Monitoring & Remediation》2021,41(2):73-86

The attenuation factor (AF) of 0.03 recommended by the U.S. Environmental Protection Agency (USEPA) is increasingly being used by regulatory agencies for the development of subsurface vapor screening levels for vapor intrusion (VI). There are concerns, however, over the database used to derive the AF and the AF's applicability to building types and geographies not included in USEPA database. To derive a more technically defensible AF for subsurface vapor screening in California, a database consisting of 8415 paired indoor and subsurface vapor samples collected from 485 buildings at 36 sites in California was compiled. Filtering was applied to remove data of suspect quality that were potentially affected by background (non-VI) sources. Filtering reduced the size of the database to 788 indoor air and subsurface vapor pairs, 80% of which were trichloroethylene (TCE) measurements. An AF of 0.0008 was derived from only TCE vapor data, based on the ability of the AF to reliably identify buildings with indoor air concentrations above screening levels in 95% of cases where subsurface vapor screening levels were exceeded. The AF derived from this study demonstrated limited sensitivity to the variables typically considered important in VI characterization, which was partially attributed to relatively weak correlation of indoor air and subsurface vapor concentration data. The results of this study can be used to improve VI screening in California and other states and help focus limited resources on sites posing the greatest potential risk.  相似文献   

A Titanosaurian Sauropod Dinosaur with Opisthocoelous Caudal Vertebrae from the Early Late Cretaceous of Liaoning Province, China   总被引：4，自引：0，他引：4  

Matthew C. LAMANNA 《《地质学报》英文版》2004,78(4):907-911

We describe a new titanosaurian sauropod dinosaur, Borealosaurus wimani gen. et sp. nov., based on a distinctive mid-distal caudal vertebra from the early Late Cretaceous Sunjiawan Formation exposed in the Shuangmiao village of Beipiao in Liaoning, China. We provisionally refer an isolated tooth crown, a middle caudal vertebra, and a right humerus from the same locality and horizon to this taxon. Borealosaurus is distinguished from other sauropods in its possession of opisthocoelous mid-distal caudal vertebrae. The occurrence of opisthocoelous caudals in Borealosaurus and the Mongolian sauropod Opisthocoelicaudia raises the possibility that these taxa pertain to an as-yet unrecognized titanosaurian subclade endemic to the Cretaceous Asia.  相似文献   

A limit on the number density of bright z≈ 7 galaxies     

Elizabeth R. Stanway  Malcolm N. Bremer  Valentina Squitieri  Laura S. Douglas  Matthew D. Lehnert 《Monthly notices of the Royal Astronomical Society》2008,386(1):370-376

We present a survey of bright optical dropout sources in two deep, multiwavelength surveys comprising 11 widely separated fields, aimed at constraining the galaxy luminosity function at   z ≈ 7  for sources at  5–10  L * ( z = 6)  . Our combined survey area is 225 arcmin² to a depth of   J _AB= 24.2  (3σ) and 135 arcmin² to   J = 25.3  (4σ). We find that infrared data longwards of 2 μm are essential for classifying optical dropout sources, and in particular for identifying cool Galactic star contaminants. Our limits on the number density of high-redshift sources are consistent with current estimates of the Lyman break galaxy luminosity function at   z = 6  .  相似文献