Sensitivity of subglacial Lake Vostok's flow regime on environmental parameters     

Malte Thoma  Christoph Mayer  Klaus Grosfeld 《Earth and Planetary Science Letters》2008,269(1-2):242-247

Subglacial lakes provide unique habitats, but the exact nature of physical and geochemical conditions are still a matter of debate and await direct sampling of water. Due to its isolation from external atmospheric forcing other environmental parameters influence the flow characteristics within the lake. In this study we use an improved treatment of the physical processes at the ice–water boundary interface to identify and quantify the impact of (1) the geothermal heat flux, (2) the heat flux from the lake into the ice, (3) the influence of the salinity of the lake water, and (4) the ice thickness on the size of the freezing area and the freeze/melt rates. We show that the modelled basal mass imbalance (that is the produced melt water minus the re-frozen water) depends on the geothermal heating as well as the heat flux into the ice. The circulation and the temperature distribution within subglacial Lake Vostok are rather stable against variations of geothermal heat flux, heat flux into the ice sheet, salinity of the lake, and small changes of the ice thickness above the lake. However, the flow regime for any subglacial lake with less than 2000 m ice thickness above, will be substantially different from those that experience higher pressures. This is because the buoyancy–temperature relationship reverses at this depth.  相似文献   

Evaluation of Seasonal Factors on Petroleum Hydrocarbon Vapor Biodegradation and Intrusion Potential in a Cold Climate     

Ian Hers  Parisa Jourabchi  Matthew A. Lahvis  Paul Dahlen  E. Hong Luo  Paul Johnson  George E. DeVaull  K. Ulrich Mayer 《Ground Water Monitoring & Remediation》2014,34(4):60-78

A detailed seasonal study of soil vapor intrusion at a cold climate site with average yearly temperature of 1.9 °C was conducted at a house with a crawlspace that overlay a shallow dissolved‐phase petroleum hydrocarbon (gasoline) plume in North Battleford, Saskatchewan, Canada. This research was conducted primarily to assess if winter conditions, including snow/frost cover, and cold soil temperatures, influence aerobic biodegradation of petroleum vapors in soil and the potential for vapor intrusion. Continuous time‐series data for oxygen, pressure differentials, soil temperature, soil moisture, and weather conditions were collected from a high‐resolution monitoring network. Seasonal monitoring of groundwater, soil vapor, crawlspace air, and indoor air was also undertaken. Petroleum hydrocarbon vapor attenuation and biodegradation rates were not significantly reduced during low temperature winter months and there was no evidence for a significant capping effect of snow or frost cover that would limit oxygen ingress from the atmosphere. In the residual light nonaqueous phase liquid (LNAPL) source area adjacent to the house, evidence for biodegradation included rapid attenuation of hydrocarbon vapor concentrations over a vertical interval of approximately 0.9 m, and a corresponding decrease in oxygen to less than 1.5% v/v. In comparison, hydrocarbon vapor concentrations above the dissolved plume and below the house were much lower and decreased sharply within a few tens of centimeters above the groundwater source. Corresponding oxygen concentrations in soil gas were at least 10% v/v. A reactive transport model (MIN3P‐DUSTY) was initially calibrated to data from vertical profiles at the site to obtain biodegradation rates, and then used to simulate the observed soil vapor distribution. The calibrated model indicated that soil vapor transport was dominated by diffusion and aerobic biodegradation, and that crawlspace pressures and soil gas advection had little influence on soil vapor concentrations.  相似文献   

The 2006 Bahía Asunción Earthquake Swarm: Seismic Evidence of Active Deformation Along the Western Margin of Baja California Sur,Mexico     

Luis Munguía  Sergio Mayer  Alfredo Aguirre  Ignacio Méndez  Mario González-Escobar  Manuel Luna 《Pure and Applied Geophysics》2016,173(10-11):3615-3629

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The temporal evolution of the large equatorial plasma depletions observed during the 29–30 October 2003 storm     

Hyosub Kil  Larry J. Paxton 《Journal of Atmospheric and Solar》2010,72(4):327-333

This study investigates the temporal evolution of the large plasma depletions observed by ROCSAT-1 and DMSP near 295°E during the 29–30 October 2003 storm. The presence of a penetration electric field around the detection time of the large plasma depletions is supported by the observation of high upward ion drift velocity and formation of an intense equatorial ionization anomaly in the American sector. However, these ionospheric disturbances occur in broad longitude regions; a short-range polarization electric field may adequately explain the creation of the large plasma depletions. The penetration electric field may trigger the Rayleigh–Taylor instability and produce abnormally large plasma depletions during the storm. The TIMED/GUVI and CHAMP observations provide an insight for the evolution of the large depletions several hours after their formation. The large depletions appear as arch-shaped emission depletions in the TIMED/GUVI image and as symmetric depletions paired in the magnetic north and south in the CHAMP observation. These characteristics can be explained by the “plasma depletion shell” phenomenon (Kil et al., 2009) produced by the westward shear flow of the ionosphere during the storm.  相似文献   

Spatio-temporal variability of moisture conditions within the Urban Canopy Layer     

H. Mayer  A. Matzarakis  M. G. Iziomon 《Theoretical and Applied Climatology》2003,76(3-4):165-179

Summary ¶Re-analysed data from an urban climate research project in Munich, Germany, were used to investigate the spatio-temporal variability of moisture conditions (expressed here in vapour pressure VP) within the Urban Canopy Layer UCL. The results, which apply to three main sites and additional subsidiary ones, cover both summer and winter months. The summer month variation of VP is characterised by higher monthly mean values of VP for all three sites, howbeit with considerable inter-site differences. The temporal variability of mean VP values at diurnal time scales is also examined. With respect to the UCL, they reveal different amplitudes and times of occurrence of their extreme values. In addition, results of car traverses performed during clear sky conditions in downtown Munich show a remarkable small-scale spatio-temporal variability of VP.In relation to a sealed downtown site within a courtyard in Munich, a time-dependent urban moisture excess (UME) was formed. A positive correlation between UME and the urban heat island (UHI) could be verified in general. However, it was slightly negative with a very low coefficient of determination in the summer month when the maximum UME preceded the maximum UHI up to 5hrs. As example for the effects of air moisture on the urban climate within the UCL, the role of VP on a thermal index (physiologically equivalent temperature PET) was investigated. Based on one-year data from another urban climate project in Munich, a positive correlation between PET and VP was found, although the coefficient of determination was somewhat low. However, during a human-biometeorological case study on a typical summer day in the northern downtown of Freiburg, a medium-sized city in southwest Germany, PET and VP showed a negative correlation (possibly because the specific temporal course of VP at the measuring points was mainly influenced by thermally induced turbulence).  相似文献   

Introduction     

Wim Ostendorf  Larry Bourne  Denise Pumain 《GeoJournal》1997,43(4):303-305

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The range of validity of the two-body approximation in models of terrestrial planet accumulation: II. Gravitational cross sections and runaway accretion     

George W. Wetherill  Larry P. Cox 《Icarus》1985,63(2):290-303

The validity of the two-body approximation in calculating encounters between planetesimals has been evaluated as a function of the ratio of unperturbed planetesimal velocity (with respect to a circular orbit) to mutual escape velocity when their surfaces are in contact (V/V_e). Impact rates as a function of V/V_e are calculated to within ～20% by numerical integration of the equations of motion. It is found that when V/V_e > 0.4, the two-body approximation is a good one. At low velocities (V/V_e < 0.1) two-body “collision-course” trajectories fail to lead to impacts. On the other hand, at these low velocities many impacts result from encounter trajectories with unperturbed separation distances far beyond the two-body gravitational radius. These two effects tend to cancel, and the resulting impact rates remain within a factor of ～3 of the two-body value in spite of these major differences in the nature of the impact trajectories. Therefore, on the average, the two-body approximation is useful well below the value of V/V_e for which it fails to describe individual encounters, and the required corrections are not large. As a consequence of this “anomalous gravitational focusing” planetesimals will continue to interact even when their orbits are noncrossing. This reduces the difficulty with premature isolation of planetesimal embryos during accumulation. Quantitatively, when 0.06 ? V/V_e ? 0.2, the impact rate varies approximately with the fifth power of the radius of the larger body, and is about a factor of 3 above that predicted using the conventional two-body gravitational cross-section formula. At lower values of V/V_e , the impact rate increases less rapidly. Finally, at the lowest values of V/V_e (<.02), the impact rate increases only in proportion to the geometric cross section, as a consequence of the swarm being essentially two dimensional for large unperturbed encounter distances. The gravitational enhancement in effective cross section is thereby limited to a value of about 3000. This leads to an optimal size for growth of planetesimals from a swarm of given eccentricity, and places a limit on the extent of runaway accretion.  相似文献   

Experiment on Obtaining Mercury's Images by the Short Exposure Method     

Ksanfomaliti  L. V.  Dzhapiashvili  V. P.  Kakhiani  V. O.  Kvaratskheliya  O. I.  Korol'  A. N.  Mayer  A. K.  Petashvili  P. P. 《Solar System Research》2001,35(3):190-194

December 1–3, 1999, observations of the planet Mercury were carried out at the Abastumani Astrophysical Observatory of the Republic of Georgia by the short exposure method with the aid of a charge-coupled device (CCD) camera. The materials of these observations are presented in this paper. It is shown that the reduction of the exposure down to 10 ms eliminates the image blurring caused by the atmospheric instability and considerably improves resolution. As regards the image distortions, they can be eliminated only by selecting acceptable images from a sufficiently large number of pictures obtained. The short exposure method allows one to obtain new results from the ground-based observations of Mercury.  相似文献   

The gravitational and hydrodynamical interaction between the Large Magellanic Cloud and the Galaxy     

C. Mastropietro  B. Moore  L. Mayer  J. Wadsley  J. Stadel 《Monthly notices of the Royal Astronomical Society》2005,363(2):509-520

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Lithologic distribution and geologic history of the Apollo 17 site: The record in soils and small rock particles from the highland massifs     

Bradley L. Jolliff  Kaylynn M. Rockow  Randy L. Korotev  Larry A. Haskin 《Meteoritics & planetary science》1996,31(1):116-145

Abstract— Through analysis by instrumental neutron activation (INAA) of 789 individual lithic fragments from the 2 mm–4 mm grain-size fractions of five Apollo 17 soil samples (72443, 72503, 73243, 76283, and 76503) and petrographic examination of a subset, we have determined the diversity and proportions of rock types recorded within soils from the highland massifs. The distribution of rock types at the site, as recorded by lithic fragments in the soils, is an alternative to the distribution inferred from the limited number of large rock samples. The compositions and proportions of 2 mm–4 mm fragments provide a bridge between compositions of <1 mm fines, and types and proportions of rocks observed in large collected breccias and their clasts. The 2 mm–4 mm fraction of soil from South Massif, represented by an unbiased set of lithic fragments from station-2 samples 72443 and 72503, consists of 71% noritic impact-melt breccia, 7% incompatible-trace-element-(ITE)-poor highland rock types (mainly granulitic breccias), 19% agglutinates and regolith breccias, 1% high-Ti mare basalt, and 2% others (very-low-Ti (VLT) basalt, monzogabbro breccia, and metal). In contrast, the 2 mm–4 mm fraction of a soil from the North Massif, represented by an unbiased set of lithic fragments from station-6 sample 76503, has a greater proportion of ITE-poor highland rock types and mare-basalt fragments: it consists of 29% ITE-poor highland rock types (mainly granulitic breccias and troctolitic anorthosite), 25% impact-melt breccia, 13% high-Ti mare basalt, 31% agglutinates and regolith breccias, 1% orange glass and related breccia, and 1% others. Based on a comparison of mass-weighted mean compositions of the lithic fragments with compositions of soil fines from all Apollo 17 highland stations, differences between the station-2 and station-6 samples are representative of differences between available samples from the two massifs. From the distribution of different rock types and their compositions, we conclude the following: (1) North-Massif and South-Massif soil samples differ significantly in types and proportions of ITE-poor highland components and ITE-rich impact-melt-breccia components. These differences reflect crudely layered massifs and known local geology. The greater percentage of impact-melt breccia in the South-Massif light-mantle soil stems from derivation of the light mantle from the top of the massif, which apparently is richer in noritic impact-melt breccia than are lower parts of the massifs. (2) At station 2, the 2 mm–4 mm grain-size fraction is enriched in impact-melt breccias compared to the <1 mm fraction, suggesting that the <1 mm fraction within the light mantle has a greater proportion of lithologies such as granulitic breccias which are more prevalent lower in the massifs and which we infer to be older (pre-basin) highland components. (3) Soil from station 6, North Massif, contains magnesian troctolitic anorthosite, which is a component that is rare in station-2 South-Massif soils. (4) Compositional differences between poikilitic impact-melt breccias from the two massifs suggest broad-scale heterogeneity in impact-melt breccia interpreted by most investigators to be ejecta from the Serenitatis basin. We have found rock types not previously recognized or uncommon at the Apollo 17 site. These include (1) ITE-rich impact-melt breccias that are compositionally distinct from previously recognized “aphanitic” and “poikilitic” groups at Apollo 17; (2) regolith breccias that are free of mare components and poor in impact melt of the types associated with the main melt-breccia groups, and that, if those groups derive from the Serenitatis impact, may represent the pre-Serenitatis surface; (3) several VLT basalts, including an unusual very-high-K basaltic breccia; (4) orange-glass regolith breccias; (5) aphanitic-matrix melt breccias at station 6; (6) fragments of alkali-rich composition, including alkali anorthosite, and monzogabbro; (7) one fragment of 72275-type KREEP basalt from station 3; (8) seven lithic fragments of ferroan-anorthositic-suite rocks; and (9) a fragment of metal, possibly from an L chondrite. Some of these lithologies have been found only as lithic fragments in the soils and not among the large rock samples. In contrast, we have not found among the 2 mm–4 mm lithic fragments individual samples of certain lithologies that have been recognized as clasts in breccias (e.g., dunite and spinel troctolite). The diversity of lithologic information contained in the lithic fragments of these soils nearly equals that found among large rock samples, and most information bearing on petrographic relationships is maintained, even in such small samples. Given a small number of large samples for “petrologic ground truth,” small lithic fragments contained in soil “scoop” samples can provide the basis for interpreting the diversity of rock types and their proportions in remotely sensed geologic units. They should be considered essential targets for future automated sample-analysis and sample-return missions.  相似文献