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121.
Matthew J. Genge Matthias van Ginneken Martin D. Suttle Ralph P. Harvey 《Meteoritics & planetary science》2018,53(10):2051-2066
We report the discovery of a large accumulation of micrometeorites (MMs) in a supraglacial moraine at Larkman Nunatak in the Grosvenor Mountains of the Transantarctic Range in Antarctica. The MMs are present in abundances of ~600 particles kg−1 of moraine sediment and include a near-complete collection of MM types similar to those observed in Antarctic blue ice and within bare-rock traps in the Antarctic. The size distribution of the observed particles is consistent with those collected from snow collections suggesting the moraine has captured a representative collection of cosmic spherules with significant loss of only the smallest particles (<100 μm) by wind. The presence of microtektites with compositions similar to those of the Australasian strewn field suggests the moraine has been accumulating for 780 ka with dust-sized debris. On the basis of this age estimate, it is suggested that accumulation occurs principally through ice sublimation. Direct infall of fines is suggested to be limited by snow layers that act as barriers to accumulation and can be removed by wind erosion. MM accumulation in many areas in Antarctica, therefore, may not be continuous over long periods and can be subject to climatic controls. On the basis of the interpretation of microtektites as Australasian, Larkman Nunatak deposit is the oldest known supraglacial moraine and its survival through several glacial maxima and interglacial periods is surprising. We suggest that stationary ice produced by the specific ice flow conditions at Larkman Nunatak explains its longevity and provides a new type of record of the East Antarctic ice sheet. 相似文献
122.
Marc S. Humphries Claudia R. Benitez-Nelson Michael Bizimis Timothy J. Ralph Zacchary T. Larkin Terence S. McCarthy 《地球表面变化过程与地形》2020,45(8):1705-1716
Dust plays a globally important role in supplying biologically essential elements to landscapes underlain by nutrient-poor substrates. Here we show that dust may play a significant role in sustaining productivity in the vast wetlands of the Okavango Delta in southern Africa, one of the world's richest biodiversity hotspots. Dust accumulates preferentially on tree-covered islands in the seasonal swamps of the Delta, creating pockets of fine-grained, nutrient-rich material within the semi-arid landscape of the Kalahari Desert. Strontium and neodymium isotopes reveal that this dust likely originates predominantly from the Makgadikgadi salt pans, located 300 km away, and contributes 10–80% of the fine-grained material present in Okavango island soils. Surface material sourced from the Makgadikgadi Pans contains relatively high amounts of bioavailable phosphorus and iron, potentially influencing Okavango Delta biological productivity. We propose that long-term ecosystem productivity and nutrient availability in the Okavango may be strongly mediated by regional dust inputs. Understanding the influence of dust deposition on nutrient loads and biogeochemical cycling is thus critical for predicting the response of the Okavango Delta to future changes in climate. We suggest that dust inputs may play a significant role in the supply of nutrients to other large, global wetland systems located in dryland environments. © 2020 John Wiley & Sons, Ltd 相似文献
123.
Lauren C. Wye Howard A. Zebker Richard D. West Ralph D. Lorenz the Cassini RADAR Team 《Icarus》2007,188(2):367-385
We report regional-scale low-resolution backscatter images of Titan's surface acquired by the Cassini RADAR scatterometer at a wavelength of 2.18-cm. We find that the average angular dependence of the backscatter from large regions and from specific surface features is consistent with a model composed of a quasi-specular Hagfors term plus a diffuse cosine component. A Gaussian quasi-specular term also fits the data, but less well than the Hagfors term. We derive values for the mean dielectric constant and root-mean-square (rms) slope of the surface from the quasi-specular term, which we ascribe to scattering from the surface interface only. The diffuse term accommodates contributions from volume scattering, multiple scattering, or wavelength-scale near-surface structure. The Hagfors model results imply a surface with regional mean dielectric constants between 1.9 and 3.6 and regional surface roughness that varies between 5.3° and 13.4° in rms-slope. Dielectric constants between 2 and 3 are expected for a surface composed of solid simple hydrocarbons, water ice, or a mixture of both. Smaller dielectric constants, between 1.6 and 1.9, are consistent with liquid hydrocarbons, while larger dielectric constants, near 4.5, may indicate the presence of water-ammonia ice [Lorenz, R.D., 1998. Icarus 136, 344-348] or organic heteropolymers [Thompson, W.R., Squyres, S.W., 1990. Icarus 86, 336-354]. We present backscatter images corrected for angular effects using the model residuals, which show strong features that correspond roughly to those in 0.94-μm ISS images. We model the localized backscatter from specific features to estimate dielectric constant and rms slope when the angular coverage is within the quasi-specular part of the backscatter curve. Only two apparent surface features are scanned with angular coverage sufficient for accurate modeling. Data from the bright albedo feature Quivira suggests a dielectric constant near 2.8 and rms slope near 10.1°. The dark albedo feature Shangri-La is best fit by a Hagfors model with a dielectric constant close to 2.4 and an rms slope near 9.5°. From the modeled backscatter curves, we find the average radar albedo in the same linear (SL) polarization to be near 0.34. We constrain the total-power albedo in order to compare the measurements with available groundbased radar results, which are typically obtained in both senses of circular polarization. We estimate an upper limit of 0.4 on the total-power albedo, a value that is significantly higher than the 0.21 total albedo value measured at 13 cm [Campbell, D., Black, G., Carter, L., Ostro, S., 2003. Science 302, 431-434]. This is consistent with a surface that has more small-scale structure and is thus more reflective at 2-cm than 13-cm. We compare results across overlapping observations and observe that the reduction and analysis are repeatable and consistent. We also confirm the strong correlations between radar and near-infrared images. 相似文献
124.
Laramie V. POTTS Ralph R. B. von FRESE C. K. SHUM 《Meteoritics & planetary science》2002,37(9):1197-1207
Abstract— We use satellite altitude free‐air and terrain gravity correlations to differentiate regional variations in crustal viscosity and transient cavity diameters of impact basins on the Moon and Mars that we combine with surface roughness for a rheological assessment of the crust of Mercury. For the Moon and Mars, we separate the free‐air anomalies into terrain‐correlated and terrain‐decorrelated components using the spectral properties of the free‐air and computed terrain gravity effects. Adjusting the terrain effects for the terrain‐correlated anomalies yields compensated terrain effects that we evaluate for crustal thickness variations of the impact basins. We interpret the terrain‐correlated anomalies for uncompensated elements of the crustal thickness variations that we find are strongly correlated with the distribution of basin rings from photogeologic analyses. Hence, we estimate the transient cavity diameter from the innermost diameter of the gravity‐inferred rings. Comparing these diameters with the related crustal thickness estimates clearly differentiates regional variations in the crustal rheologies. For the Moon, the analysis points to a farside crust that was significantly more rigid than the nearside crust during bombardment time. For Mars, the growth in transient cavity diameters with apparent crustal age also reflects increased viscosity due to crustal cooling. These results are also consistent with local estimates of surface roughness developed from the root‐mean‐squared topography over 64 times 64° patches centered on the basins. Hence for Mercury where gravity observations are lacking, rheological inferences on its crust may result from comparing photometric estimates of transient cavity diameter and local surface roughness with the lunar and martian estimates. These results for the Beethoven Basin, a typical multi‐ring impact feature of Mercury, suggest that the viscosity of the mercurian crust was relatively great during bombardment time. This enhanced rigidity, despite crustal temperatures that were probably much hotter than those of the Moon and Mars, may reflect an extremely dry crust for Mercury in its early development. 相似文献
125.
Ralph E. Pudritz 《Astrophysics and Space Science》2002,281(1-2):493-496
We present the results of very high resolution CDM simulations of galaxy formation designed to follow the formation and evolution
of self-gravitating, supergiant star-forming clouds. We find that the mass spectrum of these clouds is identical to that of
globular clusters and GMCs; dN/dM ∝ M
-1.7 ± 0.1.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
126.
Ralph M. Gailis Alexander Hill Eugene Yee Trevor Hilderman 《Boundary-Layer Meteorology》2007,122(3):577-607
Fluctuating plume models provide a useful conceptual paradigm in the understanding of plume dispersion in a turbulent flow.
In particular, these models have enabled analytical predictions of higher-order concentration moments, and the form of the
one-point concentration probability density function (PDF). In this paper, we extend the traditional formalism of these models,
grounded in the theory of homogeneous and isotropic turbulent flow, to two cases: namely, a simple sheared boundary layer
and a large array of regular obstacles. Some very high-resolution measurements of plume dispersion in a water channel, obtained
using laser-induced fluorescence (LIF) line-scan techniques are utilised. These data enable us to extract time series of plume
centroid position (plume meander) and dispersion in the relative frame of reference in unprecedented detail. Consequently,
experimentally extracted PDFs are able to be directly compared with various theoretical forms proposed in the literature.
This includes the PDF of plume centroid motion, the PDF of concentration in the relative frame, and a variety of concentration
moments in the absolute and relative frames of reference. The analysis confirms the accuracy of some previously proposed functional
forms of model components used in fluctuating plume models, as well as suggesting some new forms necessary to deal with the
complex boundary conditions in the spatial domain. 相似文献
127.
Ralph Kahn 《Icarus》1982,49(1):71-85
We show how crater size-density counts may be used to help constrain the history of the Venus atmosphere, based on the predictions of simple but reasonable models for crater production, surface erosion, and the effects of atmospheric drag and breakup on incident meteors in the Venus atmosphere. If the atmosphere is old, we may also be able to determine the importance of breakup as a mechanism for destroying incident meteors in a dense fluid. In particular, if the atmosphere is young, the old (uneroded) surfaces will have crater densities upward of 10?4 km?2 and a ratio of small (4 km) craters to large (128 km) craters near 103. If the atmosphere is old and the breakup mechanism is dominant, absolute crater densities on Venus surfaces will be diminished by several orders of magnitude relative to the young atmosphere case. If atmospheric drag is dominant and the atmosphere is old, the absolute crater density will be lowered by perhaps an order of magnitude relative to the young atmosphere case, and the ratio of small to large craters will be reduced to a value near 101.5 according to the models. The comparison of crater populations on young, as well as old, surfaces on Venus can help in distinguishing the young and old atmosphere scenarios, especially since the situation may be complicated by currently undetermined erosional and tectonic processes. Once a large fraction of Venus surface has been imaged at kilometer resolution, as the VOIR project promises to do, it could be possible to make an early determination of the age of the Venus atmosphere. 相似文献
128.
129.
130.
Nodularia spumigena periodically proliferates to cause toxic algal blooms with some aquatic animals enduring and consuming high densities of the blue green algae or toxic lysis. N. spumigena contains toxic compounds such as nodularin and lipopolysaccharides. This current work investigates physiological effects of exposure from bloom conditions of N. spumigena cells and a post-bloom lysis. Biochemical and antioxidative biomarkers were comparatively studied over an acute 3-day exposure. In general, a post-bloom N. spumigena lysis caused opposite physiological responses to bloom densities of N. spumigena. Specifically, increases in glutathione (GSH) and glutathione peroxidase (GPx) and decreases in glutathione S-transferase (GST) were observed from the N. spumigena lysis. In contrast, N. spumigena cell densities decreased GSH and increased GST and lipid peroxidation (LPO) in mussels. Findings also suggest that at different stages of a toxic bloom, exposure may result in toxic stress to specific organs in the mussel. 相似文献