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111.
C. T. Russell F. Capaccioni A. Coradini M. C. De Sanctis W. C. Feldman R. Jaumann H. U. Keller T. B. McCord L. A. McFadden S. Mottola C. M. Pieters T. H. Prettyman C. A. Raymond M. V. Sykes D. E. Smith M. T. Zuber 《Earth, Moon, and Planets》2007,101(1-2):65-91
The initial exploration of any planetary object requires a careful mission design guided by our knowledge of that object as gained by terrestrial observers. This process is very evident in the development of the Dawn mission to the minor planets 1 Ceres and 4 Vesta. This mission was designed to verify the basaltic nature of Vesta inferred both from its reflectance spectrum and from the composition of the howardite, eucrite and diogenite meteorites believed to have originated on Vesta. Hubble Space Telescope observations have determined Vesta’s size and shape, which, together with masses inferred from gravitational perturbations, have provided estimates of its density. These investigations have enabled the Dawn team to choose the appropriate instrumentation and to design its orbital operations at Vesta. Until recently Ceres has remained more of an enigma. Adaptive-optics and HST observations now have provided data from which we can begin to confidently plan the mission. These observations reveal a rotationally symmetric body with little surface relief, an ultraviolet bright point that can be used as a control point for determining the pole and anchoring a geographic coordinate system. They also reveal albedo and color variations that provide tantalizing hints of surface processes. 相似文献
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Eric Christopher Merten Jacques Finlay Lucinda Johnson Raymond Newman Heinz Stefan Bruce Vondracek 《水文研究》2011,25(4):593-602
Wood deposited in streams provides a wide variety of ecosystem functions, including enhancing habitat for key species in stream food webs, increasing geomorphic and hydraulic heterogeneity and retaining organic matter. Given the strong role that wood plays in streams, factors that influence wood inputs, retention and transport are critical to stream ecology. Wood entrapment, the process of wood coming to rest after being swept downstream at least 10 m, is poorly understood, yet important for predicting stream function and success of restoration efforts. Data on entrapment were collected for a wide range of natural wood pieces (n = 344), stream geomorphology and hydraulic conditions in nine streams along the north shore of Lake Superior in Minnesota. Locations of pieces were determined in summer 2007 and again following an overbank stormflow event in fall 2007. The ratio of piece length to effective stream width (length ratio) and the weight of the piece were important in a multiple logistic regression model that explained 25% of the variance in wood entrapment. Entrapment remains difficult to predict in natural streams, and often may simply occur wherever wood pieces are located when high water recedes. However, this study can inform stream modifications to discourage entrapment at road crossings or other infrastructure by applying the model formula to estimate the effective width required to pass particular wood pieces. Conversely, these results could also be used to determine conditions (e.g. pre‐existing large, stable pieces) that encourage entrapment where wood is valued for ecological functions. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
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Ismaïla Ngounouno Bernard Déruelle Raymond Montigny Daniel Demaiffe 《Comptes Rendus Geoscience》2006,338(8):537-544
Camptonites have been discovered at Mount Cameroon. They contain phenocrysts of olivine + clinopyroxene + plagioclase ± kaersutite ± bi and ocelli of nepheline or analcite and carbonates. 87Sr/86Sri values are similar to those of the basalts of Mount Cameroon . The camptonites are interpreted as resulting from fractional crystallization of a basaltic magma generated by a low degree of melting of an infra-lithospheric metasomatized mantle tapped by huge lithospheric cracks. To cite this article: I. Ngounouno et al., C. R. Geoscience 338 (2006). 相似文献
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We analyzed Holocene sedimentary records from two lakes in the Lofoten Islands, northern Norway to evaluate environmental changes during the Holocene related to northern North Atlantic climate dynamics. The lakes are located in different geomorphological settings, and thus provide a contrast in their response to regional climate change. Environmental changes at both lakes were interpreted based on magnetic susceptibility, organic-matter flux, C/N, ??13Corg, Ti concentrations, and mass accumulation rates. Chronologies were established using 16 AMS radiocarbon dates, and average deposition rates in both environments are higher than 0.2?mm/year throughout the Holocene. At Vikjordvatnet, sedimentary geochemical properties define three distinct phases of sedimentation related to changes in aquatic productivity and gradual landscape development. Following deglaciation, during the early Holocene (11.6?C7.2?ka), aquatic productivity increased and the landscape stabilized as regional temperatures increased in response to higher summer insolation and increasing inflow of warm Atlantic water into the Norwegian Sea. Centennial-scale intervals of decreased organic-matter flux, from 10.9 to 10.2?ka and 9.2 to 8.0?ka, record episodes of instability during the early Holocene. These may represent regional cooling events related to freshwater forcing and a slowdown of the northward transport of warm water into the North Atlantic. During the mid-Holocene (7.2?C4.8?ka) organic-matter properties show less variability and the timing of this phase corresponds with the regional Holocene thermal maximum. The late Holocene sediments (4.8?ka?Cpresent) record a transition to colder climate conditions. The record from Fiskeb?lvatnet captures periodic changes in clastic input related to runoff and exhibits high-frequency variations over the last 9.5?ka. The most significant change in sedimentation was during the late Holocene (4.3?ka?Cpresent) when the frequency and magnitude of runoff events show an abrupt transition to wetter conditions. The timing of this shift corresponds to other regional reconstructions that indicate wetter and colder conditions during the late Holocene. 相似文献
116.
We identify mechanisms controlling the distribution of methane convection and large-scale circulation in a simplified, axisymmetric model atmosphere of Titan forced by gray radiation and moist (methane) convection. The large-scale overturning circulation, or Hadley cell, is global in latitudinal extent and provides fundamental control of precipitation and tropospheric winds. The precipitating, large-scale updraft regularly oscillates in latitude with seasons. The distance of greatest poleward excursion of the Hadley cell updraft is set by the mass of the convective layer of the atmosphere; convection efficiently communicates seasonal warming of the surface through the cold and dense lower atmosphere, increasing the heat capacity of the system. The presence of deep, precipitating convection introduces three effects relative to the case with no methane latent heating: (1) convection is narrowed and enhanced in the large-scale updraft of the Hadley cell; (2) the latitudinal amplitude of Hadley cell updraft oscillations is decreased; and (3) a time lag is introduced. These effects are observable in the location and timing of convective methane clouds in Titan’s atmosphere as a function of season. A comparison of simulations over a range of convective regimes with available observations suggest methane thermodynamic-dynamic feedback is important in the Titan climate. 相似文献
117.
Dany AZAR Raymond GèZE Antoine El-Samrani Jacqueline Maalouly André NEL 《《地质学报》英文版》2010,84(4):977-983
<正>Reports of amber predating the Lower Cretaceous are unusual and scarce;they mostly refer to amber pieces of millimetric dimension.In the present study,we report the discovery of 10 new outcrops of Jurassic amber in Lebanon.Some of these had large centimetric-sized pieces of amber.The new localities are described,amber is characterized,and its infrared spectra given.Although the new Jurassic amber yielded to date no more than fungal inclusions,this material is significant and promising.The discovery of several Jurassic outcrops provides crucial information on the prevailing paleoenvironment of that time. 相似文献
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