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排序方式: 共有790条查询结果,搜索用时 218 毫秒
1.
Philippe CARREZ Karine DEMYK Patrick CORDIER Lon GENGEMBRE Jean GRIMBLOT Louis D'HENDECOURT Anthony P. JONES Hugues LEROUX 《Meteoritics & planetary science》2002,37(11):1599-1614
Abstract— We present the results of irradiation experiments aimed at understanding the structural and chemical evolution of silicate grains in the interstellar medium. A series of He+ irradiation experiments have been performed on ultra‐thin olivine, (Mg,Fe)2SiO4, samples having a high surface/volume (S/V) ratio, comparable to the expected S/V ratio of interstellar dust. The energies and fluences of the helium ions used in this study have been chosen to simulate the irradiation of interstellar dust grains in supernovae shock waves. The samples were mainly studied using analytical transmission electron microscopy. Our results show that olivine is amorphized by low‐energy ion irradiation. Changes in composition are also observed. In particular, irradiation leads to a decrease of the atomic ratios O/Si and Mg/Si as determined by x‐ray photoelectron spectroscopy and by x‐ray energy dispersive spectroscopy. This chemical evolution is due to the differential sputtering of atoms near the surfaces. We also observe a reduction process resulting in the formation of metallic iron. The use of very thin samples emphasizes the role of surface/volume ratio and thus the importance of the particle size in the irradiation‐induced effects. These results allow us to account qualitatively for the observed properties of interstellar grains in different environments, that is, at different stages of their evolution: chemical and structural evolution in the interstellar medium, from olivine to pyroxene‐type and from crystalline to amorphous silicates, porosity of cometary grains as well as the formation of metallic inclusions in silicates. 相似文献
2.
The aim of this interdisciplinary study is to examine a component of the hydrological cycle in Galapagos by characterizing
soil properties. Nine soil profiles were sampled on two islands. Their physical and hydrodynamic properties were analyzed,
along with their mineralogical composition. Two groups of soils were identified, with major differences between them. The
first group consists of soils located in the highlands (>350 m a.s.l.), characterized by low hydraulic conductivity (<10−5 m s−1) and low porosity (<25%). These soils are thick (several meters) and homogeneous without coarse components. Their clay fraction
is considerable and dominated by gibbsite. The second group includes soils located in the low parts of the islands (<300 m
a.s.l.). These soils are characterized by high hydraulic conductivity (>10−3 m s−1) and high porosity (>35%). The structure of these soils is heterogeneous and includes coarse materials. The physical properties
of the soils are in good agreement with the variations of the rainfall according to the elevation, which appears as the main
factor controlling the soil development. The clayey alteration products constrain soils physical and hydrodynamic properties
by reducing the porosity and consequently the permeability and also by increasing water retention. 相似文献
3.
S. Torres V. Cañon R. Casas A. Umaña C. Tello T. Villela M. Bersanelli M. Bensadoun G. De Amici M. Limon G. Smoot C. Witebsky 《Astrophysics and Space Science》1996,240(2):225-234
The GEM (Galactic Emission Mapping) project is an international collaboration established with the aim of surveying the full sky at long wavelengths with a multi-frequency radio telescope. A total of 745 hours of observation at 408 MHz were completed from an Equatorial site in Colombia. The observations cover the celestial band 0
h
<<24
h
, and –24° 22<<+35° 37. Preliminary results of this partial survey will be discussed. A review of the instrumental setup and a 10° resolution sky map at 408 MHz is presented.Presented by S. Torres at the UN/ESA Workshop on Basic Space Sciences: From Small Telescopes to Space Missions, Colombo, Sri Lanka 11–14 January 1996 相似文献
4.
Ice and snow have often helped physicists understand the world. On the contrary it has taken them a very long time to understand the flow of the glaciers. Naturalists only began to take an interest in glaciers at the beginning of the 19th century during the last phase of glacier advances. When the glacier flow from the upslope direction became obvious, it was then necessary to understand how it flowed. It was only in 1840, the year of the Antarctica ice sheet discovery by Dumont d'Urville, that two books laid the basis for the future field of glaciology: one by Agassiz on the ice age and glaciers, the other one by canon Rendu on glacier theory. During the 19th century, ice flow theories, adopted by most of the leading scientists, were based on melting/refreezing processes. Even though the word ‘fluid’ was first used in 1773 to describe ice, more the 130 years would have to go by before the laws of fluid mechanics were applied to ice. Even now, the parameter of Glen's law, which is used by glaciologists to model ice deformation, can take a very wide range of values, so that no unique ice flow law has yet been defined. To cite this article: F. Rémy, L. Testut, C. R. Geoscience 338 (2006). 相似文献
5.
6.
7.
From 1993 to 1996, three oceanographic moorings were deployed in the north-western Barents Sea, each with a current meter and an upward-looking sonar for measuring ice drafts. These yielded three years of currents and two years of ice draft measurements. An interannual variability of almost I m was measured in the average ice draft. Causes for this variability are explored, particularly its possible connection to changes in atmospheric circulation patterns. We found that the flow of Northern Barents Atlantic-derived Water and the transport of ice from the Central Arctic into the Barents Sea appears to be controlled by winds between Nordaustlandet and Franz Josef Land, which in turn may be influenced by larger-scale variations such as the Arctic Oscillation/North Atlantic Oscillation. 相似文献
8.
In South Africa attempts are being made to address the socio-spatial distortions of the apartheid era through a more equitable
distribution of resources, and the re-drawing of municipal geographical boundaries. However, boundaries are not neutral geographic
lines. Boundary changes are often associated with a redistribution of political power and resources. The aim of this paper
is to analyse the effects of the contemporary territorial and administrative restructuring on urban dynamics in South Africa.
More specifically, the focus is on how the process of territorial restructuring impacted on metropolitan areas as well as
on secondary cities and their hinterlands. Examining and elucidating the manner in which various social, economic and political
forces have manifested themselves in the process of boundary delimitation in a major metropolitan centre as well as adjacent
rural areas is a central theme of this paper. There were considerable contestations over the delimitation of new local government
boundaries. Affluent metro authorities like that in Durban were opposed to the spatial extension of their boundaries because
of the costs of the providing services and infrastructure in the deprived margins. Similarly, there was concern that incorporation
of rural areas will result in increased municipal service charges being imposed on these communities. Tensions were heightened
between urban and rural regions because traditional leaders believed that their territorial jurisdiction and authority were
being undermined. In other parts of the country, the merger of traditionally white and black fragments of secondary cities
often resulted in many black locations continuing to be marginalized. There appears to be neither the political will nor the
economic capacity to upgrade these zones of marginalized urban communities. While the Municipal Demarcation Board was largely
successful in eliminating the political geography of apartheid at a macro- scale, this paper suggests that the greater challenge
for government and policy makers is to reduce the socio-spatial and economic inequalities which appears to be still very high
and perhaps increasing.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
Frédérick Delay Hélène Housset-Resche Gilles Porel Ghislain de Marsily 《Mathematical Geology》1996,28(1):45-71
A new method for solving the transport equation based on the management of a large numbe of particles in a discretized 2-D domain is presented. The method uses numerical variables to represent the number of particles in a given mesh and is more complex than the 1-D problem. The first part of the paper focuses on the specific management of particles in a 2-D problem. The method also would be valid for three dimensions as long as the medium can be modeled similar to a layered system. As the particles are no longer tracked individually, the algorithm is fast and does not depend on the number of particles present. The numerical tests show that the method is nearly numerical dispersion free and permits accurate calculations even for simulations of low-concentration transport. Because each mesh is considered as a closed system between two successive time steps, it is easy to add adsorption phenomenon without any problem of numerical stability. The model is tested under conditions that are extremely demanding for its operating mode and gives a good fit to analytical solutions. The conditions in which it can be used to best advantage are discussed. 相似文献
10.
内蒙古科尔康油田沙海组储集层的岩石学特征 总被引:1,自引:0,他引:1
根据碎屑岩铸体薄片和扫描电镜观察分析资料,研究了科尔康油田沙海组储集层的岩石学特征。科尔康油田沙海组储集层的岩石类型主要为岩屑砂岩和岩屑砂砾岩。碎屑岩的岩性特征对沙海组储集层的成岩作用具有显著的影响作用。 相似文献