Silt-sized quartz particles: a proposed source     

P.A. Riezebos  L. Van der Waals 《Sedimentary Geology》1974,12(4):279-285

The origin of quartz-silt particles is attributed especially to the development and the breakdown of the cracked, marginal parts of sand-sized grains and particles. Generally the breakdown of these cracked grain parts will take place subsequently to their genesis, but these processes may partly overlap one another. This breakdown may succesfully occur in glacial, but likewise in other environments.  相似文献   

Do comets have satellites?     

T.C. Van Flandern 《Icarus》1981,47(3):480-486

The recent evidence that many minor planets may have satellites, together with recently iscovered physical, chemical, and lightcurve similarities between minor planets and comets, lead naturally to the question, “Might comets have satellites also?” This paper explores several puzzling features of comets which do not fit easily into conventional cometary models, but which can be satisfactorily explained if it is assumed that comets have a full range of gravitationally bound masses, from dust size to the size of the nucleus, in orbit around the principal nucleus. This discussion also implies a higher probability of destruction of a spacecraft near a comet than is usually assumed.  相似文献   

Temperature variations from Hubble Space Telescope spectroscopy of the Orion Nebula     

R. H. Rubin  P. G. Martin  R. J. Dufour  G. J. Ferland  K. P. M. Blagrave  X.-W. Liu  J. F. Nguyen  J. A. Baldwin 《Monthly notices of the Royal Astronomical Society》2003,340(2):362-374

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Seismic response of slopes subjected to incident SV wave by an improved boundary element approach     

Behrouz Gatmiri  Khoa‐Van Nguyen  Kaveh Dehghan 《国际地质力学数值与分析法杂志》2007,31(10):1183-1195

In this paper, an improved boundary element approach for 2D elastodynamics in time‐domain is presented. This approach consists in the truncation of time integrations, based on the rapid decrease of the fundamental solutions with time. It is shown that an important reduction of the computation time as well as the storage requirement can be achieved. Moreover, for half‐plane problems, the size of boundary element (BE) meshes and the computation time can be significantly reduced. The proposed approach is used to study the seismic response of slopes subjected to incident SV waves. It is found that large amplifications take place on the upper surface close to the slope, while attenuations are produced on the lower surface. The results also show that surface motions become very complex when the incident wavelength is comparable with the size of the slope or when the slope is steep. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Circumstellar absorption and emission in the post-AGB stars HR4049 and HD213985     

C. Waelkens  L. B. F. M. Waters  H. Van Winckel  K. Daems 《Astrophysics and Space Science》1995,224(1-2):357-360

The evolved C-rich low-mass stars HR 4049 and HD 213985 present variable circumstellar extinction and emission. Observations of HR 4049 strengthen the link between the far-UV extinction rise and the presence of small C-rich particles. Variability of the UV absorption bump in HD 213985 is reported here for the first time and is discussed.  相似文献   

The salts of Mars     

Benton C. Clark  Daniel C. Van Hart 《Icarus》1981,45(2):370-378

Salt compounds are apparently an important component of the finite-grained regolith on Mars. Salt enrichment may be explained either as a secondary concentration of chemical weathering products or as direct incorporation of planetary released volatiles. Geochemical measurements and chemical relationships constrain the salt species and resultant physicochemical consequences. A likely assemblage is dominated by (Mg,Na)SO₄, NaCl, and (Mg,Ca)CO₃. Formation of brine in equilibrium with such a salt mixture is unlikely under the temperature and water-vapor restrictions prevalent over most, if not all, of the Martian surface. Acidic conditions, accompanying salt formation, favor the preferential destruction of susceptible igneous minerals.  相似文献   

An overlooked fragment of the Santa Catharina ataxite     

R. Van Tassel  H. Dillen  R. Vochten  E. DeGrave  J. Hertogen 《Meteoritics & planetary science》1992,27(4):467-469

Abstract— An undocumented mass of 28 kg, recovered from a museum collection, is examined by several methods: optical microscopy, scanning electron microscopy, X-ray diffraction, Mössbauer spectroscopy and chemical analyses by atomic absorption, energy- and wavelength-dispersive X-ray spectrometry and, with respect to the trace elements Ir, As, Au, by neutron activation. The structure and the mineralogical chemical composition of the mass identify it as a fragment of the Santa Catharina iron meteorite from Brazil.  相似文献   

Late quaternary of the middle Caquetá River area (Colombian Amazonia)     

Thomas Van Der Hammen  Joost F. Duivenvoorden  Johanna M. Lips  Ligia E. Urrego  Nohora Espejo 《第四纪科学杂志》1992,7(1):45-55

The late Quaternary history of the middle Caquetá River area in Colombia, northwestern Amazonia is described, based on observations of river bank sections, radiocarbon dates and palynological analyses of organic layers in floodplain and low terrace sediments of the Caquetá River. It is shown that the Late Pleistocene and Holocene climatic changes that took place in the Andean Cordilleras, were related to the depositional and erosional history of the Caquetá River in the Colombian Amazonian lowlands. The low terrace sediments consist of sandy and gravelly deposits covered by clays that sometimes contain lenses of peaty material. From these organic low terrace sediments, seven finite radiocarbon dates were obtained of Middle Pleniglacial age, between 56 000 and 30 000 yr BP. The coarse textured basal deposits of the low terrace apparently stem from the early part of the Middle Pleniglaciai period, during which the effective rainfall in the Andes was relatively high and the Andean glaciers had a considerable extension. Palynological data from silty sediments with organic remains at one site, show an interval when drier and more open types of vegetation on poor soils must have covered a larger area than today, but Amazonian forest was still the dominating type of vegetation. This interval might correspond to one of the Middle Pleniglacial savanna intervals from eastern Amazonia (Carajas). No organic sediments from the Upper Pleniglacial period were found and hence radiocarbon dates were not obtained. In the Andes this period had a very cold climate with low effective rainfall and in the east Amazonian Carajas area it is characterised by the relative extension of open savanna vegetation. The river run-off and sediment transport must have been much lower than in the Middle Pleniglacial and the Caquetá River cut itself down in its own sediments. Two Late-glacial radiocarbon datings obtained at one site (ca. 12 500 yr BP) indicate the existence of a Late-glacial sedimentation phase, separated from the Holocene sequence by a minor erosional phase. Organic layers in the Holocene floodplain sediments yielded 28 radiocarbon dates between 10 000 and 355 yr BP. Holocene sedimentation started with the rapid deposition of (sandy) clay possibly in a partly permanently inundated Caquetá valley. During the major part of the Holocene (silty) clays were deposited, with a dominant seasonal inundation cycle.  相似文献   

Weak lensing predictions at intermediate scales     

Ludovic Van Waerbeke  Takashi Hamana  †  Román Scoccimarro  Stephane Colombi  Francis Bernardeau 《Monthly notices of the Royal Astronomical Society》2001,322(4):918-926

As pointed out in previous studies, the measurement of the skewness of the convergence field κ will be useful in breaking the degeneracy among the cosmological parameters constrained from weak lensing observations. The combination of shot noise and finite survey volume implies that such a measurement is likely to be performed in a range of intermediate scales (0.5 to 20 arcmin) where neither perturbation theory nor the hierarchical ansatz applies. Here we explore the behaviour of the skewness of κ at these intermediate scales, based on results for the non-linear evolution of the mass bispectrum. We combined different ray-tracing simulations to test our predictions, and we find that our calculations describe accurately the transition from the weakly non-linear to the strongly non-linear regime. We show that the single lens-plane approximation remains accurate even in the non-linear regime, and we explicitly calculate the corrections to this approximation. We also discuss the prospects of measuring the skewness in upcoming weak lensing surveys.  相似文献   

Le volcanisme et sa prévention. H. Tazieff and M. Derruau. Publisher Masson,Paris 1990 (256 pp) FF 206 ISBN 2-225-82122-4     

Jan Van Mourik 《第四纪科学杂志》1995,10(4):402-402

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