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271.
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The effect of the small fraction of clays on the rheological behaviour of alpine debris flow is poorly understood. This is partly due to the complexity of the debris flow mineralogy and the broad particle size distribution. This study has investigated this issue by simulating an alpine debris flow with a mixture of well characterized fractions and then varying the clay fraction composition. Four samples were tested, ranging from a clay fraction made up of only kaolinite (1:1 type clay) to samples where 80 per cent of the kaolinite is replaced by bedeillite (a 2:1 type clay similar to smectite). Changing the content of 2:1 type clay has a strong influence on the behaviour of the whole material, despite its low weight fraction of around 2 per cent. The tests carried out on these reconstituted materials were compared with the results obtained for natural debris flow materials and showed some common trends: in particular, the rheological parameters for materials with and without 2:1 clays with respect to yield stress as a function of solid content. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
274.
What have Stardust samples told us about the early solar system? Phil A Bland, Anton T Kearsley, P J Wozniakiewicz, M J Burchell, M Gounelle, M E Zolensky and Matt J Genge have some of the answers – and a few more questions.  相似文献   
275.
We present reflectance spectra from 0.4 to 2.4 μm of Asteroid (101955) 1999 RQ36, the target of the OSIRIS-REx spacecraft mission. The visible spectral data were obtained at the McDonald Observatory 2.1-m telescope with the ES2 spectrograph. The infrared spectral data were obtained at the NASA Infrared Telescope Facility using the SpeX instrument. The average visible spectrum is combined with the average near-infrared wavelength spectrum to form a composite spectrum. We use three methods to constrain the compositional information in the composite spectrum of Asteroid (101955) 1999 RQ36 (hereafter RQ36). First, we perform a least-squares search for meteorite spectral analogs using 15,000 spectra from the RELAB database. Three most likely meteorite analogs are proposed based on the least-squares search. Next, six spectral parameters are measured for RQ36 and their values are compared with the ranges in parameter values of the carbonaceous chondrite meteorite classes. A most likely meteorite analog group is proposed based on the depth of overlap in parameter values. The results of the least-squares search and the parametric comparisons point to CIs and/or CMs as the most likely meteorite analogs for RQ36, and COs and CHs as the least likely. RQ36 has a spectrally “blue” continuum slope that is also observed in carbonaceous chondrites containing magnetite. We speculate that RQ36 is composed of a “CM1”-like material. Finally, we compare RQ36 to other B-type asteroids measured by Clark et al. (Clark, B.E. et al. [2010]. J. Geophys. Res. 115, E06005). The results of this comparison are inconclusive. RQ36 is comparable to Themis spectral properties in terms of its albedo, visible spectrum, and near-infrared spectrum from 1.1 to 1.45 μm. However, RQ36 is more similar to Pallas in terms of its near-infrared spectrum from 1.6 to 2.3 μm. Thus it is possible that B-type asteroids form a spectral continuum and that RQ36 is a transitional object, spectrally intermediate between the two end-members. This is particularly interesting because Asteroid 24 Themis was recently discovered to have H2O ice on the surface (Rivkin, A., Emery, J. [2010]. Nature 464, 1322–1323; Campins, H. et al. [2010a]. Nature 464, 1320–1321).  相似文献   
276.
Zoned crystals can be important recorders of magmatic processes in space and time. However, in most situations, the temporal dimension is difficult to quantify. Here, we have employed secondary ion mass spectrometry depth profiling to excavate parallel pits into non-polished crystal faces of zircon to obtain ~5 μm resolution U–Th disequilibrium ages (one pit) that can be correlated with trace element zoning at sub-μm resolution derived from a second pit. Data from 17 crystals representing each of the four rhyolite eruptions of Tarawera volcano, an intra-caldera edifice within the Okataina Volcanic Centre, reveal diverse zircon growth conditions over time. Most crystals display rimward depletions in Zr/Hf and Ti, broadly consistent with cooling and crystallization. However, a significant fraction of crystals lacks these patterns and displays rimward trace element variations consistent with isothermal or prograde crystallization. Oscillatory zonation patterns in Y, Th, and U are superimposed on the Zr/Hf and Ti trends. Despite the limited number of crystals analyzed in this way, the striking lack of ubiquitous trace element zoning patterns in crystals from the same hand sample implies that fractional crystallization upon cooling was punctuated by magma recharge and crystal mixing affecting different parts of the magma reservoir. By combining data from all crystals, a systematic change to more heterogeneous trace element abundances is revealed by zircon crystal domains <45 ka following the Rotoiti caldera-forming eruption. This contrasts with the more uniform conditions of zircon crystallization lasting >100 ka prior to caldera formation and is best explained by the post-caldera system consisting of small, isolated melt pockets that evolved independently. An important conclusion is that the zircon ‘cargo’ in volcanic rocks reflects thermally and compositionally divergent processes that act near simultaneously in a magma storage region and not exclusively the conditions in the eruptible magma.  相似文献   
277.
Many gaseous exoplanets in short-period orbits are on the verge or are in the process of Roche-lobe overflow (RLO). Moreover, orbital stability analysis shows tides can drive many hot Jupiters to spiral inevitably toward their host stars. Thus, the coupled processes of orbital evolution and RLO likely shape the observed distribution of close-in exoplanets and may even be responsible for producing some of the short-period rocky planets. However, the exact outcome for an overflowing planet depends on its internal response to mass loss, and the accompanying orbital evolution can act to enhance or inhibit RLO. In this study, we apply the fully-featured and robust Modules for Experiments in Stellar Astrophysics suite to model RLO of short-period gaseous planets. We show that, although the detailed evolution may depend on several properties of the planetary system, it is largely determined by the core mass of the overflowing gas giant. In particular, we find that the orbital expansion that accompanies RLO often stops and reverses at a specific maximum period that depends on the core mass. We suggest that RLO may often strand the remnant of a gas giant near this orbital period, which provides an observational prediction that can corroborate the hypothesis that short-period gas giants undergo RLO. We conduct a preliminary comparison of this prediction to the observed population of small, short-period planets and find some planets in orbits that may be consistent with this picture. To the extent that we can establish some short-period planets are indeed the remnants of gas giants, that population can elucidate the properties of gas giant cores, the properties of which remain largely unconstrained.  相似文献   
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