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201.
Secondary ion mass spectrometry (SIMS) U‐Pb ages of Ca‐phosphates from four texturally distinct breccia samples (72255, 76055, 76015, 76215) collected at the Apollo 17 landing site were obtained in an attempt to identify whether they represent a single or several impact event(s). The determined ages, combined with inferences from petrologic relationships, may indicate two or possibly three different impact events at 3920 ± 3 Ma, 3922 ± 5 Ma, and 3930 ± 5 Ma (all errors 2σ). Searching for possible sources of the breccias by calculating the continuous ejecta radii of impact basins and large craters as well as their expected ejecta thicknesses, we conclude that Nectaris, Crisium, Serenitatis, and Imbrium are likely candidates. If the previous interpretation that the micropoikilitic breccias collected at the North Massif represent Serenitatis ejecta is correct, then the average 207Pb/206Pb age of 3930 ± 5 Ma (2σ) dates the formation of the Serenitatis basin. The occurrence of zircon in the breccias sampled at the South Massif, which contain Ca‐phosphates yielding an age of 3922 ± 5 Ma (2σ), may indicate that the breccia originated from within the Procellarum KREEP terrane (PKT) and the Imbrium basin appears to be the only basin that could have sourced them. However, this interpretation implies that all basins suggested to fall stratigraphically between Serenitatis and Imbrium formed within a short (<11 Ma) time interval, highlighting serious contradictions between global stratigraphic constraints, sample interpretation, and chronological data. Alternatively, the slightly older age of the two micropoikilitic breccias may be a result of incomplete resetting of the U‐Pb system preserved in some phosphate grains. Based on the currently available data set this possibility cannot be excluded.  相似文献   
202.
Northwest Africa 7533, a polymict Martian breccia, consists of fine‐grained clast‐laden melt particles and microcrystalline matrix. While both melt and matrix contain medium‐grained noritic‐monzonitic material and crystal clasts, the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars, microbasaltic clasts, vitrophyric and microcrystalline spherules, and shards. The clast‐laden melt rocks contain clump‐like aggregates of orthopyroxene surrounded by aureoles of plagioclase. Some shards of vesicular melt rocks resemble the pyroxene‐plagioclase clump‐aureole structures. Submicron size matrix grains show some triple junctions, but most are irregular with high intergranular porosity. The noritic‐monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths, and cooled more slowly than rocks with zoned‐pyroxene or fine grain size. Noritic material contains orthopyroxene or inverted pigeonite, augite, calcic to intermediate plagioclase, and chromite to Cr‐bearing magnetite; monzonitic clasts contain augite, sodic plagioclase, K feldspar, Ti‐bearing magnetite, ilmenite, chlorapatite, and zircon. These feldspathic rocks show similarities to some rocks at Gale Crater like Black Trout, Mara, and Jake M. The most magnesian orthopyroxene clasts are close to ALH 84001 orthopyroxene in composition. All these materials are enriched in siderophile elements, indicating impact melting and incorporation of a projectile component, except for Ni‐poor pyroxene clasts which are from pristine rocks. Clast‐laden melt rocks, spherules, shards, and siderophile element contents indicate formation of NWA 7533 as a regolith breccia. The zircons, mainly derived from monzonitic (melt) rocks, crystallized at 4.43 ± 0.03 Ga (Humayun et al. 2013 ) and a 147Sm‐143Nd isochron for NWA 7034 yielding 4.42 ± 0.07 Ga (Nyquist et al. 2016 ) defines the crystallization age of all its igneous portions. The zircon from the monzonitic rocks has a higher Δ17O than other Martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration (Nemchin et al. 2014 ). This record of protolith interaction with atmosphere‐hydrosphere during regolith formation before melting demonstrates a thin atmosphere, a wet early surface environment on Mars, and an evolved crust likely to have contaminated younger extrusive rocks. The latest events recorded when the breccia was on Mars are resetting of apatite, much feldspar and some zircons at 1.35–1.4 Ga (Bellucci et al. 2015 ), and formation of Ni‐bearing pyrite veins during or shortly after this disturbance (Lorand et al. 2015 ).  相似文献   
203.
Data from several cases of radiation fog occurring at the Met Office field site at Cardington, Bedfordshire, UK have been analyzed with a view to elucidating the typical evolution in its static stability from formation to dissipation. Typically the early stages of radiation fog are characterized by a stable thermal profile and a relatively shallow depth. However, when the fog reached approximately 100 m depth it was observed to become optically thick (to longwave radiation), with a subsequent change over several hours to a saturated adiabatic stability profile. At this time turbulence levels were seen to increase significantly. The mechanisms involved appear to be radiative cooling from fog top and a positive heat flux to the atmosphere from the soil. The importance of this change in stability for numerical modelling of fog episodes is discussed. Several case studies are made to gain some insight into how common this transition is. Droplet spectra were measured at 2-m height for many of the cases considered, and their evolution is discussed. It is found that distributions fall into an initial phase with small drop sizes (approximately ≤ 10 μm diameter) and concentration, and a mature phase with the appearance of much larger drop sizes with a mean diameter of approximately 15−20 μm. It is found that the appearance of the mature phase does not coincide with the change in stability from stable to saturated adiabatic, but there is some evidence that once a saturated adiabatic profile is established, the droplet spectra variations are significantly less than for the stable period. The observed evolution of these spectra brings into question the suitability of microphysical schemes that assume constant spectral shape, drop diameter, and number density.  相似文献   
204.
The effects of Cd on the adsorption of an aquatic fulvic acid (FA) to the surface of Bacillus subtilis were investigated from pH 2.5 to 7.0, at fixed ionic strength (0.1 M NaClO4) and at ambient temperature (22 °C). Cd (14 mg/l) had no effect on FA adsorption at pH<5 but increased FA adsorption at pH>6. The effects of Cd (0, 14 mg/l) on FA adsorption to B. subtilis were further examined as a function of initial FA concentration (0–45 mg C/l) at pH 6.5. FA adsorption isotherms also were measured at pH 6.5 as a function of dissolved Cd concentration (0–14 mg/l) at three initial FA concentrations (4, 8, 22 mg C/l). At all FA concentrations studied at pH 6.5, FA adsorption increased with increasing initial total Cd concentration.

Under all studied conditions, preferential adsorption of high- to intermediate-molecular-weight FA components to B. subtilis resulted in a fractionation of the FA pool, with lower-molecular-weight components remaining in solution. At pH>6, Cd further enhanced the adsorption of high- to intermediate-molecular-weight FA components but did not significantly enhance the adsorption of lower-molecular-weight components. Hence, the overall process of adsorptive fractionation was not altered significantly by the presence of Cd.

Overall, the results of this study (1) demonstrate that FA adsorption to bacterial surfaces can be altered by the presence of a metal cation, and (2) provide further evidence that microbe–metal–ligand interactions may significantly affect the mobility and fate of natural organic matter in the subsurface.  相似文献   

205.
206.
Chesil Beach (Dorset) is one of the most famous coastal landforms on the British coast. The gravel beach is over 18 km long and is separated for much of its length from land by a tidal lagoon known as The Fleet. The beach links the Isle of Portland in the east to the mainland in the west. Despite its iconic status there is little available information on its internal geometry and evolutionary history. Here we present a three-fold model for the evolution of Chesil Beach based on a series of nine ground penetrating radar (GPR) traverses located at three sites along its length at Abbotsbury, Langton Herring and at Ferry Bridge. The GPR traverses reveal a remarkably consistent picture of the internal structure of this barrier beach. The first phase of evolution involves the landward transgression of a small sand and gravel beach which closed upon the coast leading to deposition of freshwater peat between 5 and 7 k yr BP. The second evolutionary phase involves the ‘bulking-out’ of the beach during continued sea level rise, but in the presence of abundant gravel supplied by down-drift erosion of periglacial slope deposits. This episode of growth was associated with a series of washover fans which accumulated on the landward flank of the barrier increasing its breadth and height but without significant landward transgression of the barrier as a whole. The final phase in the evolution of Chesil Beach involves the seaward progradation of the beach crest and upper beach face associated with continued sediment abundance, but during a still-stand or slight fall in relative sea level. This phase may provide further evidence of a slight fall in relative sea level noted elsewhere along the South Coast of Britain and dated to between 1.2 and 2.4 k yr BP. Subsequently the barrier appears to have become largely inactive, except for the reworking of sediment on the beach face during storm events. The case study not only refines the evolutionary picture of Chesil Beach, but illustrates the importance of the subtle interplay between relative sea level and sediment supply in the evolution of a barrier system. In addition, it also illustrates the potential of GPR in resolving the evolutionary history of gravel-rich coastal landforms such as Chesil Beach.  相似文献   
207.
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209.
RX J0720.4–3125 has recently been identified as a pulsating soft X-ray source in the ROSAT all-sky survey with a period of 8.391 s. Its spectrum is well characterized by a blackbody with a temperature of 8 × 105 K. We propose that the radiation from this object is thermal emission from a cooling neutron star. For this blackbody temperature we can obtain a robust estimate of the object's age of ∼ 3 × 105 yr, yielding a polar field ∼ 1014 G for magnetic dipole spin-down and a value of P compatible with current observations.  相似文献   
210.
Recent observations of the compact source embedded within the supernova remnant RCW 103 rekindle interest in the origin of this object's emission. We contrast several models in which neutron-star cooling powers RCW 103. Specifically, either the presence of an accreted envelope or a sufficiently intense magnetic field can account for the X-ray emission from this object.  相似文献   
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