全文获取类型
收费全文 | 1751篇 |
免费 | 85篇 |
国内免费 | 48篇 |
专业分类
测绘学 | 60篇 |
大气科学 | 132篇 |
地球物理 | 375篇 |
地质学 | 622篇 |
海洋学 | 122篇 |
天文学 | 421篇 |
综合类 | 10篇 |
自然地理 | 142篇 |
出版年
2024年 | 5篇 |
2023年 | 13篇 |
2022年 | 10篇 |
2021年 | 37篇 |
2020年 | 44篇 |
2019年 | 42篇 |
2018年 | 83篇 |
2017年 | 68篇 |
2016年 | 80篇 |
2015年 | 75篇 |
2014年 | 80篇 |
2013年 | 114篇 |
2012年 | 68篇 |
2011年 | 107篇 |
2010年 | 83篇 |
2009年 | 94篇 |
2008年 | 91篇 |
2007年 | 93篇 |
2006年 | 79篇 |
2005年 | 63篇 |
2004年 | 64篇 |
2003年 | 50篇 |
2002年 | 48篇 |
2001年 | 37篇 |
2000年 | 45篇 |
1999年 | 34篇 |
1998年 | 21篇 |
1997年 | 23篇 |
1996年 | 19篇 |
1995年 | 14篇 |
1994年 | 14篇 |
1993年 | 11篇 |
1992年 | 15篇 |
1991年 | 8篇 |
1990年 | 10篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 10篇 |
1985年 | 12篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 5篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1973年 | 5篇 |
排序方式: 共有1884条查询结果,搜索用时 16 毫秒
831.
832.
Anders Meibom Kevin Righter Nancy Chabot Greg Dehn Angelo Antignano Timothy J. McCoy Alexander N. Krot Michael E. Zolensky Michael I. Petaev Klaus Keil 《Meteoritics & planetary science》2005,40(9-10):1377-1391
Abstract— We have studied the CB carbonaceous chondrites Queen Alexandra Range (QUE) 94411, Hammadah al Hamra (HH) 237, and Bencubbin with an emphasis on the petrographical and mineralogical effects of the shock processing that these meteorite assemblages have undergone. Iron‐nickel metal and chondrule silicates are the main components in these meteorites. These high‐temperature components are held together by shock melts consisting of droplets of dendritically intergrown Fe,Ni‐metal/sulfide embedded in silicate glass, which is substantially more FeO‐rich (30–40 wt%) than the chondrule silicates (FeO <5 wt%). Fine‐grained matrix material, which is a major component in most other chondrite classes, is extremely scarce in QUE 94411 and HH 237, and has not been observed in Bencubbin. This material occurs as rare, hydrated matrix lumps with major and minor element abundances roughly similar to the ferrous silicate shock melts (and CI). We infer that hydrated, fine‐grained material, compositionally similar to these matrix lumps, was originally present between the Fe,Ni‐metal grains and chondrules, but was preferentially shock melted. Other shock‐related features in QUE 94411, HH 237, and Bencubbin include an alignment and occasionally strong plastic deformation of metal and chondrule fragments. The existence of chemically zoned and metastable Fe,Ni‐metal condensates in direct contact with shock melts indicates that the shock did not substantially increase the average temperature of the rock. Because porphyritic olivine‐pyroxene chondrules are absent in QUE 94411, HH 237, and Bencubbin, it is difficult to determine the precise shock stage of these meteorites, but the shock was probably relatively light (S2–S3), consistent with a bulk temperature increase of the assemblages of less than ?300 °C. The apparently similar shock processing of Bencubbin, Weatherford, Gujba (CBa) and QUE 94411/HH 237 (CBb) supports the idea of a common asteroidal parent body for these meteorites. 相似文献
833.
834.
835.
Georgi Pavlovski Michael D. Smith Mordecai–Mark Mac Low Alexander Rosen 《Astrophysics and Space Science》2004,292(1-4):69-75
Here we present results from simulations of turbulence in star forming environments obtained by coupling three-dimensional hydrodynamical models with appropriate chemical processes. We investigate regimes of decaying high-speed molecular turbulence. Here we analyse PDFs of density for the volume, mass, molecular mass and the energy distribution over the range of scales. We compare our results to those previously obtained for isothermal turbulence and suggest possible explanations. 相似文献
836.
Abstract— Anorthite‐rich chondrules in CR and CH carbonaceous chondrites consist of magnesian low‐Ca pyroxene and forsterite phenocrysts, FeNi‐metal nodules, interstitial anorthite, Al‐Ti‐Cr‐rich low‐Ca and high‐Ca pyroxenes, and crystalline mesostasis composed of silica, anorthite and high‐Ca pyroxene. Three anorthite‐rich chondrules contain relic calcium‐aluminum‐rich inclusions (CAIs) composed of anorthite, spinel, ±Al‐diopside, and ± forsterite. A few chondrules contain regions which are texturally and mineralogically similar to magnesian (type I) chondrules and consist of forsterite, low‐Ca pyroxene and abundant FeNi‐metal nodules. Anorthite‐rich chondrules in CR and CH chondrites are mineralogically similar to those in CV and CO carbonaceous chondrites, but contain no secondary nepheline, sodalite or ferrosilite. Relatively high abundances of moderately‐volatile elements such as Cr, Mn and Si in the anorthite‐rich chondrules suggest that these chondrules could not have been produced by volatilization of the ferromagnesian chondrule precursors or by melting of the refractory materials only. We infer instead that anorthite‐rich chondrules in carbonaceous chondrites formed by melting of the reduced chondrule precursors (olivine, pyroxenes, FeNi‐metal) mixed with the refractory materials, including relic CAIs, composed of anorthite, spinel, high‐Ca pyroxene and forsterite. The observed mineralogical and textural similarities of the anorthite‐rich chondrules in several carbonaceous chondrite groups (CV, CO, CH, CR) may indicate that these chondrules formed in the region(s) intermediate between the regions where CAIs and ferromagnesian chondrules originated. This may explain the relative enrichment of anorthite‐rich chondrules in 16O compared to typical ferromagnesian chondrules (Russell et al., 2000). 相似文献
837.
Tenuous dust clouds of Jupiter's Galilean moons Io, Europa, Ganymede and Callisto have been detected with the in-situ dust detector on board the Galileo spacecraft. The majority of the dust particles have been sensed at altitudes below five radii of these lunar-sized satellites. We identify the particles in the duut clouds surrounding the moons by their impact direction, impact velocity, and mass distribution. Average particle sizes are between 0.5 and 1 μm, just above the detector threshold, indicating a size distribution with decreasing numbers towards bigger particles. Our results imply that the particles have been kicked up by hypervelocity impacts of micrometeoroids onto the satellites' surfaces. The measured radial dust density profiles are consistent with predictions by dynamical modeling for satellite ejecta produced by interplanetary impactors (Krivov et al., 2003, Planet. Space Sci. 51, 251-269), assuming yield, mass and velocity distributions of the ejecta from laboratory measurements. A comparison of all four Galilean moons (data for Ganymede published earlier; Krüger et al., 2000, Planet. Space Sci. 48, 1457-1471) shows that the dust clouds of the three outer Galilean moons have very similar properties and are in good agreement with the model predictions for solid ice-silicate surfaces. The dust density in the vicinity of Io, however, is more than an order of magnitude lower than expected from theory. This may be due to a softer, fluffier surface of Io (volcanic deposits) as compared to the other moons. The log-log slope of the dust number density in the clouds vs. distance from the satellite center ranges between −1.6 and −2.8. Appreciable variations of number densities obtained from individual flybys with varying geometry, especially at Callisto, are found. These might be indicative of leading-trailing asymmetries of the clouds due to the motion of the moons with respect to the field of impactors. 相似文献
838.
Bojan Nikolic Paul Alexander Garret Cotter Malcolm Longair Marcel Clemens 《Astrophysics and Space Science》2003,284(2):573-576
We present preliminary results of recent spatially resolved mid infrared (Mid-IR) spectroscopy with the MICHELLE instrument
on UKIRT of NGC 7469, observed as part of a larger programme to study a sample of starburst and AGN in the Mid-IR.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
839.
P. Alexander 《Monthly notices of the Royal Astronomical Society》2006,368(3):1404-1410
Observations of compact symmetric double sources suggest that these objects with physical scales of order tens of parsecs to kiloparsecs are young radio active galactic nuclei. There is, in general, a striking similarity between the structures of these compact objects and the structures of large evolved radio galaxies although the latter are two to three orders of magnitude larger. This has led to the use of self-similar models of the evolution of radio sources as a framework for interpreting observational data. However, the assumptions on which the self-similar models are based become increasingly less valid on the physical scales which are probed by the observations of the smallest sources. In this paper, the dynamics of sources on these small scales is examined and a model developed which extends earlier work in a self-consistent way to small physical scales. The limit of applicability of the models is identified as is the transition from an early evolutionary phase to the self-similar phase of expansion. 相似文献
840.
Martina Queck Alexander V. Krivov Miodrag Sremčević Philippe Thébault 《Celestial Mechanics and Dynamical Astronomy》2007,99(3):169-196
We consider a belt of small bodies (planetesimals, asteroids, dust particles) around a star, captured in one of the external
or 1:1 mean-motion resonances with a massive perturber (protoplanet, planet). The objects in the belt collide with each other.
Combining methods of celestial mechanics and statistical physics, we calculate mean collisional velocities and mean collisional
rates, averaged over the belt. The results are compared to collisional velocities and rates in a similar, but non-resonant
belt, as predicted by the particle-in-a-box method. It is found that the effect of the resonant lock on the velocities is
rather small, while on the rates more substantial. At low to moderate eccentricities and libration amplitudes of tens of degrees,
which are typical of many astrophysical applications, the collisional rates between objects in an external resonance are by
about a factor of two higher than those in a similar belt of objects not locked in a resonance. For Trojans under the same
conditions, the collisional rates may be enhanced by up to an order of magnitude. The collisional rates increase with the
decreasing libration amplitude of the resonant argument, depend on the eccentricity distribution of objects, and vary from
one resonance to another. Our results imply, in particular, shorter collisional lifetimes of resonant Kuiper belt objects
in the solar system and higher efficiency of dust production by resonant planetesimals in debris disks around other stars. 相似文献