全文获取类型
收费全文 | 9243篇 |
免费 | 839篇 |
国内免费 | 1338篇 |
专业分类
测绘学 | 279篇 |
大气科学 | 248篇 |
地球物理 | 626篇 |
地质学 | 3541篇 |
海洋学 | 852篇 |
天文学 | 5063篇 |
综合类 | 330篇 |
自然地理 | 481篇 |
出版年
2024年 | 20篇 |
2023年 | 58篇 |
2022年 | 184篇 |
2021年 | 188篇 |
2020年 | 216篇 |
2019年 | 269篇 |
2018年 | 211篇 |
2017年 | 181篇 |
2016年 | 217篇 |
2015年 | 247篇 |
2014年 | 462篇 |
2013年 | 503篇 |
2012年 | 527篇 |
2011年 | 645篇 |
2010年 | 705篇 |
2009年 | 964篇 |
2008年 | 898篇 |
2007年 | 786篇 |
2006年 | 731篇 |
2005年 | 642篇 |
2004年 | 530篇 |
2003年 | 442篇 |
2002年 | 349篇 |
2001年 | 313篇 |
2000年 | 256篇 |
1999年 | 240篇 |
1998年 | 159篇 |
1997年 | 49篇 |
1996年 | 49篇 |
1995年 | 39篇 |
1994年 | 41篇 |
1993年 | 61篇 |
1992年 | 16篇 |
1991年 | 23篇 |
1990年 | 26篇 |
1989年 | 18篇 |
1988年 | 21篇 |
1987年 | 5篇 |
1986年 | 15篇 |
1985年 | 21篇 |
1984年 | 19篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 6篇 |
1980年 | 11篇 |
1979年 | 2篇 |
1978年 | 5篇 |
1977年 | 15篇 |
1877年 | 1篇 |
1875年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
991.
Szilárd Csizmadia Zsolt Kővári Péter Klagyivik 《Astrophysics and Space Science》2006,304(1-4):355-357
We carried out optical and Hα photometry of two contact binaries (V861 Herculis, EQ Tauri). The light curve modeling revealed stellar spots in both contact systems and strong Hα excess in the position of the observed stellar spots. A correlation was found between the V−R and R−Hα colour indices of V861 Her. 相似文献
992.
M.L. Litvak I.G. Mitrofanov A.B. Sanin W.V. Boynton D. Hamara R.S. Saunders 《Icarus》2006,180(1):23-37
In this paper, we have analyzed neutron spectroscopy data gathered by the High Energy Neutron Detector (HEND) instrument onboard Mars Odyssey for comparison of polar regions. It is known that observation of the neutron albedo of Mars provides important information about the distribution of water-ice in subsurface layers and about peculiarities of the CO2 seasonal cycle. It was found that there are large water-rich permafrost areas with contents of up to ∼50% water by mass fraction at both the north and south Mars polar regions. The water-ice layers at high northern latitudes are placed close to the surface, but in the south they are covered by a dry and relatively thick (10-20 cm) layer of soil. Analysis of temporal variations of neutron flux between summer and winter seasons allowed the estimation of the masses of the CO2 deposits which seasonally condense at the polar regions. The total mass of the southern seasonal deposition was estimated as 6.3×1015 kg, which is larger than the total mass of the seasonal deposition at the north by 40-50%. These results are in good agreement with predictions from the NASA Ames Research Center General Circulation Model (GCM). But, the dynamics of the condensation and sublimation processes are not quite as consistent with these models: the peak accumulation of the condensed mass of CO2 occurred 10-15 degrees of Ls later than is predicted by the GCM. 相似文献
993.
Evidence of volcano-ground ice interactions on Mars can provide important constraints on the timing and distribution of martian volcanic processes and climate characteristics. Northwest of the Elysium Rise is Hrad Vallis, a ∼370 m deep, 800 km long sinuous valley that begins in a source region at 34° N, 218° W. Flanking both sides of the source region is a lobate deposit that extends ∼50 km perpendicular from the source and is an average of ∼40 m thick. Previous studies have suggested the formation of the Hrad Vallis source region was the result of explosive magma-ice interaction and that the lobate deposit is a mudflow; here we use newly available MOLA, MOC, and THEMIS data to investigate the evidence supporting this hypothesis. Within the lobate deposit we have identified 12 craters with thermal infrared signatures and morphologies that are distinct from any other craters or depressions in the region. The thermally distinct craters are distinguished by their cool interiors surrounded by warm ejecta in the nighttime THEMIS IR data and warm interiors surrounded by cool ejecta in the daytime THEMIS IR data. The craters are typically 1100-1800 m in diameter (one crater is ∼2300 m across) and 30-40 m deep, but may be up to 70 m. The craters are typically circular and have central depressions (several with interior dune fill) surrounded by ∼1 to >6 concentric fracture sets. The distribution of the craters and their morphology suggests that they are likely the result of the interaction between a hot mudflow and ground ice. 相似文献
994.
995.
I. Bains T. Wong M. Cunningham P. Sparks D. Brisbin P. Calisse J. T. Dempsey G. Deragopian S. Ellingsen B. Fulton F. Herpin P. Jones Y. Kouba C. Kramer E. F. Ladd S. N. Longmore J. McEvoy M. Maller V. Minier B. Mookerjea C. Phillips C. R. Purcell A. Walsh M. A. Voronkov M. G. Burton 《Monthly notices of the Royal Astronomical Society》2006,367(4):1609-1628
996.
997.
Pedro Augusto Alastair C. Edge Claire J. Chandler 《Monthly notices of the Royal Astronomical Society》2006,367(1):366-374
We present multifrequency, multi-epoch radio imaging of the complex radio source B2151+174 in the core of the cluster, Abell 2390 ( z ≃ 0.23) . From new and literature data, we conclude that the Faranoff–Riley type II (FRII)-powerful radio source is the combination of a compact, core-dominated 'medium-sized symmetric object' (MSO) with a more extended, steeper spectrum mini-halo. B2151+174 is unusual in a number of important aspects. (i) It is one of the most compact and flat spectrum sources in a cluster core known; (ii) it shows a complex, compact twin-jet structure in a north–south orientation; (iii) the orientation of the jets is 45° misaligned with apparent structure (ionization cones and dust disc) of the host galaxy on larger scales. Since the twin-jet of the MSO has its northern half with an apparent 'twist', it might be that precession of the central supermassive black hole explains this misalignment. B2151+174 may be an example of the early stage (103 –104 yr duration) of a 'bubble' being blown into the intracluster medium where the plasma has yet to expand. 相似文献
998.
We investigate a new dynamical mechanism for producing Halley-type comets from the scattered disk of comets. Levison and Duncan [Levison, H., Duncan, M., 1997. Icarus 127, 13-32] and Duncan and Levison [Duncan, M., Levison, H., 1997. Science 276, 1670-1672] showed that a significant number of objects leave the scattered disk by evolving to semi-major axes greater than 1000 AU. We find that once these objects reach semi-major axes on the order of 104 AU, a significant fraction immediately have their perihelia driven inward by the galactic tides. Approximately 0.01% of the objects that reach 104 AU then evolve onto orbits similar to the observed Halley-like comets due to gravitational interactions with the giant planets. The orbital element distribution resulting from this process is statistically consistent with observations. We discuss the implications of this model for the number of objects in the scattered disk in the text. The model predicts a temporal variation in the influx of HTCs with a period of ∼118 Myr. At the peak, the model predicts that there should be roughly 10 times as many HTCs as currently observed (i.e., there should be weak HTC showers). However, the model may inflate the importance of these showers because it does not include the effects of passing stars and giant molecular clouds. 相似文献
999.
1000.