全文获取类型
收费全文 | 28512篇 |
免费 | 615篇 |
国内免费 | 244篇 |
专业分类
测绘学 | 638篇 |
大气科学 | 2172篇 |
地球物理 | 5790篇 |
地质学 | 10005篇 |
海洋学 | 2318篇 |
天文学 | 6457篇 |
综合类 | 63篇 |
自然地理 | 1928篇 |
出版年
2021年 | 226篇 |
2020年 | 281篇 |
2019年 | 284篇 |
2018年 | 588篇 |
2017年 | 570篇 |
2016年 | 688篇 |
2015年 | 529篇 |
2014年 | 720篇 |
2013年 | 1376篇 |
2012年 | 857篇 |
2011年 | 1187篇 |
2010年 | 1030篇 |
2009年 | 1359篇 |
2008年 | 1185篇 |
2007年 | 1155篇 |
2006年 | 1077篇 |
2005年 | 909篇 |
2004年 | 919篇 |
2003年 | 880篇 |
2002年 | 806篇 |
2001年 | 731篇 |
2000年 | 682篇 |
1999年 | 606篇 |
1998年 | 601篇 |
1997年 | 612篇 |
1996年 | 470篇 |
1995年 | 447篇 |
1994年 | 395篇 |
1993年 | 351篇 |
1992年 | 331篇 |
1991年 | 287篇 |
1990年 | 325篇 |
1989年 | 295篇 |
1988年 | 249篇 |
1987年 | 328篇 |
1986年 | 269篇 |
1985年 | 367篇 |
1984年 | 410篇 |
1983年 | 391篇 |
1982年 | 357篇 |
1981年 | 331篇 |
1980年 | 326篇 |
1979年 | 291篇 |
1978年 | 320篇 |
1977年 | 276篇 |
1976年 | 278篇 |
1975年 | 284篇 |
1974年 | 250篇 |
1973年 | 243篇 |
1972年 | 162篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
53.
Of the impact craters on Earth larger than 20 km in diameter, 10-15% (3 out of 28) are doublets, having been formed by the simultaneous impact of two well-separated projectiles. The most likely scenario for their formation is the impact of well-separated binary asteroids. If a population of binary asteroids is capable of striking the Earth, it should also be able to hit the other terrestrial planets as well. Venus is a promising planet to search for doublet craters because its surface is young, erosion is nearly nonexistent, and its crater population is significantly larger than the Earth's. After a detailed investigation of single craters separated by less than 150 km and “multiple” craters having diameters greater than 10 km, we found that the proportion of doublet craters on Venus is at most 2.2%, significantly smaller than Earth's, although several nearly incontrovertible doublets were recognized. We believe this apparent deficit relative to the Earth's doublet population is a consequence of atmospheric screening of small projectiles on Venus rather than a real difference in the population of impacting bodies. We also examined “splotches,” circular radar reflectance features in the Magellan data. Projectiles that are too small to form craters probably formed these features. After a careful study of these patterns, we believe that the proportion of doublet splotches on Venus (14%) is comparable to the proportion of doublet craters found on Earth (10-15%). Thus, given the uncertainties of interpretation and the statistics of small numbers, it appears that the doublet crater population on Venus is consistent with that of the Earth. 相似文献
54.
55.
Piero F. Spinnato Michael Fellhauer Simon F. Portegies Zwart 《Monthly notices of the Royal Astronomical Society》2003,344(1):22-32
We study the efficiency at which a black hole or dense star cluster spirals in to the Galactic Centre. This process takes place on a dynamical friction time-scale, which depends on the value of the Coulomb logarithm (ln Λ). We determine the accurate value of this parameter using the direct N -body method, a tree algorithm and a particle-mesh technique with up to two million plus one particles. The three different techniques are in excellent agreement. Our measurement for the Coulomb logarithm appears to be independent of the number of particles. We conclude that ln Λ= 6.6 ± 0.6 for a massive point particle in the inner few parsec of the Galactic bulge. For an extended object, such as a dense star cluster, ln Λ is smaller, with a value of the logarithm argument Λ inversely proportional to the object size. 相似文献
56.
57.
58.
Ralph D. Lorenz 《Meteoritics & planetary science》2004,39(4):617-623
Abstract— A model for an impact ejecta landform peculiar to Saturn's moon Titan is presented. Expansion of the ejecta plume from moderate‐sized craters is constrained by Titan's thick atmosphere. Much of the plume is collimated along the incoming bolide's trajectory, as was observed for plumes from impacts on Jupiter of P/Shoemaker‐Levy‐9, but is retained as a linear, diagonal ejecta cloud, unlike on Venus where the plume “blows out.” On Titan, the blowout is suppressed because the vertically‐extended atmosphere requires a long wake to reach the vacuum of space, and the modest impact velocities mean plume expansion along the wake is slow enough to allow the wake to close off. Beyond the immediate ejecta blanket around the crater, distal ejecta is released into the atmosphere from an oblique line source: this material is winnowed by the zonal wind field to form streaks, with coarse radar‐bright particles transported less far than fine radar‐dark material. Thus, the ejecta form two distinct streaks faintly reminiscent of dual comet tails, a sharply W‐E radar‐dark one, and a less swept and sometimes comma‐shaped radar‐bright one. 相似文献
59.
60.
Non-Newtonian topographic relaxation on Europa 总被引:1,自引:0,他引:1
Models of topographic support on Europa by lateral shell thickness variations have previously assumed a Newtonian ice viscosity. Here I show that using a more realistic stress-dependent viscosity gives relaxation times which can be significantly different. Topography of wavelength 100 km cannot be supported by lateral shell thickness variations for ∼50 Myr, unless the shell thickness is <10 km or the ice grain size >10 mm. Shorter wavelength topography would require even thinner shells, but may be supported elastically. Global-scale variations in shell thickness, however, can be supported for geological timescales if the shell thickness is O(10 km). 相似文献