全文获取类型
收费全文 | 84001篇 |
免费 | 1155篇 |
国内免费 | 596篇 |
专业分类
测绘学 | 2371篇 |
大气科学 | 6189篇 |
地球物理 | 16250篇 |
地质学 | 31716篇 |
海洋学 | 6860篇 |
天文学 | 17884篇 |
综合类 | 288篇 |
自然地理 | 4194篇 |
出版年
2021年 | 552篇 |
2020年 | 601篇 |
2019年 | 697篇 |
2018年 | 4026篇 |
2017年 | 3743篇 |
2016年 | 3140篇 |
2015年 | 1144篇 |
2014年 | 1765篇 |
2013年 | 3298篇 |
2012年 | 2814篇 |
2011年 | 4814篇 |
2010年 | 4346篇 |
2009年 | 5157篇 |
2008年 | 4322篇 |
2007年 | 4826篇 |
2006年 | 2592篇 |
2005年 | 2344篇 |
2004年 | 2240篇 |
2003年 | 2144篇 |
2002年 | 2032篇 |
2001年 | 1676篇 |
2000年 | 1605篇 |
1999年 | 1324篇 |
1998年 | 1293篇 |
1997年 | 1370篇 |
1996年 | 1092篇 |
1995年 | 1053篇 |
1994年 | 1030篇 |
1993年 | 859篇 |
1992年 | 797篇 |
1991年 | 794篇 |
1990年 | 787篇 |
1989年 | 776篇 |
1988年 | 715篇 |
1987年 | 812篇 |
1986年 | 730篇 |
1985年 | 892篇 |
1984年 | 1001篇 |
1983年 | 933篇 |
1982年 | 910篇 |
1981年 | 831篇 |
1980年 | 797篇 |
1979年 | 711篇 |
1978年 | 706篇 |
1977年 | 635篇 |
1976年 | 585篇 |
1975年 | 586篇 |
1974年 | 553篇 |
1973年 | 586篇 |
1972年 | 399篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
Ralph B. Baldwin 《Icarus》1985,61(1):63-91
This paper contains a reasonably successful attempt to determine relative ages and then absolute ages of individual craters younger than Imbrium, and the rate of infalls onto the Moon as a function of time. After the tail of the massive premare bombardment became depleted before 3 aeons (1 aeon = 109 years) ago, there was a period of minimal numbers of infalls. The rate of infalls increased rather steadily from this minimum to the present. The rate in the geologically recent past (0.3 aeon) was about two times that found for the period immediately after the last of the major lave outpourings (3.2 aeons). Absolute ages were determined for large craters (?8 km) from crater counts on the surfaces within and on the rims of the large craters. Key dates were 0 and 0.3 aeon for terrestrial meteoritic craters, 3.2, 3.5, 3.8, and 3.82 aeons for the various mare surfaces according to the determinations of D.E. Wilhelms (1980, Geologic history of the Moon, U.S. Geol. Surv. Prof. Pap.) and 3.85 aeons from the formation of Imbrium. 相似文献
172.
173.
Philip B. James 《Icarus》1985,64(2):249-264
The Martian CO2 cycle, which includes the seasonal condensation and subsequent sublimation of up to 30% of the planet's atmosphere, produces meridional winds due to the consequent mass flux of CO2. These winds currently display strong seasonal and hemispheric asymmetries due to the large asymmetries in the distribution of insolation on Mars. It is proposed that asymmetric meridional advection of water vapor on the planet due to these CO2 condensation winds is capable of explaining the observed dessication of Mars' south polar region at the current time. A simple model for water vapor transport is used to verify this hypothesis and to speculate on the effects of changes in orbital parameters on the seasonal water cycle. 相似文献
174.
175.
The statistical results presented by Achong and Stahl (1984) may alternatively be interpreted as demonstrating a strong dependence of SID flare production on Mt. Wilson magnetic class of the parent sunspot group. 相似文献
176.
177.
The Apollo orbital geochemistry, photogeologic, and other remote sensing data sets were used to identify and characterize geochemical anomalies on the eastern limb and farside of the Moon and to investigate the processes responsible for their formation. The anomalies are located in the following regions: (1) Balmer basin, (2) terrain northeast of Mare Smythii, (3) near Langemak crater, (4) Pasteur crater, (5) terrain northwest of Milne basin, (6) northeast of Mendeleev basin, (7) north and northeast of Korolev basin, (8) terrain north of Taruntius crater, and (9) terrain north of Orientale basin. The anomalies are commonly associated with Imbrian- or Nectarian-aged light plains units which exhibit dark-haloed impact craters. The results of recent spectral reflectance studies of dark-haloed impact craters plus consideration of the surface chemistry of the anomalies strongly indicate that those geochemical anomalies associated with light plains deposits which display dark-haloed impact craters result from the presence of basaltic units that are either covered by varying thickness of highland debris or have a surface contaminated with significant amounts of highlands material. The burial or contamination of ancient volcanic surfaces by varying amounts of highland material appears to have been an important (though not the dominant) process in the formation of lunar light plains. Basaltic volcanism on the eastern limb and farside of the Moon was more extensive in both space and time than has been accepted. 相似文献
178.
The FAST/SKA site selection in Guizhou province 总被引:1,自引:0,他引:1
Many karst depressions with diameters of 300 m to 500 m, suitable for constructing Arecibo-style radio telescopes, were identified
in the south of Guizhou Province by Remote Sensing (RS) and Geographic Information System (GIS) technologies together with
field investigations. Fundamental topography and landform databases were established for 391candidate depressions, and using
GIS the3-dimensional images of depressions, at a scale of 1:10000, were then simulated to fit a spherical antenna.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
179.
M. A. Iatì C. Cecchi-Pestellini D. A. Williams F. Borghese P. Denti R. Saija S. Aiello 《Monthly notices of the Royal Astronomical Society》2001,322(4):749-756
Recent observations of stellar composition suggest that elements in the Sun are significantly more abundant than in other stars. The reduction in the available element budget implies a drastic revision in current models of interstellar dust. Theoretical models are therefore exploring fluffy, porous physical structure for the grain material. Since a detailed exact treatment of extinction cross-sections is mandatory for a correct understanding of the nature of interstellar dust, we present a technique based on the multipole expansions of the electromagnetic field, which has proven to be general, flexible and powerful in treating scattering of light by porous, composite, arbitrarily shaped particles. The results of this study speak in favour of core–mantle structures characterized by the presence of porosities. 相似文献
180.
High nitrate contents (up to 200 mgl) have been observed in wells drilled into fractured aquifers lying beneath layers of weathered and decayed rock in the humid tropics where annual rainfall is over 1000 mm and where there is no notable pollution. The source of the nitrates is linked mainly with localized deforestation by man. 相似文献