全文获取类型
收费全文 | 1590篇 |
免费 | 63篇 |
国内免费 | 26篇 |
专业分类
测绘学 | 58篇 |
大气科学 | 125篇 |
地球物理 | 346篇 |
地质学 | 546篇 |
海洋学 | 117篇 |
天文学 | 335篇 |
综合类 | 8篇 |
自然地理 | 144篇 |
出版年
2022年 | 23篇 |
2021年 | 22篇 |
2020年 | 26篇 |
2019年 | 24篇 |
2018年 | 52篇 |
2017年 | 54篇 |
2016年 | 38篇 |
2015年 | 40篇 |
2014年 | 48篇 |
2013年 | 84篇 |
2012年 | 46篇 |
2011年 | 68篇 |
2010年 | 72篇 |
2009年 | 76篇 |
2008年 | 70篇 |
2007年 | 64篇 |
2006年 | 63篇 |
2005年 | 41篇 |
2004年 | 52篇 |
2003年 | 41篇 |
2002年 | 57篇 |
2001年 | 27篇 |
2000年 | 36篇 |
1999年 | 22篇 |
1998年 | 28篇 |
1997年 | 26篇 |
1996年 | 17篇 |
1995年 | 21篇 |
1994年 | 14篇 |
1993年 | 18篇 |
1992年 | 23篇 |
1991年 | 14篇 |
1990年 | 14篇 |
1989年 | 10篇 |
1987年 | 21篇 |
1986年 | 17篇 |
1985年 | 23篇 |
1984年 | 30篇 |
1983年 | 27篇 |
1982年 | 33篇 |
1981年 | 24篇 |
1980年 | 25篇 |
1979年 | 22篇 |
1978年 | 9篇 |
1977年 | 15篇 |
1976年 | 16篇 |
1975年 | 8篇 |
1974年 | 15篇 |
1973年 | 9篇 |
1971年 | 8篇 |
排序方式: 共有1679条查询结果,搜索用时 218 毫秒
911.
Tucson is an unusual iron meteorite which contains highly reduced silicate inclusions and elemental silicon and chromium in solution. The metal matrix of Tucson was found to be chemically uniform, suggesting that homogenization has occurred at elevated temperatures. The microstructure of the metal consists of plessite and thin ribbons of kamacite. Nickel and phosphorus concentrations indicate that kamacite nucleated along prior taenite grain boundaries at ~650°C, and grew upon cooling to 500°C. Kamacite growth calculations show that Tucson cooled at ~1°C/1000 yr, a rate which corresponds to a depth of burial at the center of a 15 km radius parent body or closer to the surface of parent bodies of larger sizes. The shapes of the Tucson irons, and the presence and distribution of silicate inclusions in the Fe-Ni matrix appear to be a result of a solidification process. 相似文献
912.
While it has been known for a long time that in general the albedo of a surface depends on incidence angle, this fact is commonly neglected in many calculations of planetary surface temperatures. We show that the effect is especially pronounced for bright surfaces. For objects such as Ganymede and Io, the effect produces substantially cooler temperatures near the poles and terminators that would be calculated under the assumption of a constant albedo—a factor which may be important in determining the stability of frosts on such surfaces. 相似文献
913.
914.
915.
Clark R. ChapmanWilliam J. Merline Peter C. ThomasJonathan Joseph Andrew F. ChengNoam Izenberg 《Icarus》2002,155(1):104-118
Preliminary measurements of craters and boulders have been made in various locations on Eros from images acquired during the first nine months of NEAR Shoemaker's orbital mission, including the October 2000 low altitude flyover. (We offer some very preliminary, qualitative analysis of later LAF images and very high-resolution images obtained during NEAR's landing on 12 February 2001). Craters on Eros >100 m diameter closely resemble the saturated crater population of Ida; Eros is more heavily cratered than Gaspra but lacks the saturated giant craters of Mathilde. These craters and the other large-scale geological features were formed over a duration of very roughly 2 Gyr while Eros was in the main asteroid belt, between the time when its parent body was disrupted and Eros was injected into an Earth-approaching orbit (probably tens of Myr ago). Saturation equilibrium had been expected to shape Eros' crater population down to very small sizes, as on the lunar maria. However, craters <200 m diameter are instead progressively depleted toward smaller sizes and are a factor of ∼200 below empirical saturation at diameters of 4 m. Conversely, boulders and positive relief features (PRFs) rise rapidly in numbers (differential power-law index ∼−5) and those <10 m in size dominate the landscape at high resolutions. The pervasive boulders and minimal craters on Eros is radically different from the lunar surface at similar scales. This may be partly explained by a major depletion of meter-scale projectiles in the asteroid belt (due to the Yarkovsky Effect: Bell 2001), which thus form few small craters and destroy few boulders. Additionally, the small size and low gravity of Eros may result in redistribution or loss of ejecta due to seismic shaking, thus preferentially destroying small craters formed in such regolith. Possibly Eros has only a patchy, thin regolith of mobile fines; the smaller PRFs may then reflect exposures of fractured bedrock or piles of large ejecta blocks, which might further inhibit formation of craters <10 m in size. Eros may well have been largely detached dynamically and collisionally from the main asteroid belt for the past tens of Myr, in which case its cratering rate would have dropped by two orders of magnitude, perhaps enhancing the relative efficacy of other processes that would normally be negligible in competition with cratering. Such processes include thermal creep, electrostatic levitation and redistribution of fines, and space weathering (e.g., bombardment by micrometeorites and solar wind particles). Combined with other small-body responses to impact cratering (e.g., greater widespread distribution of bouldery ejecta), such processes may also help explain the unexpected small-scale character of geology on Eros. If there was a recent virtual hiatus in cratering of Eros (during which only craters <∼300 m diameter would be expected to have formed), space weathering may have reached maturity, thus explaining Eros' remarkable spectral homogeneity compared with Ida. 相似文献
916.
917.
NUTRIENT LOAD ESTIMATION METHODS FOR RIVERS 总被引:2,自引:0,他引:2
Huai-en LI. Joseph Hun-Wei LEE Ming CM Professor Environmental Science Institute Xi''''an University of Technology. Xi''''an . Shaanxi P.R. China.Professor Department of Civil Engineering The University of Hong Kong Pokfulam Road Hong Kong.P.R. China Ph.D. student in Hydrology Water Resources. Environmental Science Institute Xi''''an University ofTechnology Xi''''an Shaanxi P.R. China 《国际泥沙研究》2003,18(4)
Pollutant load estimation is essential for watershed management and water pollution control. For most watersheds, only sparse water quality measurements (e.g. monthly data) are available. The influence of input data on the accuracy of non-point source pollution load estimation is studied using the water quality and stream flow data from a small watershed in Hong Kong. Comparison and analysis of the results using 8 different methods show that the accuracy of stream-flow runoff is the single most important factor for the calculation of pollutant load. Based on the results, the stream flow correction coefficient is advanced to provide a more reliable load estimation. The improved method of pollutant load estimation can be easily applied in practice since the stream-flow runoff can be measured by hydrological station or estimated with various hydrological methods. 相似文献
918.
919.
920.
Oxygen isotopic composition and U-Pb discordance in zircon 总被引:4,自引:0,他引:4
Amanda L. Booth Yehoshua Kolodny Michael McWilliams Joseph Wooden 《Geochimica et cosmochimica acta》2005,69(20):4895-4905
We have investigated U-Pb discordance and oxygen isotopic composition of zircon using high-spatial resolution δ18O measurement by ion microprobe. δ18O in both concordant and discordant zircon grains provides an indication of the relationship between fluid interaction and discordance. Our results suggest that three characteristics of zircon are interrelated: (1) U-Pb systematics and concomitant age discordance, (2) δ18O and the water-rock interactions implied therein, and (3) zircon texture, as revealed by cathodoluminescence and BSE imaging. A key observation is that U-Pb-disturbed zircons are often also variably depleted in 18O, but the relationship between discordance and δ18O is not systematic. δ18O values of discordant zircons are generally lighter but irregular in their distribution. Textural differences between zircon grains can be correlated with both U-Pb discordance and δ18O. Discordant grains exhibit either a recrystallized, fractured, or strongly zoned CL texture, and are characteristic of 18O depletion. We interpret this to be a result of metamictization, leading to destruction of the zircon lattice and an increased susceptibility to lead loss. Conversely, grains that are concordant have less-expressed zoning and a smoother CL texture and are enriched in 18O. From this it is apparent that various stages of water-rock interaction, as evidenced by systematic variations in δ18O, leave their imprint on both the texture and U-Pb systematics of zircon. 相似文献