全文获取类型
| 收费全文 | 38684篇 |
| 免费 | 5535篇 |
| 国内免费 | 8309篇 |
专业分类
| 测绘学 | 3731篇 |
| 大气科学 | 4422篇 |
| 地球物理 | 8145篇 |
| 地质学 | 18189篇 |
| 海洋学 | 4796篇 |
| 天文学 | 6190篇 |
| 综合类 | 2414篇 |
| 自然地理 | 4641篇 |
出版年
| 2024年 | 125篇 |
| 2023年 | 397篇 |
| 2022年 | 996篇 |
| 2021年 | 1196篇 |
| 2020年 | 1445篇 |
| 2019年 | 1810篇 |
| 2018年 | 1309篇 |
| 2017年 | 1549篇 |
| 2016年 | 1583篇 |
| 2015年 | 1723篇 |
| 2014年 | 2350篇 |
| 2013年 | 2712篇 |
| 2012年 | 2418篇 |
| 2011年 | 2672篇 |
| 2010年 | 2311篇 |
| 2009年 | 2939篇 |
| 2008年 | 2889篇 |
| 2007年 | 2883篇 |
| 2006年 | 2791篇 |
| 2005年 | 2288篇 |
| 2004年 | 2074篇 |
| 2003年 | 1772篇 |
| 2002年 | 1379篇 |
| 2001年 | 1279篇 |
| 2000年 | 1125篇 |
| 1999年 | 1048篇 |
| 1998年 | 973篇 |
| 1997年 | 726篇 |
| 1996年 | 600篇 |
| 1995年 | 554篇 |
| 1994年 | 472篇 |
| 1993年 | 460篇 |
| 1992年 | 293篇 |
| 1991年 | 242篇 |
| 1990年 | 202篇 |
| 1989年 | 176篇 |
| 1988年 | 150篇 |
| 1987年 | 89篇 |
| 1986年 | 85篇 |
| 1985年 | 89篇 |
| 1984年 | 60篇 |
| 1983年 | 32篇 |
| 1982年 | 40篇 |
| 1981年 | 27篇 |
| 1980年 | 34篇 |
| 1978年 | 19篇 |
| 1977年 | 36篇 |
| 1976年 | 26篇 |
| 1973年 | 17篇 |
| 1971年 | 15篇 |
排序方式： 共有10000条查询结果，搜索用时 31 毫秒
31.
32.
33.
We present results from a new simulation code that accounts for the evolution of the reservoirs of carbon dioxide on Mars, from its early years to the present. We establish a baseline model parameter set that produces results compatible with the present (i.e., Patm?6.5 mbar with permanent CO2 ice cap) for a wide range of initial inventories. We find that the initial inventory of CO2 broadly determines the evolutionary course of the reservoirs of CO2. The reservoirs include the atmosphere, ice cap, adsorbed CO2 in the regolith, and carbonate rocks. We track the evolution of the free inventory: the atmosphere, ice cap and regolith. Simulations begin at 4.53 Gyr before present with a rapid loss of free inventory to space in the early Noachian. Models that assume a relatively small initial inventory (?5 bar) have pronounced minima in the free inventory of CO2 toward the end of the Noachian. Under baseline parameters, initial inventories below ∼4.5 bar result in a catastrophic loss of the free inventory to space. The current free inventory would be then determined by the balance between outgassing, sputtering losses and chemical weathering following the end of the late bombardment. We call these “thin” models. They generically predict small current free inventories in line with expectations of a small present CO2 ice cap. For “thick” models, with initial inventories ?5 bar, a surplus of 300-700 mbar of free CO2 remains during the late-Noachian. The histories of free inventory in time for thick models tend to converge within the last 3.5 Gyr toward a present with an ice cap plus atmospheric inventory of about 100 mbar. For thick models, the convergence is largely due to the effects of chemical weathering, which draws down higher free inventories more rapidly than the low. Thus, thick models have ?450 mbar carbonate reservoirs, while thin models have ?200 mbar. Though both thick and thin scenarios can reproduce the current atmospheric pressure, the thick models imply a relatively large current CO2 ice cap and thin models, little or none. While the sublimation of a massive cap at a high obliquity would create a climate swing of greenhouse warming for thick models, under the thin model, mean temperatures and pressures would be essentially unaffected by increases in obliquity. 相似文献
34.
35.
36.
37.
38.
39.
In the first paper of this series, we presented EBAS – Eclipsing Binary Automated Solver, a new fully automated algorithm to analyse the light curves of eclipsing binaries, based on the ebop code. Here, we apply the new algorithm to the whole sample of 2580 binaries found in the Optical Gravitational Lensing Experiment (OGLE) Large Magellanic Cloud (LMC) photometric survey and derive the orbital elements for 1931 systems. To obtain the statistical properties of the short-period binaries of the LMC, we construct a well-defined subsample of 938 eclipsing binaries with main-sequence B-type primaries. Correcting for observational selection effects, we derive the distributions of the fractional radii of the two components and their sum, the brightness ratios and the periods of the short-period binaries. Somewhat surprisingly, the results are consistent with a flat distribution in log P between 2 and 10 d. We also estimate the total number of binaries in the LMC with the same characteristics, and not only the eclipsing binaries, to be about 5000. This figure leads us to suggest that (0.7 ± 0.4) per cent of the main-sequence B-type stars in the LMC are found in binaries with periods shorter than 10 d. This frequency is substantially smaller than the fraction of binaries found by small Galactic radial-velocity surveys of B stars. On the other hand, the binary frequency found by Hubble Space Telescope ( HST ) photometric searches within the late main-sequence stars of 47 Tuc is only slightly higher and still consistent with the frequency we deduced for the B stars in the LMC. 相似文献
40.
A. Habib J. Vernin Z. Benkhaldoun H. Lanteri 《Monthly notices of the Royal Astronomical Society》2006,368(3):1456-1462
In this paper, we present a new method to estimate, for each turbulent layer labelled i , the horizontal wind speed v ( h i ) , the standard deviation of the horizontal wind speed fluctuations σ v ( hi ) and the integrated value of C 2 n over the thickness Δ hi of the turbulent layer C 2 n ( hi )Δ hi , where hi is the altitude of the turbulent layer. These parameters are extracted from single star scintillation spatiotemporal cross-correlation functions of atmospheric speckles obtained within the generalized mode. This method is based on the simulated annealing algorithm to find the optimal solution required to solve the problem. Astrophysics parameters for adaptive optics are also calculated using C 2 n ( hi ) and v ( hi ) values. The results of other techniques support this new method. 相似文献