全文获取类型
收费全文 | 262136篇 |
免费 | 5550篇 |
国内免费 | 4066篇 |
专业分类
测绘学 | 7339篇 |
大气科学 | 19492篇 |
地球物理 | 54935篇 |
地质学 | 92233篇 |
海洋学 | 21786篇 |
天文学 | 56668篇 |
综合类 | 1192篇 |
自然地理 | 18107篇 |
出版年
2021年 | 2363篇 |
2020年 | 2682篇 |
2019年 | 2935篇 |
2018年 | 3848篇 |
2017年 | 3533篇 |
2016年 | 5935篇 |
2015年 | 4331篇 |
2014年 | 7057篇 |
2013年 | 14386篇 |
2012年 | 6851篇 |
2011年 | 8391篇 |
2010年 | 7365篇 |
2009年 | 9986篇 |
2008年 | 8745篇 |
2007年 | 8195篇 |
2006年 | 9851篇 |
2005年 | 7878篇 |
2004年 | 7702篇 |
2003年 | 7190篇 |
2002年 | 6788篇 |
2001年 | 6050篇 |
2000年 | 5995篇 |
1999年 | 5229篇 |
1998年 | 5268篇 |
1997年 | 5059篇 |
1996年 | 4690篇 |
1995年 | 4432篇 |
1994年 | 4117篇 |
1993年 | 3872篇 |
1992年 | 3652篇 |
1991年 | 3612篇 |
1990年 | 3775篇 |
1989年 | 3531篇 |
1988年 | 3312篇 |
1987年 | 3855篇 |
1986年 | 3424篇 |
1985年 | 4231篇 |
1984年 | 4752篇 |
1983年 | 4426篇 |
1982年 | 4318篇 |
1981年 | 3945篇 |
1980年 | 3650篇 |
1979年 | 3516篇 |
1978年 | 3496篇 |
1977年 | 3278篇 |
1976年 | 3044篇 |
1975年 | 2961篇 |
1974年 | 2920篇 |
1973年 | 3080篇 |
1972年 | 2028篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
531.
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
532.
533.
The Dergaon (H5) chondrite: Fall,classification, petrological and chemical characteristics,cosmogenic effects,and noble gas records 总被引:1,自引:0,他引:1
P. N. SHUKLA A. D. SHUKLA V. K. RAI S. V. S. MURTY N. BHANDARI J. N. GOSWAMI A. C. MAZUMDAR P. PHUKON K. DUORAH R. E. GREENWOOD I. A. FRANCHI 《Meteoritics & planetary science》2005,40(4):627-637
Abstract— A multiple fall of a stony meteorite occurred near the town of Dergaon in Assam, India, on March 2, 2001. Several fragments weighing <2 kg and a single large fragment weighing ~10 kg were recovered from the strewn field, which extended over several tens of square kilometers. Chemical, petrographic, and oxygen isotopic studies indicate it to be, in most aspects, a typical H5 chondrite, except the unusually low K content of ~340 ppm. A cosmic ray exposure of 9.7 Ma is inferred from the cosmogenic noble gas records. Activities of eleven cosmogenic radionuclides were measured. 26Al and 22Na activities as well as the 22Na/26Al activity ratio are close to the values expected on the basis of solar modulation of galactic cosmic rays. The low 60Co activity (<1 dpm/kg) is indicative of a small preatmospheric size of the meteorite. Cosmic ray heavy nuclei track densities in olivine grains range from ~106 cm?2 in samples from the largest fragment to approximately (4–9) × 105 cm?2 in one of the smaller fragments. The combined track, radionuclide, and noble gas data suggest a preatmospheric radius of ~20 cm for the Dergaon meteorite. 相似文献
534.
The dynamics of clumps observed in planetary nebulae are considered. The possibility that SiO maser spots in evolved stars and the planetary nebula clumps are formed by the Parker instability behind shocks in pulsating stars' atmospheres is raised. Molecular observations of the clumps are suggested. The effects of the ablation of clumps on the global flow structure of a more tenuous plasma in which they are embedded are reviewed. 相似文献
535.
Two-dimensional (2D) compressible magnetohydrodynamic simulations are performed to explore the idea that the asymmetric reconnection between newly emerging intranetwork magnetic field flux and pre-existing network flux causes the explosive events in the solar atmosphere. The dependence of the reconnection rate as a function of time on the density and temperature of the emerging flux are investigated. For a Lundquist number of L
u= 5000 we find that the tearing mode instability can lead to the formation and growth of small magnetic islands. Depending on the temperature and density ratio of the emerging plasma, the magnetic island can be lifted upward and convected out of the top boundary, or is suppressed downward and convected out of the top boundary, or is suppressed downward nad submerged below the bottom boundary. The motions of the magnetic islands with different direction are accompanied respectively with upward or downward high velocity flow which might be associated with the red- and blue-shifted components detected in the explosive events. 相似文献
536.
537.
M. Madkour 《Astrophysics and Space Science》1990,174(1):1-11
Radiation transfer problem in the slowing-down region for cylindrical geometry with diffuse reflectivity and internal source is connected with source-free problem with isotropic boundary condition. Modelled kernels are used to represent the slowing-down kernel. Calculations are performed for the radiation flux at the boundary of the cylinder. Numerical results are obtained using the bi-variation technique. 相似文献
538.
539.
PÅL HAREMO ARILD ANDRESEN HENNING DYPVIK JENÖ NAGY ERS ELVERHøI TOR ARNE EIKELAND HALVOR JOHANSEN 《Polar research》1990,8(2):195-216
The Billefjorden Fault Zone represents a major lineament on Spitsbergen with a history of tectonic activity going back into the Devonian and possibly earlier. Recent structural, sedimcntological and stratigraphical investigations indicate that most of the stratigraphic thickness variations within the Mesozoic strata along the Billefjorden Fault Zone south of Isfjordcn are due to Tertiary compressional tectonics related to the transpressive Eocene West-Spitsbergen Orogeny. No convincing evidence of distinct Mesozoic extensional events, as suggested by previous workers, has been recognized. Tertiary compressional tectonics are characterized by a combined thin-skinned/thick-skinned structural style. Decollement zones arc recognized in the Triassic Sassendalen Group (tower Décollement Zone) and in the Jurassic/Cretaceous Janusfjellet Subgroup (Upper Décollement Zone). East-vergent folding and reverse faulting associated with these decollement' zones have resulted in the development of compressional structures, of which the major arc the Skolten and Tronfjellct Anticlines and the Advcntelva Duplex. Movements on one or more high angle east-dipping reverse faults in the pre-Mesozoic basement have resulted in the development of the Juvdalskampcn Monocline, and are responsible for out-of-sequence thrusting and thinning of the Mesozoic sequence across the Billefjorden Fault Zone. Preliminary shortening calculations indicate an eastward displacement of minimum 3-4 km, possibly as much as 10 km for the Lower Cretaceous and younger rocks across the Billefjorden Fault Zone. 相似文献
540.
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