全文获取类型
收费全文 | 261篇 |
免费 | 41篇 |
国内免费 | 25篇 |
专业分类
测绘学 | 52篇 |
大气科学 | 9篇 |
地球物理 | 48篇 |
地质学 | 118篇 |
海洋学 | 12篇 |
天文学 | 53篇 |
综合类 | 13篇 |
自然地理 | 22篇 |
出版年
2023年 | 1篇 |
2022年 | 8篇 |
2021年 | 10篇 |
2020年 | 4篇 |
2019年 | 6篇 |
2018年 | 4篇 |
2017年 | 9篇 |
2016年 | 7篇 |
2015年 | 10篇 |
2014年 | 21篇 |
2013年 | 18篇 |
2012年 | 19篇 |
2011年 | 15篇 |
2010年 | 22篇 |
2009年 | 7篇 |
2008年 | 11篇 |
2007年 | 17篇 |
2006年 | 10篇 |
2005年 | 32篇 |
2004年 | 19篇 |
2003年 | 8篇 |
2002年 | 7篇 |
2001年 | 7篇 |
2000年 | 10篇 |
1999年 | 5篇 |
1998年 | 10篇 |
1997年 | 3篇 |
1996年 | 1篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1982年 | 1篇 |
排序方式: 共有327条查询结果,搜索用时 968 毫秒
171.
Efi Meletlidou Simos Ichtiaroglou Fabian Josef Winterberg 《Celestial Mechanics and Dynamical Astronomy》2001,80(2):145-156
We prove that Hill's lunar problem does not possess a second analytic integral of motion, independent of the Hamiltonian. In order to obtain this result, we avoid the usual normalization in which the angular velocity of the rotating reference frame is put equal to unit. We construct an artificial Hamiltonian that includes an arbitrary parameter b and show that this Hamiltonian does not possess an analytic integral of motion for in an open interval around zero. Then, by selecting suitable values of , b and using the invariance of the Hamiltonian under scaling in the units of length and time, we show that the Hamiltonian of Hill's problem does not possess an integral of motion, analytically continued from the integrable two–body problem in a rotating frame. 相似文献
172.
Marc D. Norman Vickie C. BennettGraham Ryder 《Earth and Planetary Science Letters》2002,202(2):217-228
Highly siderophile element compositions of lunar impact melt breccias provide a unique record of the asteroid population responsible for large cratering events in the inner Solar System. Melt breccias associated with the 3.89 Ga Serenitatis impact basin resolve at least two separate impact events. KREEP-rich melt breccias representing the Apollo 17 poikilitic suite are enriched in highly siderophile elements (3.6-15.8 ppb Ir) with CI-normalized patterns that are elevated in Re, Ru and Pd relative to Ir and Pt. The restricted range of lithophile element compositions combined with the coherent siderophile element signatures indicate formation of these breccias in a single impact event involving an EH chondrite asteroid, probably as melt sheet deposits from the Serenitatis Basin. One exceptional sample, a split from melt breccia 77035, has a distinctive lithophile element composition and a siderophile element signature more like that of ordinary chondrites, indicating a discrete impact event. The recognition of multiple impact events, and the clear signatures of specific types of meteoritic impactors in the Apollo 17 melt breccias, shows that the lunar crust was not comprehensively reworked by prior impacts from 3.9 to 4.5 Ga, an observation more consistent with a late cataclysm than a smoothly declining accretionary flux. Late accretion of enstatite chondrites during a 3.8-4.0 Ga cataclysm may have contributed to siderophile element heterogeneity on the Earth, but would not have made a significant contribution to the volatile budget of the Earth or oxidation of the terrestrial mantle. Siderophile element patterns of Apollo 17 poikilitic breccias become more fractionated with decreasing concentrations, trending away from known meteorite compositions to higher Re/Ir and Pd/Pt ratios. The compositions of these breccias may be explained by a two-stage impact melting process involving: (1) deep penetration of the Serenitatis impactor into meteorite-free lower crust, followed by (2) incorporation of upper crustal lithologies moderately contaminated by prior meteoritic infall into the melt sheet. Trends to higher Re/Ir with decreasing siderophile element concentrations may indicate an endogenous lunar crustal component, or a non-chondritic late accretionary veneer in the pre-Serenitatis upper crust. 相似文献
173.
174.
Wu Ji Li Dihui Zhang Xiaohui Jiang Jingshan A. T. Altyntsev B. I. Lubyshev 《Journal of Earth System Science》2005,114(6):627-632
Among many scientific objectives of lunar exploration, investigations on lunar soil become attractive due to the existence
of He3 and ilmenite in the lunar soil and their possible utilization as nuclear fuel for power generation. Although the composition
of the lunar surface soil can be determined by optical and γ/X-ray spectrometers, etc., the evaluation of the total reserves
of He3 and ilmenite within the regolith and in the lunar interior are still not available. In this paper, we give a rough analysis
of the microwave brightness temperature images of the lunar disc observed using the NRAO 12 meter Telescope and Siberian Solar
Radio Telescope. We also present the results of the microwave dielectric properties of terrestrial analogues of lunar soil
and, discuss some basic relations between the microwave brightness temperature and lunar soil properties. 相似文献
175.
H. Mizutani A. Fujimura S. Tanaka H. Shiraishi T. Nakjima 《Journal of Earth System Science》2005,114(6):763-768
The scientific objective of the Lunar-A, Japanese Penetrator Mission, is to explore the lunar interior by seismic and heat-flow
experiments. Two penetrators containing two seismometers (horizontal and vertical components) and heat-flow probes will be
deployed from a spacecraft onto the lunar surface, one on the near-side and the other on the far-side of the moon. The data
obtained by the penetrators will be transmitted to the earth station via the Lunar-A mother spacecraft orbiting at an altitude
of about 200 km.
The spacecraft of a cylindrical shape, 2.2 m in maximum diameter and 1.7 m in height, is designed to be spin-stabilized. The
spacecraft will be inserted into an elliptic lunar orbit, after about a half-year cruise during which complex manoeuvering
is made using the lunar-solar gravity assist. After lunar orbit insertion, two penetrators will be separated from the spacecraft
near perilune, one by one, and will be landed on the lunar surface.
The final impact velocity of the penetrator will be about 285 m/sec; it will encounter a shock of about 8000 G at impact on
the lunar surface. According to numerous experimental impact tests using model penetrators and a lunar-regolith analog target,
each penetrator is predicted to penetrate to a depth between l and 3 m, depending on the hardness and/or particle-size distribution
of the lunar regolith. The penetration depth is important for ensuring the temperature stability of the instruments in the
penetrator and heat flow measurements. According to the results of the Apollo heat flow experiment, an insulating regolith
blanket of only 30 cm is sufficient to dampen out about 280 K lunar surface temperature fluctuation to < 3 K variation.
The seismic observations are expected to provide key data on the size of the lunar core, as well as data on deep lunar mantle
structure. The heat flow measurements at two penetrator-landing sites will also provide important data on the thermal structure
and bulk concentrations of heat-generating elements in the Moon. These data will provide much stronger geophysical constraints
on the origin and evolution of the Moon than has been obtained so far.
Currently, the Lunar-A system is being reviewed and a more robust system for communication between the penetrators and spacecraft
is being implemented according to the lessons learned from Beagle-2 and DS-2 failures. More impact tests for penetrators onto
a lunar regolith analogue target will be undertaken before its launch. 相似文献
176.
177.
Lunar and solar atmospheric tidal oscillations have been determined with reasonable accuracy from a ten-year record of hourly mercury-barometer readings, corrected to mean-sea-level, at Rarotonga (Cook Islands), 21.2°S. For the lunar semidiurnal tide, the annual determination shows an amplitude (56 b) slightly lower and a phase (51°) much smaller than the values (58 b, 72°) that would be derived, for the position of Rarotonga, from the spherical harmonic analysis given byHaurwitz andCowley (1969). The seasonal variation of this oscillation, as given by the monthly and J, E, D values, shows most of the characteristic features found in world-wide determinations. In particular, the near equality of the J, D amplitudes at Rarotonga tends to support theHaurwitz andCowley (1969) suggestion of negative J-D values in southern middle latitudes. For the solar tides, the semidiurnal and terdiurnal oscillations at Rarotonga are similar to those found at other stations in the south-west Pacific region. However, for the diurnal oscillation, the annual amplitude (232 b) is only about half the value (465 b) indicated for the position of Rarotonga by the world maps of theS
1(p) annual harmonic coefficients given byHaurwitz (1965). It thus seems likely that the relatively small area of lowS
1(p) annual amplitude in the eastern part of the south Pacific, as indicated by these maps, is much more extensive than formerly supposed. 相似文献
178.
179.
180.