全文获取类型
收费全文 | 9975篇 |
免费 | 920篇 |
国内免费 | 806篇 |
专业分类
测绘学 | 338篇 |
大气科学 | 1487篇 |
地球物理 | 2681篇 |
地质学 | 4208篇 |
海洋学 | 743篇 |
天文学 | 1323篇 |
综合类 | 254篇 |
自然地理 | 667篇 |
出版年
2023年 | 77篇 |
2022年 | 145篇 |
2021年 | 281篇 |
2020年 | 286篇 |
2019年 | 249篇 |
2018年 | 477篇 |
2017年 | 479篇 |
2016年 | 641篇 |
2015年 | 492篇 |
2014年 | 571篇 |
2013年 | 785篇 |
2012年 | 651篇 |
2011年 | 583篇 |
2010年 | 569篇 |
2009年 | 537篇 |
2008年 | 447篇 |
2007年 | 361篇 |
2006年 | 365篇 |
2005年 | 287篇 |
2004年 | 265篇 |
2003年 | 243篇 |
2002年 | 241篇 |
2001年 | 196篇 |
2000年 | 182篇 |
1999年 | 212篇 |
1998年 | 175篇 |
1997年 | 207篇 |
1996年 | 152篇 |
1995年 | 151篇 |
1994年 | 144篇 |
1993年 | 98篇 |
1992年 | 64篇 |
1991年 | 86篇 |
1990年 | 91篇 |
1989年 | 56篇 |
1988年 | 44篇 |
1987年 | 61篇 |
1986年 | 50篇 |
1985年 | 49篇 |
1984年 | 57篇 |
1983年 | 48篇 |
1982年 | 44篇 |
1981年 | 49篇 |
1980年 | 25篇 |
1979年 | 37篇 |
1978年 | 25篇 |
1977年 | 25篇 |
1975年 | 24篇 |
1974年 | 24篇 |
1973年 | 30篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Abstract– Pyroxenes are among the most common minerals in the solar system and are ideally suited for remote geochemical analysis because of the sensitivity of their distinctive spectra to mineral composition. Fe2+ is responsible for the dominant pyroxene absorptions in the visible and near‐infrared, but substitutions of other cations such as Ca2+ change the crystal structure and site geometries and thus the crystal field splitting energies of the Fe cations. To define spectral systematics resulting from major pyroxene cations (Ca2+, Mg2+, and Fe2+), we focus on a suite of pyroxenes synthesized with only Ca2+, Mg2+, and Fe2+ in the two octahedral sites, specifically examining the effect of Ca2+ on pyroxene absorption bands. The modified Gaussian model is used to deconvolve pyroxene spectra into component bands that can then be linked directly to crystal field absorptions. In orthopyroxenes and low‐Ca clinopyroxenes, Ca2+‐content has a strong and predictable effect on the positions of the absorption bands. At a threshold of Wo30, the crystal field environment stagnates and the M2 bands cease to change significantly as more Ca2+ is added. At Wo50, when most of the M2 sites are filled by Ca2+, band positions do not change drastically, although the presence and strengths of the 1 and 2 μm bands are affected by even trace amounts of Fe2+ in the M2 site. It is thus apparent that next‐nearest neighbors and the distortions they impose on the pyroxene lattice affect the electronic states around the Fe2+ cations and control absorption band properties. 相似文献
992.
To determine where to search for life in our solar system or in other extrasolar systems, the concept of habitability has
been developed, based on the only sample we have of a biological planet—the Earth. Habitability can be defined as the set
of the necessary conditions for an active life to exist, even if it does not exist. In astronomy, a habitable zone (HZ) is
the zone defined around a sun/star, where the temperature conditions allow liquid water to exist on its surface. This habitability
concept can be considered from different scientific perspectives and on different spatial and time scales. Characterizing
habitability at these various scales requires interdisciplinary research. In this article, we have chosen to develop the geophysical,
geological, and biological aspects and to insist on the need to integrate them, with a particular focus on our neighboring
planets, Mars and Venus. Important geodynamic processes may affect the habitability conditions of a planet. The dynamic processes,
e.g., internal dynamo, magnetic field, atmosphere, plate tectonics, mantle convection, volcanism, thermo-tectonic evolution,
meteorite impacts, and erosion, modify the planetary surface, the possibility to have liquid water, the thermal state, the
energy budget, and the availability of nutrients. They thus play a role in the persistence of life on a planet. Earth had
a liquid water ocean and some continental crust in the Hadean between 4.4 and 4.0 Ga (Ga: billions years ago), and may have
been habitable very early on. The origin of life is not understood yet; but the oldest putative traces of life are early Archean
(~3.5 Ga). Studies of early Earth habitats documented in the rock record hosting fossil life traces provide information about
possible habitats suitable for life beyond Earth. The extreme values of environmental conditions in which life thrives today
can also be used to characterize the “envelope” of the existence of life and the range of potential extraterrestrial habitats.
The requirement of nutrients by life for biosynthesis of cellular constituents and for growth, reproduction, transport, and
motility may suggest that a dynamic and rocky planet with hydrothermal activity and formation of relief, liquid water alteration,
erosion, and runoff is required to replenish nutrients and to sustain life (as we know it). The concept of habitability is
very Earth-centric, as we have only one biological planet to study. However, life elsewhere would most probably be based on
organic chemistry and leave traces of its past or recent presence and metabolism by modifying microscopically or macroscopically
the physico-chemical characteristics of its environment. The extent to which these modifications occur will determine our
ability to detect them in astrobiological exploration. Looking at major steps in the evolution of life may help determining
the probability of detecting life (as we know it) beyond Earth and the technology needed to detect its traces, be they morphological,
chemical, isotopic, or spectral. 相似文献
993.
Martin Nekola René Hudec Martin Jelínek Matůš Kocka Petr Kubánek Filip Münz Cyril Polášek Vojtěch Šimon Jan Štrobl 《Experimental Astronomy》2010,28(1):79-85
We report on two small aperture robotic telescopes called BART and D50 operated in Ondřejov. Both telescopes are capable of
automatic observation of gamma ray burst (GRB) optical afterglows. Coordinates of GRBs are taken from alerts distributed via
Internet. Telescopes observe other interesting high energy sources when there is not any alert. The smaller telescope BART
has aperture D = 254 mm. The bigger telescope D50 has a primary mirror of diameter D = 500 mm. Both telescopes are controlled
by free software package RTS2 and are accessible through Internet. We describe the two telescopes and related software and
show some results such as our first observed optical counterpart of GRB. 相似文献
994.
Jiang-Pei Dou De-Qing Ren Yong-Tian Zhu National Astronomical Observatories / Nanjing Institute of Astronomical Optics & Technology Chinese Academy of Sciences Nanjing China Key Laboratory of Astronomical Optics & Technology Nanjing Institute of Astronomical Optics & Technology China Physics & Astronomy Department California State University Northridge Nordhoff Street Northridge California – 《中国天文和天体物理学报》2010,10(2)
We propose a high-contrast coronagraph for direct imaging of young Jupiter-like planets orbiting nearby bright stars. The coronagraph employs a steptransmission filter in which the intensity is apodized with a finite number of steps with identical transmission in each step. It should be installed on a large ground-based telescope equipped with a state-of-the-art adaptive optics system. In this case, contrast ratios around 10-6 should be accessible within 0.1 arcsec of the central star. In recent progress, a... 相似文献
995.
Sarah K. NOBLE Lindsay P. KELLER Carlé M. PIETERS 《Meteoritics & planetary science》2010,45(12):2007-2015
Abstract– Space weathering products, such as agglutinates and nanophase iron‐bearing rims are easily preserved through lithification in lunar regolith breccias, thus such products, if produced, should be preserved in asteroidal regolith breccias as well. A study of representative regolith breccia meteorites, Fayetteville (H4) and Kapoeta (howardite), was undertaken to search for physical evidence of space weathering on asteroids. Amorphous or npFe0‐bearing rims cannot be positively identified in Fayetteville, although possible glass rims were found. Extensive friction melt was discovered in the meteorite that is difficult to differentiate from weathered materials. Several melt products, including spherules and agglutinates, as well as one irradiated rim and one possible npFe0‐bearing rim were identified in Kapoeta. The existence of these products suggests that lunar‐like space weathering processes are, or have been, active on asteroids. 相似文献
996.
Bengt Gustafsson Jorge Meléndez Martin Asplund David Yong 《Astrophysics and Space Science》2010,328(1-2):185-191
The question whether the solar chemical composition is typical for solar-type stars is analysed by comparing the Sun with different stellar samples, including a sample of stars with very similar parameters, solar twins. Although typical in terms of overall metallicity for stars of solar age and galactic orbit, the solar atmosphere is found to have abundances, as compared with solar twins, that indicate that its gas has once been affected by dust formation and dust separation. It is concluded that this may be related to the formation of the solar planetary system and its special properties. 相似文献
997.
M. M. Bisi A. R. Breen B. V. Jackson R. A. Fallows A. P. Walsh Z. Mikić P. Riley C. J. Owen A. Gonzalez-Esparza E. Aguilar-Rodriguez H. Morgan E. A. Jensen A. G. Wood M. J. Owens M. Tokumaru P. K. Manoharan I. V. Chashei A. S. Giunta J. A. Linker V. I. Shishov S. A. Tyul’bashev G. Agalya S. K. Glubokova M. S. Hamilton K. Fujiki P. P. Hick J. M. Clover B. Pintér 《Solar physics》2010,265(1-2):49-127
We report the results of a multi-instrument, multi-technique, coordinated study of the solar eruptive event of 13 May 2005. We discuss the resultant Earth-directed (halo) coronal mass ejection (CME), and the effects on the terrestrial space environment and upper Earth atmosphere. The interplanetary CME (ICME) impacted the Earth’s magnetosphere and caused the most-intense geomagnetic storm of 2005 with a Disturbed Storm Time (Dst) index reaching ?263 nT at its peak. The terrestrial environment responded to the storm on a global scale. We have combined observations and measurements from coronal and interplanetary remote-sensing instruments, interplanetary and near-Earth in-situ measurements, remote-sensing observations and in-situ measurements of the terrestrial magnetosphere and ionosphere, along with coronal and heliospheric modelling. These analyses are used to trace the origin, development, propagation, terrestrial impact, and subsequent consequences of this event to obtain the most comprehensive view of a geo-effective solar eruption to date. This particular event is also part of a NASA-sponsored Living With a Star (LWS) study and an on-going US NSF-sponsored Solar, Heliospheric, and INterplanetary Environment (SHINE) community investigation. 相似文献
998.
Experimental hydraulic heads and electrical (self-potential) signals associated with a pumping test were used in an inverse model to estimate the transmissivity distribution of a real aquifer. Several works reported in the literature show that there is a relatively good linear relationship between the hydraulic heads in the aquifer and electrical signals measured at the ground surface. In this experimental test field, first, the current coupling coefficient was determined by the best fit between experimental and modeled self-potential signals at the end of the pumping phase. Soon afterward, with the hydraulic heads obtained from the self-potential signals, the transmissivity distribution of the aquifer was conditioned by means an inverse model based on the successive linear estimator (SLE). To further substantiate the estimated T field from the SLE analysis, we analyzed the drawdown rate, the derivative of the drawdown with respect to the ln(t), because the drawdown rate is highly sensitive to the variability in the transmissivity field. In our opinion, these results show that self-potential signals allow the monitoring of subsurface flow in the course of pumping experiments, and that electrical potentials serve as a good complement to piezometric observations to condition and characterize the transmissivity distribution of an aquifer. 相似文献
999.
Occurrence and fate of polycyclic aromatic hydrocarbons in the coastal surface microlayer 总被引:2,自引:0,他引:2
Polycyclic aromatic hydrocarbons (PAHs) were measured in the coastal sea surface microlayer (SML), the sub-surface waters (SSW) and the overlying atmosphere in order to investigate the influence of the SML on contaminant enrichment and air-sea exchange. Samples were collected at two contrasting locations of the NW Mediterranean, one urban influenced (off Barcelona, Spain) and another comparatively clean (off Banyuls-sur-Mer, France). Statistical data analysis confirmed the accumulation of PAHs in the SML with respect to the SSW (20.3+/-9.1 vs. 13.1+/-10.0 ng L(-1) in the dissolved phase; 709+/-207 vs. 158+/-111 ng g(-1) in the particulate phase). This accumulation was higher at the contaminated location (Barcelona station) compared with the more pristine one, with PAH enrichments 1.5 and 4.5 times higher for the dissolved and particulate phases, respectively, indicating that the enrichment of PAHs in the SML is dominated by particle transport processes. 相似文献
1000.