全文获取类型
收费全文 | 389篇 |
免费 | 0篇 |
专业分类
系统科学 | 6篇 |
教育与普及 | 2篇 |
现状及发展 | 91篇 |
研究方法 | 50篇 |
综合类 | 229篇 |
自然研究 | 11篇 |
出版年
2020年 | 2篇 |
2018年 | 2篇 |
2017年 | 2篇 |
2016年 | 3篇 |
2014年 | 3篇 |
2013年 | 7篇 |
2012年 | 14篇 |
2011年 | 23篇 |
2010年 | 7篇 |
2009年 | 2篇 |
2008年 | 16篇 |
2007年 | 17篇 |
2006年 | 18篇 |
2005年 | 14篇 |
2004年 | 16篇 |
2003年 | 12篇 |
2002年 | 17篇 |
2001年 | 17篇 |
2000年 | 7篇 |
1999年 | 8篇 |
1998年 | 2篇 |
1996年 | 2篇 |
1992年 | 7篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 11篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 9篇 |
1978年 | 8篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1974年 | 8篇 |
1972年 | 2篇 |
1971年 | 6篇 |
1970年 | 11篇 |
1969年 | 8篇 |
1968年 | 14篇 |
1967年 | 8篇 |
1966年 | 4篇 |
1965年 | 9篇 |
1960年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有389条查询结果,搜索用时 265 毫秒
151.
152.
Mustard JF Murchie SL Pelkey SM Ehlmann BL Milliken RE Grant JA Bibring JP Poulet F Bishop J Dobrea EN Roach L Seelos F Arvidson RE Wiseman S Green R Hash C Humm D Malaret E McGovern JA Seelos K Clancy T Clark R Marais DD Izenberg N Knudson A Langevin Y Martin T McGuire P Morris R Robinson M Roush T Smith M Swayze G Taylor H Titus T Wolff M 《Nature》2008,454(7202):305-309
Phyllosilicates, a class of hydrous mineral first definitively identified on Mars by the OMEGA (Observatoire pour la Mineralogie, L'Eau, les Glaces et l'Activitié) instrument, preserve a record of the interaction of water with rocks on Mars. Global mapping showed that phyllosilicates are widespread but are apparently restricted to ancient terrains and a relatively narrow range of mineralogy (Fe/Mg and Al smectite clays). This was interpreted to indicate that phyllosilicate formation occurred during the Noachian (the earliest geological era of Mars), and that the conditions necessary for phyllosilicate formation (moderate to high pH and high water activity) were specific to surface environments during the earliest era of Mars's history. Here we report results from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) of phyllosilicate-rich regions. We expand the diversity of phyllosilicate mineralogy with the identification of kaolinite, chlorite and illite or muscovite, and a new class of hydrated silicate (hydrated silica). We observe diverse Fe/Mg-OH phyllosilicates and find that smectites such as nontronite and saponite are the most common, but chlorites are also present in some locations. Stratigraphic relationships in the Nili Fossae region show olivine-rich materials overlying phyllosilicate-bearing units, indicating the cessation of aqueous alteration before emplacement of the olivine-bearing unit. Hundreds of detections of Fe/Mg phyllosilicate in rims, ejecta and central peaks of craters in the southern highland Noachian cratered terrain indicate excavation of altered crust from depth. We also find phyllosilicate in sedimentary deposits clearly laid by water. These results point to a rich diversity of Noachian environments conducive to habitability. 相似文献
153.
Hocking WK Carey-Smith T Tarasick DW Argall PS Strong K Rochon Y Zawadzki I Taylor PA 《Nature》2007,450(7167):281-284
Stratospheric ozone attenuates harmful ultraviolet radiation and protects the Earth's biosphere. Ozone is also of fundamental importance for the chemistry of the lowermost part of the atmosphere, the troposphere. At ground level, ozone is an important by-product of anthropogenic pollution, damaging forests and crops, and negatively affecting human health. Ozone is critical to the chemical and thermal balance of the troposphere because, via the formation of hydroxyl radicals, it controls the capacity of tropospheric air to oxidize and remove other pollutants. Moreover, ozone is an important greenhouse gas, particularly in the upper troposphere. Although photochemistry in the lower troposphere is the major source of tropospheric ozone, the stratosphere-troposphere transport of ozone is important to the overall climatology, budget and long-term trends of tropospheric ozone. Stratospheric intrusion events, however, are still poorly understood. Here we introduce the use of modern windprofiler radars to assist in such transport investigations. By hourly monitoring the radar-derived tropopause height in combination with a series of frequent ozonesonde balloon launches, we find numerous intrusions of ozone from the stratosphere into the troposphere in southeastern Canada. On some occasions, ozone is dispersed at altitudes of two to four kilometres, but on other occasions it reaches the ground, where it can dominate the ozone density variability. We observe rapid changes in radar tropopause height immediately preceding these intrusion events. Such changes therefore serve as a valuable diagnostic for the occurrence of ozone intrusion events. Our studies emphasize the impact that stratospheric ozone can have on tropospheric ozone, and show that windprofiler data can be used to infer the possibility of ozone intrusions, as well as better represent tropopause motions in association with stratosphere-troposphere transport. 相似文献
154.
155.
Venus as a more Earth-like planet 总被引:1,自引:0,他引:1
Venus is Earth's near twin in mass and radius, and our nearest planetary neighbour, yet conditions there are very different in many respects. Its atmosphere, mostly composed of carbon dioxide, has a surface temperature and pressure far higher than those of Earth. Only traces of water are found, although it is likely that there was much more present in the past, possibly forming Earth-like oceans. Here we discuss how the first year of observations by Venus Express brings into focus the evolutionary paths by which the climates of two similar planets diverged from common beginnings to such extremes. These include a CO2-driven greenhouse effect, erosion of the atmosphere by solar particles and radiation, surface-atmosphere interactions, and atmospheric circulation regimes defined by differing planetary rotation rates. 相似文献
156.
Massey R Rhodes J Ellis R Scoville N Leauthaud A Finoguenov A Capak P Bacon D Aussel H Kneib JP Koekemoer A McCracken H Mobasher B Pires S Refregier A Sasaki S Starck JL Taniguchi Y Taylor A Taylor J 《Nature》2007,445(7125):286-290
Ordinary baryonic particles (such as protons and neutrons) account for only one-sixth of the total matter in the Universe. The remainder is a mysterious 'dark matter' component, which does not interact via electromagnetism and thus neither emits nor reflects light. As dark matter cannot be seen directly using traditional observations, very little is currently known about its properties. It does interact via gravity, and is most effectively probed through gravitational lensing: the deflection of light from distant galaxies by the gravitational attraction of foreground mass concentrations. This is a purely geometrical effect that is free of astrophysical assumptions and sensitive to all matter--whether baryonic or dark. Here we show high-fidelity maps of the large-scale distribution of dark matter, resolved in both angle and depth. We find a loose network of filaments, growing over time, which intersect in massive structures at the locations of clusters of galaxies. Our results are consistent with predictions of gravitationally induced structure formation, in which the initial, smooth distribution of dark matter collapses into filaments then into clusters, forming a gravitational scaffold into which gas can accumulate, and stars can be built. 相似文献
157.
The aim of this paper is to demonstrate how project-based learning can be used from a critical systems perspective in data warehousing education. Data warehousing is a discipline in information technology focusing on providing data-driven decision support systems for strategic decision making. In this study we used action research from a critical (emancipative) perspective to reflect on our current instructional design of the data warehousing module before redesigning it to better serve the needs of the involved and affected. We used critical systems heuristics and project-based learning as frameworks of understanding to guide our intervention. Project-based learning is a learning/teaching approach aimed at organising the learning experience in terms of a project. We used written interpretive interviews in the diagnosis and evaluation of success phases of our action research cycle. Our reflection is according to the action research model of Checkland reflecting on our success in the area of application (data warehousing instruction) as well as our methodology (action research from a critical social theory perspective) and our framework of ideas (project-based learning and critical social heuristics). 相似文献
158.
TJ Pugh SD Weeraratne TC Archer DA Pomeranz Krummel D Auclair J Bochicchio MO Carneiro SL Carter K Cibulskis RL Erlich H Greulich MS Lawrence NJ Lennon A McKenna J Meldrim AH Ramos MG Ross C Russ E Shefler A Sivachenko B Sogoloff P Stojanov P Tamayo JP Mesirov V Amani N Teider S Sengupta JP Francois PA Northcott MD Taylor F Yu GR Crabtree AG Kautzman SB Gabriel G Getz N Jäger DT Jones P Lichter SM Pfister TM Roberts M Meyerson SL Pomeroy YJ Cho 《Nature》2012,488(7409):106-110
159.
Traditional models of the supercontinent cycle predict that the next supercontinent--'Amasia'--will form either where Pangaea rifted (the 'introversion' model) or on the opposite side of the world (the 'extroversion' models). Here, by contrast, we develop an 'orthoversion' model whereby a succeeding supercontinent forms 90° away, within the great circle of subduction encircling its relict predecessor. A supercontinent aggregates over a mantle downwelling but then influences global-scale mantle convection to create an upwelling under the landmass. We calculate the minimum moment of inertia about which oscillatory true polar wander occurs owing to the prolate shape of the non-hydrostatic Earth. By fitting great circles to each supercontinent's true polar wander legacy, we determine that the arc distances between successive supercontinent centres (the axes of the respective minimum moments of inertia) are 88° for Nuna to Rodinia and 87° for Rodinia to Pangaea--as predicted by the orthoversion model. Supercontinent centres can be located back into Precambrian time, providing fixed points for the calculation of absolute palaeolongitude over billion-year timescales. Palaeogeographic reconstructions additionally constrained in palaeolongitude will provide increasingly accurate estimates of ancient plate motions and palaeobiogeographic affinities. 相似文献