首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   99999篇
  免费   1919篇
  国内免费   944篇
测绘学   2698篇
大气科学   7113篇
地球物理   20457篇
地质学   34971篇
海洋学   8638篇
天文学   22925篇
综合类   297篇
自然地理   5763篇
  2021年   921篇
  2020年   1040篇
  2019年   1130篇
  2018年   2562篇
  2017年   2390篇
  2016年   3018篇
  2015年   1752篇
  2014年   2890篇
  2013年   5207篇
  2012年   3065篇
  2011年   4159篇
  2010年   3582篇
  2009年   4818篇
  2008年   4354篇
  2007年   4226篇
  2006年   3964篇
  2005年   3144篇
  2004年   3084篇
  2003年   2867篇
  2002年   2753篇
  2001年   2522篇
  2000年   2397篇
  1999年   1971篇
  1998年   2067篇
  1997年   2005篇
  1996年   1607篇
  1995年   1609篇
  1994年   1393篇
  1993年   1287篇
  1992年   1181篇
  1991年   1138篇
  1990年   1202篇
  1989年   1100篇
  1988年   983篇
  1987年   1183篇
  1986年   1017篇
  1985年   1262篇
  1984年   1415篇
  1983年   1312篇
  1982年   1269篇
  1981年   1122篇
  1980年   1020篇
  1979年   965篇
  1978年   971篇
  1977年   855篇
  1976年   820篇
  1975年   798篇
  1974年   771篇
  1973年   782篇
  1972年   537篇
排序方式: 共有10000条查询结果,搜索用时 474 毫秒
51.
52.
53.
54.
Recent findings by the MER rover opportunity confirming the presence of iron minerals that can only be formed in the presence of water emphasize the study of analogous environments to Mars on Earth. The study of chemolithoautotrophic communities living in acidic iron-rich habitats is highly relevant in order to identify Mars analog environment-specific biomarkers. Iron oxidizing bacteria like Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans have ways of life for which it is feasible to identify a past or present hypothetical niche on Mars. We have developed a strategy for biomarker identification based on: (i) search for biosignatures on acid and metal-rich environments; (ii) development of an immunosensor microarray; and (iii) integration into an instrument for autonomous and remote operation. The instrument that we have built, called Signs Of LIfe Detector (SOLID), is capable of processing a variety of samples for the detection of specific biomarkers. Antibodies against several bacterial strains have been developed and tested in a microarray biosensor on SOLID. Tests with field samples have been successfully performed, allowing the detection of L. ferrooxidans, A. ferrooxidans present in sediment samples.  相似文献   
55.
We consider the particular solutions of the evolutionary system of equations in elements that correspond to planar and spatial circular orbits of the singly averaged Hill problem. We analyze the stability of planar and spatial circular orbits to inclination and eccentricity, respectively. We construct the instability regions of both particular solutions in the plane of parameters of the problem.  相似文献   
56.
The Hubble Space Telescope (HST), a large optical telescope having an aperture of 2.4 meters and a length of 8.8 meters, is being developed by the National Aeronautics and Space Administration. This telescope will be placed into earth orbit by the space shuttle. Astrometric observations with the HST are made using a Fine Guidance Sensor which is capable of measuring the position of one object relative to another with an accuracy of ±0.002 arcseconds. The astrometric user of HST will be provided with an Astrometric Data Reduction Software package (ADRS). The variety of astrometric problems to be investigated with HST is discussed.  相似文献   
57.
The chemical composition of several thermal springs around Changbaishan area has been investigated. Cenozoic basaltic rocks are widely distributed in Northeast China and geothermal characteristics have been described. About one hundred hot springs exist around Changhaishan Volcano at the border between China and D.P.R.Korea with high temperature about 82~C. The pH values of the spring water range from 6.9 to 7.1 and the total flow rate is about 4.8 L/sec. The chemical composition of the thermal springs is sodium carbonate; the high-mineral contents of thermal water are believed to have medicinal properties. Bathhouses are already built along the hot springs to take the advantage of the supposed healing properties. The high quality of those hot springs is believed to be utilitized for mineral water. The chemical equilibrium temperatures were estimated at about 160℃ based on the Na-K-Ca geochemical thermometer.  相似文献   
58.
59.
60.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号