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   264201篇
  免费   5250篇
  国内免费   3442篇
测绘学   7211篇
大气科学   19606篇
地球物理   55865篇
地质学   92379篇
海洋学   22098篇
天文学   56361篇
综合类   1048篇
自然地理   18325篇
  2021年   2229篇
  2020年   2654篇
  2019年   2888篇
  2018年   4321篇
  2017年   3910篇
  2016年   6185篇
  2015年   4305篇
  2014年   7017篇
  2013年   14336篇
  2012年   6847篇
  2011年   8559篇
  2010年   7439篇
  2009年   10109篇
  2008年   8975篇
  2007年   8476篇
  2006年   9833篇
  2005年   7964篇
  2004年   7896篇
  2003年   7384篇
  2002年   6938篇
  2001年   6145篇
  2000年   6039篇
  1999年   5229篇
  1998年   5261篇
  1997年   5079篇
  1996年   4716篇
  1995年   4460篇
  1994年   4144篇
  1993年   3892篇
  1992年   3658篇
  1991年   3607篇
  1990年   3781篇
  1989年   3534篇
  1988年   3317篇
  1987年   3799篇
  1986年   3277篇
  1985年   4163篇
  1984年   4657篇
  1983年   4332篇
  1982年   4254篇
  1981年   3867篇
  1980年   3611篇
  1979年   3442篇
  1978年   3444篇
  1977年   3234篇
  1976年   2985篇
  1975年   2910篇
  1974年   2877篇
  1973年   3071篇
  1972年   1997篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
142.
143.
144.
145.
Abstract— Olivine grains from selected meteorites (the Springwater pallasite, the Lowicz mesosiderite, the ALH 84025 brachinite, the Krymka LL3 chondrite, and the Calcalong Creek lunar meteorite) and terrestrial rocks (San Carlos forsterite and Rockport fayalite) were studied by optical microscopy and high-precision electron microprobe analysis. Detailed microprobe traverses revealed regular igneous zoning in the Krymka and Calcalong Creek olivines. Traverses across the San Carlos forsterite grain are flat and display no chemical variations larger than the 2σ range of counting error (±0.2 mol% Fa). Traverses across olivine grains in the ALH 84025, Lowicz, and Springwater meteorites show regular patterns of periodic or wavy chemical variations well exceeding the 2σ uncertainty range. However, no lamellar structure was seen in backscattered electron images. It is suggested that the periodic chemical variations may be due to spinodal decomposition of primary, more or less homogeneous grains. I conclude that the absence of earlier reports of such variations simply means that olivine grains in equilibrated meteorites have not been examined closely enough to detect them.  相似文献   
146.
Long-term changes in growth of Norwegian spring-spawning herring   总被引:3,自引:1,他引:2  
  相似文献   
147.
The growth of Glasgow from a small town into a heavily industrialised conurbation depended greatly on its local geological riches. Extensive mining and quarrying of a range of minerals took place from the 18th century onwards. The early underground stoop and room (pillar and stall) workings, and the backfilled quarries, together with variably consolidated natural superficial deposits, have bequeathed to the city a heritage of unwanted engineering problems which cannot easily be quantified. Recent work by the British Geological Survey (BGS) illustrates the scale of both geological and man-made problems.  相似文献   
148.
We discuss long-time changes of polar activity of the Sun using the new observational data sets in the optical range during 1872–2001. A study of the secular and cycle variations of the magnetic activity at the high-latitude regions is the main goal that includes polar magnetic field reversals during 1872–2001 and secular changes of the duration of polar activity cycles. The secular increase of the area of polar zones during the minimum activity in the last 120 years and as consequence a decrease of coronal temperature of the Sun in the high-latitude zones during the last 50 years. Correlation between the polar cycles of Caii-K bright points with the Wolf sunspot numbers cycles, W(t), and the 22-year polar magnetic cycles of Caii-K bright points at the high latitudes during 1905–1995 is discussed.  相似文献   
149.
150.
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   
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