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   111921篇
  免费   1829篇
  国内免费   1672篇
测绘学   3381篇
大气科学   8133篇
地球物理   22047篇
地质学   43211篇
海洋学   8827篇
天文学   21097篇
综合类   2373篇
自然地理   6353篇
  2022年   425篇
  2021年   790篇
  2020年   894篇
  2019年   958篇
  2018年   7675篇
  2017年   6933篇
  2016年   5483篇
  2015年   1731篇
  2014年   2457篇
  2013年   4198篇
  2012年   3943篇
  2011年   7248篇
  2010年   6180篇
  2009年   7356篇
  2008年   6306篇
  2007年   6946篇
  2006年   3263篇
  2005年   3135篇
  2004年   3106篇
  2003年   2979篇
  2002年   2508篇
  2001年   2027篇
  2000年   1988篇
  1999年   1538篇
  1998年   1626篇
  1997年   1506篇
  1996年   1223篇
  1995年   1262篇
  1994年   1080篇
  1993年   963篇
  1992年   935篇
  1991年   820篇
  1990年   939篇
  1989年   790篇
  1988年   726篇
  1987年   902篇
  1986年   738篇
  1985年   960篇
  1984年   1041篇
  1983年   980篇
  1982年   920篇
  1981年   793篇
  1980年   731篇
  1979年   675篇
  1978年   661篇
  1977年   611篇
  1976年   605篇
  1975年   562篇
  1974年   560篇
  1973年   530篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Dry valleys are a striking geographic landscape in Hengduan Mountains Region and are characterized by low rainfall, desert type of vegetation and fragile environment. Past efforts and resources have been concentrated mainly on rehabilitation of degraded ecosystem and fragile environment, particularly reforestation, while socio-economic development has been largely overlooked. Despite successes in pocket areas, the overall trend of unsustainability and environmental deterioration are continuing. It is important to understand that uplift of the Tibetan Plateau is the root cause of development of dry valleys, and development and formation of dry valleys is a natural process. Human intervention has played a secondary role in development of dry valleys and degradation of dry valleys though human intervention in many cases has speeded up environmental degradation of the dry valleys. It is important to understand that dry valleys are climatic enclaves and an integrated approach that combines rehabilitation of degraded ecosystems and socio-economic development should be adopted if the overall goal of sustainable development of dry valleys is to be achieved. Promotion of niche-based cash crops, rural energy including hydropower, solar energy, biogas and fuelwood plantation is recommended as the priority activities.  相似文献   
42.
43.
44.
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   
45.
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   
46.
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   
47.
48.
The development of coal forests during the Carboniferous is one of the best-known episodes in the history of life. Although often reconstructed as steamy tropical rainforests, these ancient ecosystems were a far cry from anything we might encounter in the Amazon today. Bizarre giant club-mosses, horsetails and tree ferns were the dominant plants, not flowering trees as in modern rainforests. At their height, coal forests stretched all the way from Kansas to Kazakhstan, spanning the entire breadth of tropical Pangaea. Most of what we know of their biodiversity and ecology has been quite literally mined out of the ground through two centuries of hard labour. Without coal mining, our knowledge would be greatly impoverished. Over the past few years, we've been exploring underground coal mines in the United States, where entire forested landscapes have been preserved intact over huge areas. Never before have geologists had the opportunity to walk out through mile upon mile of fossilized forest. In this feature article, we describe some of our recent explorations and attempt to shed new light on these old fossils.  相似文献   
49.
50.
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