全文获取类型
收费全文 | 8080篇 |
免费 | 689篇 |
国内免费 | 538篇 |
专业分类
测绘学 | 389篇 |
大气科学 | 764篇 |
地球物理 | 1926篇 |
地质学 | 3248篇 |
海洋学 | 754篇 |
天文学 | 1231篇 |
综合类 | 161篇 |
自然地理 | 834篇 |
出版年
2023年 | 65篇 |
2022年 | 131篇 |
2021年 | 209篇 |
2020年 | 199篇 |
2019年 | 258篇 |
2018年 | 302篇 |
2017年 | 284篇 |
2016年 | 357篇 |
2015年 | 306篇 |
2014年 | 327篇 |
2013年 | 543篇 |
2012年 | 368篇 |
2011年 | 471篇 |
2010年 | 393篇 |
2009年 | 488篇 |
2008年 | 431篇 |
2007年 | 374篇 |
2006年 | 326篇 |
2005年 | 317篇 |
2004年 | 295篇 |
2003年 | 257篇 |
2002年 | 274篇 |
2001年 | 160篇 |
2000年 | 189篇 |
1999年 | 135篇 |
1998年 | 142篇 |
1997年 | 115篇 |
1996年 | 103篇 |
1995年 | 100篇 |
1994年 | 97篇 |
1993年 | 78篇 |
1992年 | 97篇 |
1991年 | 69篇 |
1990年 | 61篇 |
1989年 | 59篇 |
1988年 | 59篇 |
1987年 | 56篇 |
1986年 | 61篇 |
1985年 | 70篇 |
1984年 | 65篇 |
1983年 | 81篇 |
1982年 | 59篇 |
1981年 | 68篇 |
1980年 | 57篇 |
1979年 | 72篇 |
1978年 | 60篇 |
1977年 | 36篇 |
1976年 | 30篇 |
1974年 | 30篇 |
1973年 | 34篇 |
排序方式: 共有9307条查询结果,搜索用时 15 毫秒
21.
The “Shiva Hypothesis”, in which recurrent, cyclical mass extinctions of life on Earth result from impacts of comets or asteroids, provides a possible unification of important processes in astrophysics, planetary geology, and the history of life. Collisions with Earth-crossing asteroids and comets ≥ a few km in diameter are calculated to produce widespread environmental disasters (dust clouds, wildfires), and occur with the proper frequency to account for the record of five major mass extinctions (from ≥ 108 Mt TNT impacts) and ~ 20 minor mass extinctions (from 107–108 Mt impacts) recorded in the past 540 million years. Recent studies of a number of extinctions show evidence of severe environmental disturbances and mass mortality consistent with the expected after-effects (dust clouds, wildfires) of catastrophic impacts. At least six cases of features generally considered diagnostic of large impacts (e.g., large impact craters, layers with high platinum-group elements, shock-related minerals, and/or microtektites) are known at or close to extinction-event boundaries. Six additional cases of elevated iridium levels at or near extinction boundaries are of the amplitude that might be expected from collision of relatively low-Ir objects such as comets. The records of cratering and mass extinction show a correlation, and might be explained by a combination of periodic and stochastic impactors. The mass extinction record shows evidence for a periodic component of about 26 to 30 Myr, and an ~ 30 Myr periodic component has been detected in impact craters by some workers, with recent pulses of impacts in the last 2–3 million years, and at ~ 35, 65, and 95 million years ago. A cyclical astronomical pacemaker for such pulses of impacts may involve the motions of the Earth through the Milky Way Galaxy. As the Solar System revolves around the galactic center, it also oscillates up and down through the plane of the disk-shaped galaxy with a half-cycle ~ 30±3 Myr. This cycle should lead to quasi-periodic encounters with interstellar clouds, and periodic variations in the galactic tidal force with maxima at times of plane crossing. This “galactic carrousel” effect may provide a viable perturber of the Oort Cloud comets, producing periodic showers of comets in the inner Solar System. These impact pulses, along with stochastic impactors, may represent the major punctuations in earth history. 相似文献
22.
We present the results of a three-year observational program of long-slit spectroscopy and UBVRI photometry of Triton. We find evidence for short-lived albedo variations at wavelengths less than 0.5 μm with the most notable reddening event measured in October 1997 and “normal” colors returning by May 1998. We report a possible variation in the relative 0.73/0.89 μm methane absorption band strengths, suggesting that the transport or denudation of this species may play a major role in the reddening events. 相似文献
23.
Eight-year-old Loblolly pine (Pinus taeda) plots located in South Carolina were evaluated using high resolution aircraft multispectral scanner data to determine if biomass could be accurately measured. Sixteen pine plots located on both sandy and clay soils were treated with 0, 180, or 360 kg. (approximately 0, 400, or 800 lb.) of nitrogen per plot. Indices of biomass from remote sensing data were significantly correlated with in situ biomass measurements made in each plot. A ratio of infrared (.9–1.1 μm) and red (.65–.70 μm) channels yielded the best correlation. The indices were not sensitive to differences in soil type (sandy or clay). 相似文献
24.
25.
26.
Antony J. Long Sarah A. Woodroffe Sue Dawson David H. Roberts Charlotte L. Bryant 《第四纪科学杂志》2009,24(4):345-359
In West Greenland, early and mid Holocene relative sea level (RSL) fall was replaced by late Holocene RSL rise during the Neoglacial, after 4–3 cal. ka BP (thousand calibrated years before present). Here we present the results of an isolation basin RSL study completed near to the coastal town of Sisimiut, in central West Greenland. RSL fell from 14 m above sea level at 5.7 cal. ka BP to reach a lowstand of ?4.0 m at 2.3–1.2 cal. ka BP, before rising by an equivalent amount to present. Differences in the timing and magnitude of the RSL lowstand between this and other sites in West and South Greenland record the varied interplay of local and non‐Greenland RSL processes, notably the reloading of the Earth's crust caused by a Neoglacial expansion of the Greenland Ice Sheet (GIS) and the subsidence associated with the collapse of the Laurentide Ice Sheet forebulge. This means that the timing of the sea level lowstand cannot be used to infer directly when the GIS advanced during the Neoglacial. The rise in Late Holocene RSL is contrary to recently reported bedrock uplift in the Sisimiut area, based on repeat GPS surveys. This indicates that a belt of peripheral subsidence around the current ice sheet margin was more extensive in the late Holocene, and that there has been a switch from subsidence to uplift at some point in the last thousand years or so. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
27.
Guinevere Kauffmann Timothy M. Heckman Simon D. M. White Stéphane Charlot Christy Tremonti Jarle Brinchmann Gustavo Bruzual Eric W. Peng Mark Seibert Mariangela Bernardi Michael Blanton Jon Brinkmann Francisco Castander Istvan Csábai Masataka Fukugita Zeljko Ivezic Jeffrey A. Munn Robert C. Nichol Nikhil Padmanabhan Aniruddha R. Thakar David H. Weinberg Donald York 《Monthly notices of the Royal Astronomical Society》2003,341(1):33-53
28.
29.
Andrei V. Ivanov Nataliya N. Kononkova S. Vincent Yang Michael E. Zolensky 《Meteoritics & planetary science》2003,38(5):725-737
Abstract— Clasts of alkaline (the second find in meteorites) and subalkaline rocks were found in the Kaidun meteorite. One of them (#d4A) is a large crystal of albite with inclusions of fluorapatite, arfvedsonite, aenigmatite, and wilkinsonite. The two latter minerals were previously unknown in meteorites. Another clast (#d[3–5]D) has a melt crystallization texture of mainly feldspar (oligoclase) composition and contains relict grains of both high‐Ca and low‐Ca pyroxene and fluorapatite. The mineralogical characteristics of these clasts suggest a genetic relationship and an origin from the same parent body. The textural and mineralogical characteristics of the clasts indicate origin by extensive igneous differentiation. Such processes most likely took place in a rather large differentiated body. The material of clast #d(3–5)D is similar in some mineralogical respects to basaltic shergottites. 相似文献
30.