全文获取类型
收费全文 | 775篇 |
免费 | 27篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 32篇 |
大气科学 | 46篇 |
地球物理 | 168篇 |
地质学 | 242篇 |
海洋学 | 82篇 |
天文学 | 158篇 |
综合类 | 3篇 |
自然地理 | 78篇 |
出版年
2021年 | 8篇 |
2020年 | 14篇 |
2019年 | 18篇 |
2018年 | 16篇 |
2017年 | 15篇 |
2016年 | 21篇 |
2015年 | 14篇 |
2014年 | 23篇 |
2013年 | 44篇 |
2012年 | 14篇 |
2011年 | 42篇 |
2010年 | 16篇 |
2009年 | 36篇 |
2008年 | 25篇 |
2007年 | 32篇 |
2006年 | 33篇 |
2005年 | 31篇 |
2004年 | 24篇 |
2003年 | 10篇 |
2002年 | 18篇 |
2001年 | 27篇 |
2000年 | 21篇 |
1999年 | 23篇 |
1998年 | 9篇 |
1997年 | 11篇 |
1996年 | 8篇 |
1995年 | 9篇 |
1994年 | 9篇 |
1993年 | 10篇 |
1992年 | 13篇 |
1991年 | 17篇 |
1990年 | 8篇 |
1989年 | 12篇 |
1988年 | 7篇 |
1987年 | 10篇 |
1985年 | 13篇 |
1984年 | 13篇 |
1983年 | 11篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 9篇 |
1978年 | 10篇 |
1977年 | 8篇 |
1976年 | 10篇 |
1975年 | 15篇 |
1974年 | 9篇 |
1973年 | 8篇 |
1971年 | 7篇 |
1970年 | 7篇 |
1969年 | 4篇 |
排序方式: 共有809条查询结果,搜索用时 171 毫秒
101.
The variation in azimuth of the Carlsberg Automatic Meridian Circle has been investigated using azimuth marks, pairs of circumpolar stars and three polarissimae. Preliminary results from these three methods are intercompared and some conclusions drawn about their reliability and future use. 相似文献
102.
103.
Cyrena Anne Goodrich Frank Wlotzka D. Kent Ross Rainer Bartoschewitz 《Meteoritics & planetary science》2006,41(6):925-952
Abstract— Northwest Africa (NWA) 1500 is an ultramafic meteorite dominated by coarse (?100–500 μm) olivine (95–96%), augite (2–3%), and chromite (0.6–1.6%) in an equilibrated texture. Plagioclase (0.7–1.8%) occurs as poikilitic grains (up to ?3 mm) in vein‐like areas that have concentrations of augite and minor orthopyroxene. Other phases are Cl‐apatite, metal, sulfide, and graphite. Olivine ranges from Fo 65–73, with a strong peak at Fo 68–69. Most grains are reversezoned, and also have ?10–30 μm reduction rims. In terms of its dominant mineralogy and texture, NWA 1500 resembles the majority of monomict ureilites. However, it is more ferroan than known ureilites (Fo ≥75) and other mineral compositional parameters are out of the ureilite range as well. Furthermore, neither apatite nor plagioclase have ever been observed, and chromite is rare in monomict ureilites. Nevertheless, this meteorite may be petrologically related to the rare augite‐bearing ureilites and represent a previously unsampled part of the ureilite parent body (UPB). The Mn/Mg ratio of its olivine and textural features of its pyroxenes are consistent with this interpretation. However, its petrogenesis differs from that of known augite‐bearing ureilites in that: 1) it formed under more oxidized conditions; 2) plagioclase appeared before orthopyroxene in its crystallization sequence; and 3) it equilibrated to significantly lower temperatures (800–1000 °C, from two‐pyroxene and olivine‐chromite thermometry). Formation under more oxidized conditions and the appearance of plagioclase before orthopyroxene could be explained if it formed at a greater depth on the UPB than previously sampled. However, its significantly different thermal history (compared to ureilites) may more plausibly be explained if it formed on a different parent body. This conclusion is consistent with its oxygen isotopic composition, which suggests that it is an ungrouped achondrite. Nevertheless, the parent body of NWA 1500 may have been compositionally and petrologically similar to the UPB, and may have had a similar differentiation history. 相似文献
104.
Distinct competent layers are observed in the slopes of eastern Coprates Chasma, part of the Valles Marineris system on Mars. Our observations indicate that the stratigraphy of Coprates Chasma consists of alternating thin strong layers and thicker sequences of relatively weak layers. The strong, competent layers maintain steeper slopes and play a major role in controlling the overall shape and geomorphology of the chasmata slopes. The topmost competent layer in this area is well preserved and easy to identify in outcrops on the northern rim of Coprates Chasma less than 100 m below the southern Ophir Planum surface. The volume of the topmost emplaced layer is at least 70 km3 and may be greater than 2100 km3 if the unit underlies most of Ophir Planum. The broad extent of this layer allows us to measure elevation offsets within the north rim of the chasma and in a freestanding massif within Coprates Chasma where the layer is also observed. Rim outcrop morphology and elevation differences between Ophir and Aurorae Plana may be indicative of the easternmost extent of the topmost competent layer. These observations allow an insight into the depositional processes that formed the stratigraphic stack into which this portion of the Valles Marineris is carved, and they present a picture of some of the last volcanic activity in this area. Furthermore, the elevation offsets within the layer are evidence of significant subsidence of the massif and surrounding material. 相似文献
105.
106.
A.J. Young † A.C. Fabian R.R. Ross Y. Tanaka 《Monthly notices of the Royal Astronomical Society》2001,325(3):1045-1052
The galactic black hole candidate Cygnus X-1 is observed to be in one of two X-ray spectral states: either the low/hard (low soft X-ray flux and a flat power-law tail) or high/soft (high blackbody-like soft X-ray flux and a steep power-law tail) state. The physical origin of these two states is unclear. We present here a model of an ionized accretion disc, the spectrum of which is blurred by relativistic effects, and fit it to the ASCA , Ginga and EXOSAT data of Cygnus X-1 in both spectral states. We confirm that relativistic blurring provides a much better fit to the low/hard state data and, contrary to some previous results, find the data of both states to be consistent with an ionized thin accretion disc with a reflected fraction of unity extending to the innermost stable circular orbit around the black hole. Our model is an alternative to those that, in the low/hard state, require the accretion disc to be truncated at a few tens of Schwarzschild radii, within which there is a Thomson-thin, hot accretion flow. We suggest a mechanism that may cause the changes in spectral state. 相似文献
107.
108.
Alexander Deutsch Christian Koeberl Joel D. Blum Bevan M. French Billy P. Glass Richard Grieve Peter Horn Elmar K. Jessberger Gero Kurat Wolf Uwe Reimold Jan Smit Dieter Stöffler Stuart Ross Taylor 《地学学报》1994,6(6):644-650
In a recent article in Terra Nova, Kristan-Tollmann and Tollmann (1994) suggested that the Biblical Flood can be explained by seven fragments of a comet that impacted the ocean at seven locations on Earth at 03.00h (C.E.T.) on 23 September, 9545 yr BP. We demonstrate that all the ‘geological proofs’ that allegedly support their conclusions are not supported by the available data on impact cratering. Their hypothesis is based on insufficient and ambiguous data, selective citation, and incomplete comprehension of previous research. 相似文献
109.
110.