全文获取类型
收费全文 | 1295篇 |
免费 | 43篇 |
国内免费 | 14篇 |
专业分类
测绘学 | 29篇 |
大气科学 | 95篇 |
地球物理 | 323篇 |
地质学 | 569篇 |
海洋学 | 106篇 |
天文学 | 133篇 |
综合类 | 5篇 |
自然地理 | 92篇 |
出版年
2021年 | 9篇 |
2020年 | 8篇 |
2019年 | 8篇 |
2018年 | 26篇 |
2017年 | 28篇 |
2016年 | 45篇 |
2015年 | 29篇 |
2014年 | 33篇 |
2013年 | 76篇 |
2012年 | 47篇 |
2011年 | 70篇 |
2010年 | 74篇 |
2009年 | 77篇 |
2008年 | 60篇 |
2007年 | 62篇 |
2006年 | 56篇 |
2005年 | 81篇 |
2004年 | 60篇 |
2003年 | 42篇 |
2002年 | 43篇 |
2001年 | 35篇 |
2000年 | 21篇 |
1999年 | 28篇 |
1998年 | 22篇 |
1997年 | 10篇 |
1996年 | 20篇 |
1995年 | 15篇 |
1994年 | 16篇 |
1993年 | 9篇 |
1992年 | 18篇 |
1991年 | 11篇 |
1990年 | 15篇 |
1989年 | 9篇 |
1988年 | 10篇 |
1987年 | 10篇 |
1986年 | 16篇 |
1985年 | 16篇 |
1984年 | 15篇 |
1983年 | 10篇 |
1982年 | 10篇 |
1981年 | 15篇 |
1980年 | 8篇 |
1979年 | 11篇 |
1978年 | 8篇 |
1977年 | 10篇 |
1976年 | 11篇 |
1975年 | 4篇 |
1974年 | 13篇 |
1973年 | 4篇 |
1953年 | 2篇 |
排序方式: 共有1352条查询结果,搜索用时 15 毫秒
861.
Richard A. F. Grieve Richard B. Bottomley Michel A. Bouchard P. Blyth Robertson Charles J. Orth Moses Attrep 《Meteoritics & planetary science》1991,26(1):31-39
Abstract— Approximately 1500 g of float samples of impact melt rocks have been recovered from gravel deposits ~4 km north and northeast of the rim of the 3.4 km diameter New Quebec Crater (61°17′N; 73°40′W) in northern Quebec, Canada. Previously, only two small samples of impact melt rocks were known. The newly recovered samples have cryptocrystalline to microcrystalline matrices with microlites of andesine and pigeonite. Mineral clasts of quartz and feldspar occur and, in some cases, show shock metamorphic features. The melt rocks have a normative mineralogy corresponding to ~70% quartz, orthoclase and albite and are compositionally similar. Their major element composition can be modeled as a mix of granitic gneisses that make up the target rocks. The melt rocks show enrichments, however, in Cr (21 ppm), Co (9 ppm), Ni (12 ppm) and Ir (1.5 ppb) over the target rocks. Interelement ratios suggest a chondritic impacting body, although they do not define a specific type. Assuming a C-1 chondrite, the impact melt rocks average ~2% meteoritic contamination. Stepwise 40Ar-39Ar dating using a laser on three chips from three samples give integrated ages of 0.6–2.5 Ma. From the best plateau ages, the age of the New Quebec impact is taken to be 1.4 ± 0.1 Ma, which places it before the first major northern hemisphere continental glaciation of the Pleistocene. A number of considerations suggest that the impact melt rocks were originally deposited in fractures in the crater wall and later transported to their discovery site by glacial ice and melt water. 相似文献
862.
We discuss the main mechanisms affecting the dynamical evolution of Near-Earth Asteroids (NEAs) by analyzing the results of three numerical integrations over 1 Myr of the NEA (4179) Toutatis. In the first integration the only perturbing planet is the Earth. So the evolution is dominated by close encounters and looks like a random walk in semimajor axis and a correlated random walk in eccentricity, keeping almost constant the perihelion distance and the Tisserand invariant. In the second integration Jupiter and Saturn are present instead of the Earth, and the 3/1 (mean motion) and v 6 (secular) resonances substantially change the eccentricity but not the semimajor axis. The third, most realistic, integration including all the three planets together shows a complex interplay of effects, with close encounters switching the orbit between different resonant states and no approximate conservation of the Tisserand invariant. This shows that simplified 3-body or 4-body models cannot be used to predict the typical evolution patterns and time scales of NEAs, and in particular that resonances provide some “fast-track” dynamical routes from low-eccentricity to very eccentric, planet-crossing orbits. 相似文献
863.
Abstract— We present a purely physical model for the calculation of depth‐dependent production rates in 2π exposure geometries by galactic cosmic rays (GCR). Besides the spectra of primary and secondary particles and the excitation functions of the underlying nuclear reactions, the model is based on the integral number of GCR particles in the lunar orbit. We derived this value from adjusting modeled depth profiles for 10Be, 26Al, and 53Mn to measured data from the Apollo 15 drill core. The J0,GCR value of 4.54 cm?2 s?1 and the solar modulation parameter of M = 490 MeV determined this way for 1 AU is in reasonable agreement with the J0,GCR value derived recently for the meteoroid orbits (Leya et al., 2000b). We also show that the mean GCR proton spectrum in the lunar orbit has not changed substantially over about the last 10 Ma. For the major target elements we present depth‐dependent production rates for 10Be, 14C, 26Al, 36Cl and 53Mn, as well as for the rare gas isotopes 20,21,22Ne. In addition we present production rates for 36,38Ar from Fe and Ni. The new results are consistent with the data for stony meteoroids presented recently by our group (Leya et al., 2000b), but for the rare gas isotopes the new production rates sometimes differ significantly from earlier estimates. The applicability of the 22Ne/21Ne ratio as a shielding parameter is also discussed. 相似文献
864.
Permafrost temperatures from the surface down to about 9 m from 3 bore holes distributed around Kangiqsualujjuaq village on the coast of Hudson Strait were recorded and analyzed for the period 1989-1998. The results indicate that the permafrost is getting warm along the southern shore of Hudson Strait from 1993 to 1998 though it became cooling for the past 40 a or more. The observed trend in the order of 0. 098℃/a at the 9 m depth is consistent with the long-term regional warming observed in air temperatures. It also coincides with that all the global circulation models predict an enhanced warming in polar regions associated with the increase in concentration of greenhouse gases in the atmosphere. 相似文献
865.
Michel Bouchon 《Geophysical Journal International》1976,47(3):515-530
Summary. A simple method is presented which combines the reciprocity theorem and the flat layer theory to yield teleseismic body wave radiation from seismic sources embedded in the Earth's crust. The source is represented by its equivalent body forces and can be quite general. The effect of Yucca Flat geology on explosion signals is studied in detail. In particular, the m b — M s relation is shown to be dependent on detonation medium and source depth. Application to shallow earthquake faults demonstrates the strong influence of free surface and layering on the shape of P- and S-wave teleseismic spectra. 相似文献
866.
867.
The center-of-disk reflectivity of Jupiter in the wavelength range from 1450 to 3150A?has been computed from 30 low-dispersion IUE spectra taken during solar maximum in 1978–1980. A vertically inhomogeneous radiative transfer program is used to compute model reflectivities of various stratospheric compositions for comparison. Ammonia and acetylene are well determined because they show narrow absorption bands in the ultraviolet. Above 1800A?, these two gases provide a good fit to the data, but not below. At shorter wavelengths the fit would be much improved by a small amount (5–15 ppb) of propadiene/allene (C3H4). Voyager IRIS spectra show that the IR bands of allene are not strong enough to be detected in such a small amount. Additional absorption around 1600A?can be reproduced best with the presence of cyclopropane (C3H6, <15ppb), although other absorbers (e.g., hydrocarbon molecules with more than three carbon atoms, oxygen- or nitrogen-containing molecules, or a high-attitude haze) could also explain the spectrum in this region. The data are too noisy to detect possible CO Cameron band absorption near 2000A?. 相似文献
868.
Michel Froeschlé 《Earth, Moon, and Planets》1977,17(1):47-57
The reduction of 126 photographic plates from the Moon on stellar background, gives a new determination from the Mösting A coordinates with respect to the Lunar inertial system. This determination is based on: the Eckhardt theory for the physical libration, and the ephemeride (j = 2) derived from Brown's theory. The S.A.O. star catalog for the star positions, is used for this reduction. 相似文献
869.
For the first time in decades, multiple lesions on the exoskeleton of wild caught American lobsters (Homarus americanus) were observed in eastern Canada in 2005 and 2006. These lesions were similar to those observed since the mid 1990s in the eastern United States. Of the 12 lobsters examined, 8 were males. Scanning electron microscopy showed a dense population of rod‐shaped bacteria associated with the breakdown of the first layer of cuticle. However, the ability of bacteria to fully penetrate the carapace is unknown. The shell disease did not seem to be immediately fatal and was often associated with alterations in the histoarchitecture of several internal organs (i.e., testis, vas deferens, hepatopancreas, and gills). For instance, follicle cells responsible for spermatogenesis were damaged, resulting in deformed and non‐viable spermatozoa in the testis and vas deferens. In contrast, the female reproductive organ did not show any abnormalities. R‐cells responsible for the storage of lipids and glycogen in the hepatopancreas were severely damaged or destroyed, indicating stress and starvation. Beside necrosis and calcification of gill filaments, debris between the gill filaments was also observed that would limit ion exchange and cause metabolic exhaustion. It does not seem that shell disease was the causative agent or a vector for the pathological conditions of internal organs, but one of the symptoms of a common agent affecting the internal organs. Although shell disease was observed in several areas in eastern Canada, its prevalence was low, in contrast to the epizootic shell disease observed in Long Island Sound, Rhode Island and southern Massachusetts, United States. 相似文献
870.
Michel Dedual 《新西兰海洋与淡水研究杂志》2013,47(4):809-814
The total lipid content of fry (25–30 mm) and parr (55–110 mm) of rainbow trout (Oncorhynchus mykiss Richardson 1836) was measured in spring 1996 and 1997 and in autumn 1997 in six trout‐rearing tributaries of Lake Taupo, New Zealand. The fat content of fry was variable between streams and some streams had a higher inter‐annual variation than others. The fat content of parr was also variable among streams but no difference could be found between seasons. The streams that produced fry with high fat content did not always produce parr with high fat content. 相似文献