全文获取类型
收费全文 | 5787篇 |
免费 | 225篇 |
国内免费 | 89篇 |
专业分类
测绘学 | 168篇 |
大气科学 | 546篇 |
地球物理 | 1443篇 |
地质学 | 2020篇 |
海洋学 | 549篇 |
天文学 | 754篇 |
综合类 | 22篇 |
自然地理 | 599篇 |
出版年
2021年 | 60篇 |
2020年 | 64篇 |
2019年 | 87篇 |
2018年 | 124篇 |
2017年 | 116篇 |
2016年 | 156篇 |
2015年 | 144篇 |
2014年 | 202篇 |
2013年 | 323篇 |
2012年 | 243篇 |
2011年 | 263篇 |
2010年 | 191篇 |
2009年 | 304篇 |
2008年 | 285篇 |
2007年 | 262篇 |
2006年 | 220篇 |
2005年 | 188篇 |
2004年 | 196篇 |
2003年 | 189篇 |
2002年 | 181篇 |
2001年 | 133篇 |
2000年 | 136篇 |
1999年 | 113篇 |
1998年 | 107篇 |
1997年 | 91篇 |
1996年 | 84篇 |
1995年 | 85篇 |
1994年 | 75篇 |
1993年 | 63篇 |
1992年 | 65篇 |
1991年 | 70篇 |
1990年 | 66篇 |
1989年 | 59篇 |
1988年 | 58篇 |
1987年 | 65篇 |
1986年 | 43篇 |
1985年 | 73篇 |
1984年 | 81篇 |
1983年 | 72篇 |
1982年 | 65篇 |
1981年 | 76篇 |
1980年 | 63篇 |
1979年 | 57篇 |
1978年 | 38篇 |
1977年 | 54篇 |
1976年 | 63篇 |
1975年 | 46篇 |
1974年 | 56篇 |
1973年 | 47篇 |
1972年 | 25篇 |
排序方式: 共有6101条查询结果,搜索用时 15 毫秒
891.
We have used Cassini stereo images to study the topography of Iapetus' leading side. A terrain model derived at resolutions of 4-8 km reveals that Iapetus has substantial topography with heights in the range of −10 km to +13 km, much more than observed on the other middle-sized satellites of Saturn so far. Most of the topography is older than 4 Ga [Neukum, G., Wagner, R., Denk, T., Porco, C.C., 2005. Lunar Planet. Sci. XXXVI. Abstract 2034] which implies that Iapetus must have had a thick lithosphere early in its history to support this topography. Models of lithospheric deflection by topographic loads provide an estimate of the required elastic thickness in the range of 50-100 km. Iapetus' prominent equatorial ridge [Porco, C.C., and 34 colleagues, 2005. Science 307, 1237-1242] reaches widths of 70 km and heights of up to 13 km from their base within the modeled area. The morphology of the ridge suggests an endogenous origin rather than a formation by collisional accretion of a ring remnant [Ip, W.-H., 2006. Geophys. Res. Lett. 33, doi:10.1029/2005GL025386. L16203]. The transition from simple to complex central peak craters on Iapetus occurs at diameters of 11±3 km. The central peaks have pronounced conical shapes with flanking slopes of typically 11° and heights that can rise above the surrounding plains. Crater depths seem to be systematically lower on Iapetus than on similarly sized Rhea, which if true, may be related to more pronounced crater-wall slumping (which widens the craters) on Iapetus than on Rhea. There are seven large impact basins with complex morphologies including central peak massifs and terraced walls, the largest one reaches 800 km in diameter and has rim topography of up to 10 km. Generally, no rings are observed with the basins consistent with a thick lithosphere but still thin enough to allow for viscous relaxation of the basin floors, which is inferred from crater depth-to-diameter measurements. In particular, a 400-km basin shows up-domed floor topography which is suggestive of viscous relaxation. A model of complex crater formation with a viscoplastic (Bingham) rheology [Melosh, H.J., 1989. Impact Cratering. Oxford Univ. Press, New York] of the impact-shocked icy material provides an estimate of the effective cohesion/viscosity at . The local distribution of bright and dark material on the surface of Iapetus is largely controlled by topography and consistent with the dark material being a sublimation lag deposit originating from a bright icy substrate mixed with the dark components, but frost deposits are possible as well. 相似文献
892.
The formation of organic compounds in the atmosphere of Titan is an ongoing process of the generation of complex organics from the simplest hydrocarbon, methane. Solar radiation and magnetosphere electrons are the main energy sources that drive the reactions in Titan's atmosphere. Since energy from solar radiation is 200 times greater than that from magnetosphere electrons, we have investigated the products formed by the action of UV radiation (185 and 254 nm) on a mixture of gases containing nitrogen, methane, hydrogen, acetylene, ethylene, and cyanoacetylene, the basic gas mixture (BGM) that simulates aspects of Titan's atmosphere using a flow reactor [Tran, B.N., Ferris, J.P., Chera, J.J., 2003a. Icarus 162, 114-124; Tran, B.N., Joseph, J.C., Force, M., Briggs, R.G., Vuitton, V., Ferris, J.P., 2005. Icarus 177, 106-115]. The present research extends these studies by the addition of carbon monoxide and hydrogen cyanide to the BGM. Quantum yields for the loss of reactants and the formation of volatile products were determined and compared with those measured in the absence of the hydrogen cyanide and carbon monoxide. The GCMS analyses of the volatile photolysis products from the BGM, with added hydrogen cyanide, had a composition similar to that of the BGM while the photolysis products of the BGM with added carbon monoxide contained many oxygenated compounds. The infrared spectrum of the corresponding solid product revealed the absorption band of a ketone group, which was probably formed from the reaction of carbon monoxide with the free radicals generated by photolysis of acetylene and ethylene. Of particular interest was the observation that the addition of HCN to the gas mixture only resulted in a very small change in the C/N ratio and in the intensity of the CN frequency at 2210 cm−1 in the infrared spectrum suggesting that little HCN is incorporated into the haze analog. The C/N ratio of the haze analogs was found to be in the 10-12 range. The UV spectra of the solid products formed when HCN or CO added to the BGM is similar to the UV absorption formed from the BGM alone. This result is consistent with absence of additional UV chromophores to the solid product when these mixtures are photolyzed. The following photoproducts, which were not starting materials in our photochemical studies, have been observed on Titan: acetonitrile, benzene, diacetylene, ethane, propene, propane, and propyne. 相似文献
893.
894.
895.
Peter Brown Robert J. Weryk Daniel K. Wong James Jones 《Earth, Moon, and Planets》2008,102(1-4):209-219
The Canadian Meteor Orbit Radar is a multi-frequency backscatter radar which has been in routine operation since 1999, with
an orbit measurement capability since 2002. In total, CMOR has measured over 2 million orbits of meteoroids with masses greater
than 10 μg, while recording more than 18 million meteor echoes in total. We have applied a two stage comparative technique
for identifying meteor streams in this dataset by making use of clustering in radiants and velocities without employing orbital
element comparisons directly. From the large dataset of single station echoes, combined radiant activity maps have been constructed
by binning and then stacking each years data per degree of solar longitude. Using the single-station mapping technique described
in Jones and Jones (Mon Not R Astron Soc 367:1050–1056, 2006) we have identified probable streams from these single station
observations. Additionally, using individual radiant and velocity data from the multi-station velocity determination routines,
we have utilized a wavelet search algorithm in radiant and velocity space to construct a list of probable streams. These two
lists were then compared and only streams detected by both techniques, on multiple frequencies and in multiple years were
assigned stream status. From this analysis we have identified 45 annual minor and major streams with high reliability. 相似文献
896.
Peter Jenniskens 《Earth, Moon, and Planets》2008,102(1-4):505-520
The history of associating meteor showers with asteroidal-looking objects is long, dating to before the 1983 discovery that
3200 Phaethon moves among the Geminids. Only since the more recent recognition that 2003 EH1 moves among the Quadrantids are
we certain that dormant comets are associated with meteoroid streams. Since that time, many orphan streams have found parent
bodies among the newly discovered Near Earth Objects. The seven established associations pertain mostly to showers in eccentric
or highly inclined orbits. At least 35 other objects are tentatively linked to streams in less inclined orbits that are more
difficult to distinguish from those of asteroids. There is mounting evidence that the streams originated from discrete breakup
events, rather than long episodes of gradual water vapor outgassing. If all these associations can be confirmed, they represent
a significant fraction of all dormant comets that are in near-Earth orbits, suggesting that dormant comets break at least
as frequently as the lifetime of the streams. I find that most pertain to NEOs that have not yet fully decoupled from Jupiter.
The picture that is emerging is one of rapid disintegration of comets after being captured by Jupiter, and consequently, that
objects such as 3200 Phaethon most likely originated from among the most primitive asteroids in the main belt, instead. They
too decay mostly by disintegration into comet fragments and meteoroid streams. The disintegration of dormant comets is likely
the main source of our meteor showers and the main supply of dust to the zodiacal cloud.
Editorial handling: Frans Rietmeijer. 相似文献
897.
Peter Jenniskens 《Earth, Moon, and Planets》2008,102(1-4):5-9
The International Astronomical Union at its 2006 General Assembly in Prague has adopted a set of rules for meteor shower nomenclature,
a working list with designated names (with IAU numbers and three-letter codes), and established a Task Group for Meteor Shower Nomenclature in Commission 22 (Meteors and Interplanetary Dust) to help define which meteor showers exist from well defined groups of
meteoroids from a single parent body. 相似文献
898.
The orbital evolution of the two meteorites Příbram and Neuschwanstein on almost identical orbits and also several thousand
clones were studied in the framework of the N-body problem for 5,000 years into the past. The meteorites moved on very similar
orbits during the whole investigated interval. We have also searched for photographic meteors and asteroids moving on similar
orbits. There were five meteors found in the IAU MDC database and six NEAs with currently similar orbits to Příbram and Neuschwanstein.
However, only one meteor 161E1 and one asteroid 2002 QG46 had a similar orbital evolution over the last 2,000 years. 相似文献
899.
Josep M. Trigo-Rodriguez José M. Madiedo Peter S. Gural Alberto J. Castro-Tirado Jordi Llorca Juan Fabregat Standa Vítek Pep Pujols 《Earth, Moon, and Planets》2008,102(1-4):231-240
By using high-resolution, low-scan-rate, all-sky CCD cameras, the SPanish Meteor Network (SPMN) is currently monitoring meteor
and fireball activity on a year round basis. Here are presented just a sampling of the accurate trajectory, radiant and orbital
data obtained for meteors imaged simultaneously from two SPMN stations during the continuous 2006–2007 coverage of meteor
and fireball monitoring. Typical astrometric uncertainty is 1–2 arc min, while velocity determination errors are of the order
of 0.1–0.5 km/s, which is dependent on the distance of each event to the station and its particular viewing geometry. The
cameras have demonstrated excellent performance for detecting meteor outbursts. The recent development of automatic detection
software is also providing real-time information on the global meteor activity. Finally, some examples of the all-sky CCD
cameras applications for detecting unexpected meteor activity are given. 相似文献
900.
Peter S. Gural 《Earth, Moon, and Planets》2008,102(1-4):269-275
An ever increasing variety of electronic instrumentation is being brought to bear in meteor studies and analysis, with unique
meteor detection challenges arising from the attempt to do automated and near real-time processing of the imagery. Recent
algorithm developments in the literature have been applied and implemented in software to provide reliable meteor detection
in all-sky imagers, wide-field intensified video, and narrow field-of-view telescopic systems. The algorithms that have been
employed for meteor streak detection include Hough transforms with phase coded disk, localized Hough transforms with matched
filtering, and fast moving cluster detection. They have found application in identifying meteor tracks in the Spanish Fireball
Network all-sky images, detailed analysis of video recordings during the recent Leonid meteor storms, and development of a
detection/cueing technology system for rapid slew and tracking of meteors. 相似文献