全文获取类型
收费全文 | 624篇 |
免费 | 21篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 17篇 |
大气科学 | 42篇 |
地球物理 | 168篇 |
地质学 | 195篇 |
海洋学 | 68篇 |
天文学 | 108篇 |
综合类 | 2篇 |
自然地理 | 47篇 |
出版年
2023年 | 2篇 |
2022年 | 2篇 |
2021年 | 9篇 |
2020年 | 10篇 |
2019年 | 17篇 |
2018年 | 21篇 |
2017年 | 30篇 |
2016年 | 31篇 |
2015年 | 20篇 |
2014年 | 30篇 |
2013年 | 34篇 |
2012年 | 29篇 |
2011年 | 39篇 |
2010年 | 35篇 |
2009年 | 31篇 |
2008年 | 36篇 |
2007年 | 29篇 |
2006年 | 31篇 |
2005年 | 24篇 |
2004年 | 21篇 |
2003年 | 20篇 |
2002年 | 18篇 |
2001年 | 12篇 |
2000年 | 12篇 |
1999年 | 13篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 9篇 |
1995年 | 6篇 |
1994年 | 5篇 |
1993年 | 7篇 |
1992年 | 3篇 |
1990年 | 2篇 |
1988年 | 2篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1977年 | 1篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1972年 | 2篇 |
1971年 | 3篇 |
1970年 | 4篇 |
1966年 | 2篇 |
1963年 | 4篇 |
1961年 | 2篇 |
1940年 | 2篇 |
排序方式: 共有647条查询结果,搜索用时 23 毫秒
551.
Jens Orm James M. Dohm Justin C. Ferris Alain Lepinette Alberto G. Fairn 《Meteoritics & planetary science》2004,39(2):333-346
Abstract— Observations of impact craters on Earth show that a water column at the target strongly influences lithology and morphology of the resultant crater. The degree of influence varies with the target water depth and impactor diameter. Morphological features detectable in satellite imagery include a concentric shape with an inner crater inset within a shallower outer crater, which is cut by gullies excavated by the resurge of water. In this study, we show that if oceans, large seas, and lakes existed on Mars for periods of time, marine‐target craters must have formed. We make an assessment of the minimum and maximum amounts of such craters based on published data on water depths, extent, and duration of putative oceans within “contacts 1 and 2,” cratering rate during the different oceanic phases, and computer modeling of minimum impactor diameters required to form long‐lasting craters in the seafloor of the oceans. We also discuss the influence of erosion and sedimentation on the preservation and exposure of the craters. For an ocean within the smaller “contact 2” with a duration of 100,000 yr and the low present crater formation rate, only ?1–2 detectable marine‐target craters would have formed. In a maximum estimate with a duration of 0.8 Gyr, as many as 1400 craters may have formed. An ocean within the larger “contact 1‐Meridiani,” with a duration of 100,000 yr, would not have received any seafloor craters despite the higher crater formation rate estimated before 3.5 Gyr. On the other hand, with a maximum duration of 0.8 Gyr, about 160 seafloor craters may have formed. However, terrestrial examples show that most marine‐target craters may be covered by thick sediments. Ground penetrating radar surveys planned for the ESA Mars Express and NASA 2005 missions may reveal buried craters, though it is uncertain if the resolution will allow the detection of diagnostic features of marine‐target craters. The implications regarding the discovery of marine‐target craters on Mars is not without significance, as such discoveries would help address the ongoing debate of whether large water bodies occupied the northern plains of Mars and would help constrain future paleoclimatic reconstructions. 相似文献
552.
Alberto Carramiñana 《Astrophysics and Space Science》2007,309(1-4):373-378
The Cygnus region of the Milky Way is prolific in star formation and presents extended diffuse γ-ray emission with a few γ-ray point sources. Among them is 3EG J2020+4017, the brightest of the unidentified EGRET sources, positionally coincident
with the supernova remnant G78.2+2.1. Even though the EGRET and multi-wavelength data have not provided a conclusive identification
for this γ-ray loud, but otherwise faint object, the evidence favors a pulsar like source. The EGRET photon data lack the signal-to-noise
ratio required for a period search, but will serve as a valuable timing baseline extension in the case that GLAST confirms the pulsar nature of the γ-Cygni source.
Work sponsored by CONACyT grant SEP-2003-C02-42611. 相似文献
553.
Source subtraction for the extended Very Small Array and 33-GHz source count estimates 总被引:1,自引:0,他引:1
Kieran A. Cleary † Angela C. Taylor ‡ Elizabeth Waldram Richard A. Battye Clive Dickinson § Rod D. Davies Richard J. Davis Ricardo Genova-Santos Keith Grainge Michael E. Jones ¶ Rüdiger Kneissl G. G. Pooley Rafael Rebolo José Alberto Rubiño-Martín Richard D. E. Saunders Paul F. Scott Ane Slosar David Titterington Robert A. Watson 《Monthly notices of the Royal Astronomical Society》2005,360(1):340-353
We describe the source subtraction strategy and observations for the extended Very Small Array (VSA), a cosmic microwave background interferometer operating at 33 GHz. A total of 453 sources were monitored at 33 GHz using a dedicated source subtraction baseline. 131 sources brighter than 20 mJy were directly subtracted from the VSA visibility data. Some characteristics of the subtracted sources, such as spectra and variability, are discussed. The 33-GHz source counts are estimated from a sample selected at 15 GHz. The selection of VSA fields in order to avoid bright sources introduces a bias into the observed counts. This bias is corrected and the resulting source count is estimated to be complete in the flux-density range 20–114 mJy. The 33-GHz source counts are used to calculate a correction to the VSA power spectrum for sources below the subtraction limit. 相似文献
554.
Emanuela Bianchi Janetti Monica Riva Alberto Guadagnini 《Computational Geosciences》2016,20(3):555-565
We analyze three-phase flow of immiscible fluids taking place within an elementary capillary tube with circular cross-section under water- and oil-wet conditions. We account explicitly for momentum transfer between the moving phases, which leads to the phenomenon of viscous coupling, by imposing continuity of velocity and shear stress at fluid-fluid interfaces. The macroscopic flow model which describes the system at the Darcy scale includes three-phase effective relative permeabilities, K i j,r , accounting for the flux of the ith phase due to the presence of the jth phase. These effective parameters strongly depend on phase saturations, fluid viscosities, and wettability of the solid matrix. In the considered flow setting, K i j,r reduce to a set of nine scalar quantities, K i j,r . Our results show that K i j,r of the wetting phase is a function only of the fluid phase own saturation. Otherwise, K i j,r of the non-wetting phase depends on the saturation of all fluids in the system and on oil and water viscosities. Viscous coupling effects (encapsulated in K i j,r with i ≠ j) can be significantly relevant in both water- and oil-wet systems. Wettability conditions influence oil flow at a rate that increases linearly with viscosity ratio between oil and water phases. 相似文献
555.
556.
Reducing GHG emissions and mitigating climate change would require significant investments in renewable energy technologies. Foreign direct investments (FDI) in renewable energy (RE) have increased over the last years, contributing to the diffusion of RE globally. In the field of climate policy, there are multiple policy instruments aimed at attracting investments in renewable energy. This article aims to map the FDI flows globally including source and destination countries. Furthermore, the article investigates which policy instruments attract more FDI in RE sectors such as solar, wind and biomass, based on an econometric analysis of 137 Organisation for Economic Co-operation and Development (OECD) and non-OECD countries. The results show that Feed in Tariffs (FIT) followed by Fiscal Measures (FM), such as tax incentives and Renewable Portfolio Standards (RPS), are the most significant policy instrument that attract FDI in the RE sector globally. Regarding carbon pricing instruments, based on our analysis, carbon tax proved to be correlated with high attraction of FDI in OECD countries, whereas Emissions Trading Schemes (ETS) proved to be correlated with high attraction of FDI mainly in non-OECD countries.
Key policy insights
Feed in Tariffs is the most significant policy instrument that attracts FDI in the Renewable Energy sector globally.
Fiscal Measures (FM), such as tax incentives, show a significant and positive impact on renewable energy projects by foreign investors, and particularly on solar energy.
Carbon pricing instruments, such as carbon taxation and emissions trading, proved to attract FDI in OECD and non-OECD countries respectively.
Public investments, such as government funds for renewable energy projects, proved not as attractive to foreign private investors, perhaps because public funds are not perceived as stable in the long run.
557.
High crustal diversity preserved in the lunar meteorite Mount DeWitt 12007 (Victoria Land,Antarctica)
下载免费PDF全文
![点击此处可从《Meteoritics & planetary science》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Alberto Collareta Massimo D'Orazio Maurizio Gemelli Andreas Pack Luigi Folco 《Meteoritics & planetary science》2016,51(2):351-371
The meteorite Mount DeWitt (DEW) 12007 is a polymict regolith breccia mainly consisting of glassy impact‐melt breccia particles, gabbroic clasts, feldspathic clasts, impact and volcanic glass beads, basaltic clasts, and mingled breccia clasts embedded in a matrix dominated by fine‐grained crystals; vesicular glassy veins and rare agglutinates are also present. Main minerals are plagioclase (typically An>85) and clinopyroxene (pigeonites and augites, sometimes interspersed). The presence of tranquillityite, coupled with the petrophysical data, the O‐isotope data (Δ17O = ?0.075), and the FeOtot/MnO ratios in olivine (91), pyroxene (65), and bulk rock (77) indicate a lunar origin for DEW 12007. Impactites consist of Al‐rich impact‐melt splashes and plagioclase‐rich meta‐melt clasts. The volcanic products belong to the very low titanium (VLT) or low titanium (LT) suites; an unusual subophitic fragment could be cryptomare‐related. Gabbroic clasts could represent part of a shallow intrusion within a volcanic complex with prevailing VLT affinity. DEW 12007 has a mingled bulk composition with relatively high incompatible element abundances and shows a high crustal diversity comprising clasts from the Moon's major terranes and rare lithologies. First‐order petrographic and chemical features suggest that DEW 12007 could be launch‐paired with other meteorites including Y 793274/981031, QUE 94281, EET 87521/96008, and NWA 4884. 相似文献
558.
559.
Celia Ojeda-Martínez Francisca Giménez Casalduero Just T. Bayle-Sempere Carmen Barbera Cebrián Carlos Valle Jose Luis Sanchez-Lizaso Aitor Forcada Pablo Sanchez-Jerez Pablo Martín-Sosa Jesús M. Falcón Fuensanta Salas Mariagrazia Graziano Renato Chemello Ben Stobart Pedro Cartagena Angel Pérez-Ruzafa Fréderic Vandeperre Elisabeth Rochel Serge Planes Alberto Brito 《Ocean & Coastal Management》2009,52(2):89-101
A general conceptual framework for the management of marine protected areas (MPAs) was developed. The driver-pressure-state-impacts-response (DPSIR) framework was used to determine the elements affecting MPAs. The developed evaluation framework helped to select an appropriate suite of indicators to support an ecosystem approach, an assessment of the MPAs functioning and policy decisions. Gaps derived from the management and policy responses in the MPAs were also outlined. It was concluded that the DPSIR framework can help to simplify the complexity of MPA management. This document is a tool for policy makers, scientists and general public on the relevance of indicators to monitor changes and MPAs management. 相似文献
560.
Victor Quintino Franca Sangiorgio Fernando Ricardo Renato Mamede Adília Pires Rosa Freitas Ana Maria Rodrigues Alberto Basset 《Estuarine, Coastal and Shelf Science》2009
An experimental study on Phragmites australis leaf litter decomposition was conducted in the estuarine environment, Ria de Aveiro, Western Portugal, using the leaf-bag technique, with fine- (1 mm) and coarse-mesh (5 mm) bags. The leaf bags were placed in the field sites at day 0, covering a complete salinity gradient, and replicates were collected over time, at days 3 (leaching), 7, 15, 30 and 60. The biomass loss through the leaching phase, about 20% of the initial leaf mass, was independent of both the salinity and the bag mesh size. The biomass decay pattern along the salinity gradient varied through time and presented strong similarities between the two mesh sizes, with the remaining biomass always lower in the 5 mm mesh-size bags. At days 7 and 15, the lowest remaining biomass was observed at the head of the estuary, the preferential distribution area of P. australis. At day 30, the remaining biomass was higher in the marine area and diminished under a direct relationship with salinity, reaching the lowest value in the freshwater environment, with values ranging from 66% to 44% of the initial weight in 5 mm bags, and from 79% to 51% in 1 mm bags. The largest heterogeneity in the remaining biomass among the study areas positioned along the salinity gradient was found close to days 30 (5 mm) and 40 (1 mm). The overall results indicate that the relationship between leaf decay rate and salinity depends on the decay time considered (k15, k30 or k60) and, for the later stages (k60), also on the leaf-bag mesh size. This implies that the use of leaf litter decay rates as a functional indicator in transitional waters will need to take into consideration the factor location in the salinity gradient and leaf litter stage at which the decay rate is determined. The differences between the decay rates with the mesh size acted mainly at the level of the absolute k value and not at the level of the pattern along the salinity gradient. Even so, the data obtained at the mouth of the estuary, in the area closest to a fully marine environment, indicated that after the initial biomass loss through leaching, P. australis decayed either very slowly, in the 5 mm, or not at all, in the 1 mm mesh bags. 相似文献