全文获取类型
收费全文 | 1992篇 |
免费 | 58篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 35篇 |
大气科学 | 139篇 |
地球物理 | 501篇 |
地质学 | 709篇 |
海洋学 | 155篇 |
天文学 | 343篇 |
综合类 | 12篇 |
自然地理 | 162篇 |
出版年
2022年 | 12篇 |
2021年 | 14篇 |
2020年 | 16篇 |
2019年 | 17篇 |
2018年 | 41篇 |
2017年 | 22篇 |
2016年 | 57篇 |
2015年 | 21篇 |
2014年 | 50篇 |
2013年 | 92篇 |
2012年 | 56篇 |
2011年 | 68篇 |
2010年 | 79篇 |
2009年 | 97篇 |
2008年 | 92篇 |
2007年 | 85篇 |
2006年 | 89篇 |
2005年 | 62篇 |
2004年 | 67篇 |
2003年 | 54篇 |
2002年 | 38篇 |
2001年 | 36篇 |
2000年 | 36篇 |
1999年 | 32篇 |
1998年 | 32篇 |
1997年 | 24篇 |
1996年 | 48篇 |
1995年 | 24篇 |
1994年 | 20篇 |
1993年 | 30篇 |
1992年 | 20篇 |
1991年 | 25篇 |
1990年 | 20篇 |
1989年 | 24篇 |
1988年 | 21篇 |
1987年 | 20篇 |
1986年 | 18篇 |
1985年 | 39篇 |
1984年 | 27篇 |
1983年 | 50篇 |
1982年 | 51篇 |
1981年 | 30篇 |
1980年 | 23篇 |
1979年 | 45篇 |
1978年 | 35篇 |
1977年 | 22篇 |
1976年 | 22篇 |
1975年 | 24篇 |
1974年 | 31篇 |
1973年 | 35篇 |
排序方式: 共有2056条查询结果,搜索用时 781 毫秒
991.
Juan A. Sanchez Vishnu Reddy Andreas Nathues Edward A. Cloutis Paul Mann Harald Hiesinger 《Icarus》2012,220(1):36-50
Phase reddening is an effect that produces an increase of the spectral slope and variations in the strength of the absorption bands as the phase angle increases. In order to understand its effect on spectroscopic observations of asteroids, we have analyzed the visible and near-infrared spectra (0.45–2.5 μm) of 12 near-Earth asteroids observed at different phase angles. All these asteroids are classified as either S-complex or Q-type asteroids. In addition, we have acquired laboratory spectra of three different types of ordinary chondrites at phase angles ranging from 13° to 120°. We have found that both, asteroid and meteorite spectra show an increase in band depths with increasing phase angle. In the case of the asteroids the Band I depth increases in the range of ~2° < g < 70° and the Band II depth increases in the range of ~2° < g < 55°. Using this information we have derived equations that can be used to correct the effect of phase reddening in the band depths. Of the three meteorite samples, the (olivine-rich) LL6 ordinary chondrite is the most affected by phase reddening. The studied ordinary chondrites have their maximum spectral contrast of Band I depths at a phase angle of ~60°, followed by a decrease between 60° and 120° phase angle. The Band II depths of these samples have their maximum spectral contrast at phase angles of 30–60° which then gradually decreases to 120° phase angle. The spectral slope of the ordinary chondrites spectra shows a significant increase with increasing phase angle for g > 30°. Variations in band centers and band area ratio (BAR) values were also found, however they seems to have no significant impact on the mineralogical analysis. Our study showed that the increase in spectral slope caused by phase reddening is comparable to certain degree of space weathering. In particular, an increase in phase angle in the range of 30–120° will produce a reddening of the reflectance spectra equivalent to exposure times of ~0.1 × 106–1.3 × 106 years at about 1 AU from the Sun. This increase in spectral slope due to phase reddening is also comparable to the effects caused by the addition of different fractions of SMFe. Furthermore, we found that under some circumstances phase reddening could lead to an ambiguous taxonomic classification of asteroids. 相似文献
992.
We recount the life and career of Richard Christopher Carrington (1826??C?1875) and explore his pivotal relationship with Astronomer Royal George Biddell Airy. Carrington was the pre-eminent solar astronomer of the 19th century. During a ten year span, he determined the position of the Sun??s rotation axis and made the following discoveries: i)?the latitude variation of sunspots over the solar cycle, ii)?the Sun??s differential rotation, and iii)?the first solar flare (with Hodgson). Due to the combined effects of family responsibilities, failure to secure a funded position in astronomy (reflecting Airy??s influence), and ill health, Carrington??s productive period ended when he was at the peak of his powers. 相似文献
993.
Richard C. Ghail Colin Wilson Marina Galand David Hall Chris Cochrane Philippa Mason Joern Helbert Franck MontMessin Sanjay Limaye Manish Patel Neil Bowles Daphne Stam Jan-Erik Wahlund Fabio Rocca David Waltham Tamsin A. Mather Juliet Biggs Matthew Genge Philippe Paillou Karl Mitchell Lionel Wilson Upendra N. Singh 《Experimental Astronomy》2012,33(2-3):337-363
994.
Annabelle F. Keene Scott G. Johnston Richard T. Bush Edward D. Burton Leigh A. Sullivan 《Marine pollution bulletin》2010,60(4):620-3451
This study examines the abundance of trace elements in surface sediments of a former acid sulfate soil (ASS) wetland subjected to marine tidal inundation. Sediment properties of this highly modified study site are compared with those of an adjacent unmodified, intertidal mangrove forest. Whilst some trace elements (Al, Cd, Mn, Ni and Zn) were clearly depleted due to mobilisation and leaching in the previous oxic-acidic phase, other trace elements (As and Cr) displayed significant enrichment in the tidally inundated ASS. Many trace elements were strongly associated with the reactive Fe and acid volatile sulfide (AVS) fractions, suggesting that trace elements may be adsorbed to abundant reactive Fe phases or sequestered as sulfide minerals. These findings provide an important understanding of the fate and mobility of reactive iron, AVS and trace elements during tidal remediation of a formerly acidified Great Barrier Reef (GBR) catchment. 相似文献
995.
996.
The stone crab (Menippe mercenaria) is an important component of the estuarine food web as both predator and prey. Stone crabs live in sediment, primarily consume oysters, and as a result, have the potential to accumulate significant quantities of pollutants including metals. In South Carolina, the stone crab is becoming a targeted fishery as an ecologically sustainable seafood choice. To date, no studies have reported metals in stone crab tissues. This study examined the distribution of major and minor trace elements in chelae and body muscle, gill, and hepatopancreas. Crabs were collected from three tidal areas within Charleston County, South Carolina, with differing upland use. Results were compared by collection location and by tissue type. Concentrations of some metals associated with anthropogenic activities were up to three times higher in crabs from sites adjacent to more urbanized areas. Concentrations in edible tissues were below historical FDA levels of concern. 相似文献
997.
Jessica L. Malone M. Clara Castro Chris M. Hall Peter T. Doran Fabien Kenig Chris P. McKay 《Earth and Planetary Science Letters》2010,289(1-2):112-122
Unlike other lakes in the McMurdo Dry Valleys, Antarctica, Lake Vida has a thick (~ 19 m) ice cover sealing a liquid brine body of unusually high salinity (~ 245 g/L) from the atmosphere. To constrain the conditions under which the atypical Lake Vida ice cover formed and evolved, 19 ice samples were collected down to a depth of ~ 14 m, together with three brine samples trapped in the ice at ~ 16 m for analysis of helium, neon, argon, krypton, and xenon concentrations. The broad pattern of noble gas concentrations for Lake Vida samples is fundamentally different from that of air saturated water (ASW) at 0 °C and an elevation of 340 m for salinities of 0 (ice) and 245 g/L (brine). Overall, ice samples are enriched in He and depleted in Ne with saturation relative to ASW averages of 1.38 and 0.82, respectively, and strongly depleted in Ar, Kr, and Xe with relative saturations of 0.10, 0.06, and 0.05, respectively. By contrast, brine samples are generally depleted in He and Ne (relative saturation averages of 0.33 and 0.27, respectively) but enriched in Ar, Kr, and Xe, with relative saturation averages of 1.45, 3.15, and 8.86, respectively. A three-phase freezing partitioning model generating brine, ice and bubble concentrations for all stable noble gases was tested and compared with our data. Measured brine values are best reproduced for a salinity value of 175 g/L, a pressure of 1.1 atm, and a bubble volume of 20 cm3 kg?1. Sensitivity tests for ice + bubble samples show an ideal fit for bubble volumes of ~ 1–2 cm3 kg?1. Our results show that the conditions under which ice and brine formed and evolved at Lake Vida are significantly different from other ice-covered lakes in the area. Our brine data suggest that Lake Vida may be transitioning from a wet to a dry-based lake, while the ice + bubble data suggest at least partial re-equilibration of residual liquid with the atmosphere as ice forms at the top of Lake Vida ice cover. 相似文献
998.
Evapotranspiration (ET) is the largest term after precipitation in terrestrial water budgets. Accurate estimates of ET are
needed for numerous agricultural and natural resource management tasks and to project changes in hydrological cycles due to
potential climate change. We explore recent methods that combine vegetation indices (VI) from satellites with ground measurements
of actual ET (ETa) and meteorological data to project ETa over a wide range of biome types and scales of measurement, from local to global estimates. The majority of these use time-series
imagery from the Moderate Resolution Imaging Spectrometer on the Terra satellite to project ET over seasons and years. The
review explores the theoretical basis for the methods, the types of ancillary data needed, and their accuracy and limitations.
Coefficients of determination between modeled ETa and measured ETa are in the range of 0.45–0.95, and root mean square errors are in the range of 10–30% of mean ETa values across biomes, similar to methods that use thermal infrared bands to estimate ETa and within the range of accuracy of the ground measurements by which they are calibrated or validated. The advent of frequent-return
satellites such as Terra and planed replacement platforms, and the increasing number of moisture and carbon flux tower sites
over the globe, have made these methods feasible. Examples of operational algorithms for ET in agricultural and natural ecosystems
are presented. The goal of the review is to enable potential end-users from different disciplines to adapt these methods to
new applications that require spatially-distributed ET estimates. 相似文献
999.
Edward J. Rhodes Patricia C. Fanning Simon J. Holdaway 《Quaternary Geochronology》2010,5(2-3):348-352
Research conducted by the Western New South Wales Archaeology Program (WNSWAP) provides the opportunity to assess the reliability of optically stimulated luminescence (OSL) dating of late Pleistocene and Holocene fluvial sediments and burnt stone samples from arid zone geoarchaeological contexts. A large number of radiocarbon age determinations of charcoal preserved in heat retainer hearths provides independent chronological control at these contexts. We describe a rapid OSL methodology for dating burnt hearth stones to complement previously applied radiocarbon methods, which we have tested using 37 samples from hearths with radiocarbon determinations. We propose a geoarchaeological model in which these hearths were constructed by people whose activity took place on an archaeological surface, formed by the earlier deposition of fluvial sediments. Here we demonstrate the veracity of this model by dating sediments lying stratigraphically below the hearths, and use the radiocarbon age control and chronological consistency to assess the accuracy and reliability of both small aliquot and single grain single aliquot regenerative-dose (SAR) OSL dating. While small aliquot age estimates are in most cases in agreement with independent control, the single grain determinations using a finite mixture model (FMM) appear to provide improved chronological resolution. Using single grains, we note some problems in the application of the FMM and in the dating of young samples in the range of 1–100 years. As many samples may have resided close to the surface since deposition, we have developed a mathematical function to describe gamma and cosmic dose rate contributions at burial depths down to 40 cm. These OSL age estimates allow us to reject the model of intensification of human activity as responsible for the observed pattern of archaeological radiocarbon determinations in this part of the Australian arid zone. 相似文献
1000.