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941.
Mars is the fourth planet out from the sun. It is a terrestrial planet with a density suggesting a composition roughly similar
to that of the Earth. Its orbital period is 687 days, its orbital eccentricity is 0.093 and its rotational period is about
24 hours. Mars has two small moons of asteroidal shapes and sizes (about 11 and 6 km mean radius), the bigger of which, Phobos,
orbits with decreasing semimajor orbit axis. The decrease of the orbit is caused by the dissipation of tidal energy in the
Martian mantle. The other satellite, Deimos, orbits close to the synchronous position where the rotation period of a planet
equals the orbital period of its satellite and has hardly evolved with time. Mars has a tenous atmosphere composed mostly
of CO with strong winds and with large scale aeolian transport of surface material during dust storms and in sublimation-condensation
cycles between the polar caps. The planet has a small magnetic field, probably not generated by dynamo action in the core
but possibly due to remnant magnetization of crustal rock acquired earlier from a stronger magnetic field generated by a now
dead core dynamo. A dynamo powered by thermal power alone would have ceased a few billions of years ago as the core cooled
to an extent that it became stably stratified. Mars' topography and its gravity field are dominated by the Tharsis bulge,
a huge dome of volcanic origin. Tharsis was the major center of volcanic activity, a second center is Elysium about 100° in
longitude away. The Tharsis bulge is a major contributor to the non-hydrostaticity of the planet's figure. The moment of inertia
factor together with the mass and the radius presently is the most useful constraint for geophysical models of the Martian
interior. It has recently been determined by Doppler range measurements to the Mars Pathfinder Lander to be (Folkner et al. 1997). In addition, models of the interior structure use the chemistry of the SNC meteorites which are widely
believed to have originated on Mars. According to the models, Mars is a differentiated planet with a 100 to 200 km thick basaltic
crust, a metallic core with a radius of approximately half the planetary radius, and a silicate mantle. Mantle dynamics is
essential in forming the elements of the surface tectonics. Models of mantle convection find that the pressure-induced phase
transformations of -olivine to -spinel, -spinel to -spinel, and -spinel to perovskite play major roles in the evolution of mantle flow fields and mantle temperature. It is not very likely
that the -spinel to perovskite transition is present in Mars today, but a few 100 km thick layer of perovskite may have been present
in the lower mantle immediately above the core-mantle boundary early in the Martian history when mantle temperatures were
hotter than today. The phase transitions act to reduce the number of upwellings to a few major plumes which is consistent
with the bipolar distribution of volcanic centers of Mars. The phase transitions also cause a partial layering of the lower
mantle which keeps the lower mantle and the core from extensive cooling over the past aeons. A relatively hot, fluid core
is the most widely accepted explanation for the present lack of a self-generated magnetic field. Growth of an inner core which
requires sub-liquidus temperatures in the core would have provided an efficient mechanism to power a dynamo up to the present
day.
Received 10 May 1997 相似文献
942.
红河断裂带的新生代变形机制及莺歌海盆地的实验证据 总被引:30,自引:4,他引:30
红河断裂带是印藏碰撞过程中,印支地块被顺时针旋转挤出的走滑变形带。莺歌海盆地发育于红河断裂带海上延伸带上。根据莺歌海盆地和相邻的NE向琼东南盆地在晚中新世前(5.5Ma B.P.)独立的构造发育和差异的沉降特点,认为红河断裂不可能穿越莺琼盆地界限向北东延伸,而越东断裂和中建南断裂很可能是红河断裂的延续。莺歌海盆地成盆机制的物理模拟结合红河断裂带陆上的变形特征、年代学证据与青藏高原隆升过程的研究,参考莺歌海盆地模拟过程中不同应力场下沉降中心的长轴方向,我们推断红河断裂带新生代的演化大致分4个阶段:(1)50-38Ma B.P.期间的缓慢平移运动;(2)38—25MaB.P.期间的快速左行走滑运动;(3)25—5Ma B.P.期间的左行走滑逐渐停止阶段;(4)5Ma B.P.后的右行走滑阶段。 相似文献
943.
Franco PIRAJNO Martin J. Van KRANENDONK Long XIAO 《地质科技情报》2008,27(1):1-12
最近美国航空与空间计划署(NASA)开展的卡西尼-惠更斯外空探测计划发现,在直径为500 km的卫星--土卫二上存在水冰和间隙泉的喷发活动.这一现象和在火星上工作的"机遇号"和"勇气号"漫游车所发现的液态水一起,证明了除地球以外的其他星球上过去和现在都存在水,其中的一些星体还有火山活动的证据,这意味着这些星球上可能存在过热液活动地质过程.讨论了火星、木卫二和土卫二可能存在的热液系统类型.这些热液系统类型是根据地球上的构造背景进行相似性研究后得出的,例如海底、火山和裂谷系统.将东非裂谷和贝加尔湖裂谷系统与火星Tharsis高原上巨大的水手大峡谷进行了对比,这些地区都是由地幔柱作用下构造-热液活动导致的地壳抬升、火山和裂谷作用.在火星上,地下冰或低温层会在火山活动和(或)小行星或彗星撞击作用下溶解而形成热液对流. 相似文献
944.
火星和地球雅丹形态学类比分析——以埃律西昂平原和柴达木盆地为例 总被引:1,自引:1,他引:0
作为一种风蚀地貌,雅丹地貌是对环境长期适应的一种表现,其形态特征可以反映所受内外营力的作用。对火星埃律西昂平原和地球柴达木盆地雅丹体的形态参数进行对比分析。结果显示:(1)埃律西昂平原雅丹体的长宽比(3.49)与柴达木盆地雅丹体的长宽比(3.57)较为接近,同时也与理想状态下的长宽比(4)接近,它们都具有流线型的形态,且处于雅丹体发育的成熟期。(2)柴达木盆地雅丹体是鲸背状,埃律西昂平原雅丹体是椭圆状,原因是柴达木盆地雅丹体常年受盛行风的影响,埃律西昂平原雅丹体受两种反向风的影响。柴达木盆地雅丹体表面崩裂的痕迹不明显,埃律西昂平原雅丹体表面崩裂的痕迹比较明显,原因是形成雅丹体的物质基础和温差环境不同:柴达木盆地雅丹体的物质基础是湖相沉积物,雅丹体表面有减弱风化的盐壳;埃律西昂平原雅丹体的物质基础是火山熔岩流,而且火星的昼夜温差大于地球。 相似文献
945.
我国苏州花岗岩-伟晶岩接触带岩石中,与高放废物内某些长寿命超铀铜系元素和裂变产物化学性质相似的微量元素、同位素迁移行为天然类比研究表明,伟晶岩脉中的U、Li、Sr、Be、Mo、Nb、Er、Y和234U、238U在该岩脉形成后约130Ma期间,在水-岩反应作用下,仅向花岗岩中迁移10-30cm,而Th、Pb、206Pb、207Pb、208Pb、87Rb、87Sr、86Sr则未发生明显迁移。这一研究结果为我国拟在花岗岩中处置高放废物提供了长期安全评价的依据。 相似文献
946.
947.
Three points raised by Solomon's discussion are clarified, a conservative approach is preferred, using pollen-vegetation relationships based on all taxa potentially involved for a given site. The use of techniques for comparison with modern analogues is defended, provided that appropriate methods such as Standardized Euclidian Distance are used. The model ?FORET’? used by Solomon is based on half hectare plots and is not considered ideal for aggregation to the forty hectare lake site or to the same area for pollen grains entering it. 相似文献
948.
利用国家气候中心第2代季节气候预测模式BCC_CSM1.1(m) 的1991—2010年每年2月起报的历史回算资料集,考察模式对于5个夏季风指数的预测能力,并通过发展基于经验正交函数分解与相似分析的EOF-相似误差订正方法,对5个夏季风指数的模式预测进行再修正。交叉检验和独立样本检验结果表明:该模式对1991—2010年东亚夏季风指数与西北太平洋夏季风指数预测技巧较高;EOF-相似误差订正方法适用于模式预测技巧较低的指数,这些指数经订正后预测效果均有不同程度改进,而预测技巧较高的夏季风指数经订正后改进效果不明显;在交叉检验中,线性部分订正多优于非线性部分订正效果,而对于独立试报的年份,非线性部分订正多优于线性部分订正效果,显示出良好的应用前景。 相似文献
949.
The Tulul Al Hammam area in central Jordan is a natural analogue site where to study long-term Cd2+ retention during the weathering of Cd-enriched combustion-metamorphic marbles with clinker-like mineralogy. Natural Cd-rich portlandite (Ca0.67−0.45Cd0.33−0.55)[OH]2 was discovered and characterised by single-crystal XRD, SEM/EDS, and microprobe analyses, and Raman spectroscopy. The Ca[OH]2 – Cd[OH]2 solid solution ((Ca1−xCdx)[OH]2) is isostructural with Ca[OH]2 and β-Cd[OH]2 crystals in the CdI2-type of structure. Cd-rich portlandite is a common secondary phase in slightly altered massive marbles, where it replaces and rims single-crystals of the high-temperature precursor (Ca1−xCdx)O. The Cd:Ca molar ratios of (Ca1−xCdx)[OH]2 are inherited from the precursor phase (Ca1−xCdx)O. Reliable long-term immobilisation of Cd in (Ca1−xCdx)[OH]2 is assumed for pore water pH values from 10.5 up to 12 buffered by C-S-Hs formed mainly during spurrite hydration. The Tulul Al Hammam site may have some bearing on long-term Cd2+ retention in the concrete matrix. 相似文献
950.
我国南极陨石研究与展望 总被引:2,自引:0,他引:2
继1998~2000年我国第15、16次南极科考队在南极格罗夫山发现32块陨石之后,2002~2003年第19次科考队成立了以回收陨石为中心任务的格罗夫山综合考察分队,在同一地区成功回收4448块陨石。我国的南极陨石回收工作不但实现了零的突破,而且成为继日本和美国之后拥有南极陨石数量最多的国家之一。通过对第15、16次队回收的32块陨石以及第19次队4448块陨石中的38块代表性样品的化学一岩石类型划分工作,除平衡型普通球粒陨石外,发现了2块火星陨石、2块橄辉无球粒陨石、6块非平衡L3型陨石、4块碳质球粒陨石和1块非平衡型顽辉石球粒陨石等特殊类型陨石。本文主要介绍了南极陨石的回收和研究进展,以及我国在南极格罗夫山回收陨石的情况和已取得的初步研究成果。同时对我国今后的陨石回收与研究工作提出初步设想。 相似文献