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11.
Farside explorer: unique science from a mission to the farside of the moon   总被引:4,自引:0,他引:4  
Farside Explorer is a proposed Cosmic Vision medium-size mission to the farside of the Moon consisting of two landers and an instrumented relay satellite. The farside of the Moon is a unique scientific platform in that it is shielded from terrestrial radio-frequency interference, it recorded the primary differentiation and evolution of the Moon, it can be continuously monitored from the Earth–Moon L2 Lagrange point, and there is a complete lack of reflected solar illumination from the Earth. Farside Explorer will exploit these properties and make the first radio-astronomy measurements from the most radio-quiet region of near-Earth space, determine the internal structure and thermal evolution of the Moon, from crust to core, and quantify impact hazards in near-Earth space by the measurement of flashes generated by impact events. The Farside Explorer flight system includes two identical solar-powered landers and a science/telecommunications relay satellite to be placed in a halo orbit about the Earth–Moon L2 Lagrange point. One lander would explore the largest and oldest recognized impact basin in the Solar System— the South Pole–Aitken basin—and the other would investigate the primordial highlands crust. Radio astronomy, geophysical, and geochemical instruments would be deployed on the surface, and the relay satellite would continuously monitor the surface for impact events.  相似文献   
12.
Magnetic signature of different vegetation species in polluted environment   总被引:2,自引:0,他引:2  
Detailed magnetic study on vegetation samples from several strongly polluted and clean sites in Bulgaria is carried out in order to evaluate suitability of different species as passive dust collectors in magnetometry. From each location, available species among lichens, mosses, poplar leaves, dandelion, needles have been sampled. Magnetic susceptibility calculated on mass-specific basis shows wide variability between diamagnetic signal up to 846 × 10−8 m3/kg. Lichens and mosses are found to be the species, showing magnetic signals with the strongest contrast between clean and polluted environment. The main magnetic phase is magnetite-like according to the results from thermomagnetic analysis of susceptibility on magnetic extracts. Scanning electron microscopy (SEM) microphotographs reveal the presence of abundant particulate matter on vegetation surface both with anthropogenic (spherules) and lithogenic origin. Magnetic grain size deduced by the ratio of saturation remanent magnetization (SIRM) and mass-specific magnetic susceptibility (χ) and coercivities (Bc and Bcr) suggest that different species accumulate preferentially small SD-like grains from pollution emissions. Contrasting relationship of the ratio of anhysteretic remanent magnetization (ARM) and χ for polluted vs clean sites deduced by needles and lichens may be related to transformation of the accumulated dust particles within lichens’ tissue. This finding indicates that the exact species used as biological dust collector is of importance when studying spatial grain size distribution of magnetic dust particles. Pilot study on polycyclic aromatic hydrocarbons (PAH) content and its relation to magnetic parameters shows good correspondence between high levels of PAHs and high SIRM values for locations affected by non-ferrous industrial production.  相似文献   
13.
In the framework of the Deep Electromagnetic Soundings for Mineral Exploration project, we conducted ground-based long-offset transient-electromagnetic measurements in a former mining area in eastern Thuringia, Germany. The large-scale survey resulted in an extensive dataset acquired with multiple high-power transmitters and a high number of electric and magnetic field receivers. The recorded data exhibit a high data quality over several decades of time and orders of magnitude. Although the obtained subsurface models indicate a strong multi-dimensional subsurface with variations in resistivity over three orders of magnitude, the electrical field step-on transients are well fitted using a conventional one-dimensional inversion. Due to superimposed induced polarization effects, the transient step-off data are not interpretable with conventional electromagnetic inversion. For further interpretation in one and two dimensions, a new approach to evaluate the long-offset transient-electromagnetic data in frequency domain is realized. We present a detailed workflow for data processing in both domains and give an overview of technical obstructions that can occur in one domain or the other. The derived one-dimensional inversion models of frequency-domain data show strong multi-dimensional effects and are well comparable with the conventional time domain inversion results. To adequately interpret the data, a 2.5D frequency-domain inversion using the open source algorithm MARE2DEM (Modeling with Adaptively Refined Elements for 2-D EM) is carried out. The inversion leads to a consistent subsurface model with shallow and deep conductive structures, which are confirmed by geology and additional geophysical surveys.  相似文献   
14.
A marine seismic method based on continuous source and receiver wavefields has been developed. The method requires continuous recording of the seismic data. The source that may consist of multiple source elements can emit signals continuously while moving. The ideal source wavefield to be used with this method should be as white as possible both in a temporal and a spatial sense to avoid deep notches in the spectrum enabling a stable multi-dimensional deconvolution. White noise has such properties. However, equipment that can generate white noise does not exist. In order to generate a continuous source wavefield that is approaching the properties of white noise using existing equipment onboard marine seismic vessels, individual air-guns can be triggered with short randomized time intervals in a near-continuous fashion. The main potential benefits with the method are to reduce the environmental impact of marine seismic surveys and to improve acquisition efficiency. The peak sound pressure levels are significantly reduced by triggering one air-gun at a time compared to conventional marine seismic sources. Sound exposure levels are also reduced in most directions. Since the method is based on continuous recording of seismic data and the air-guns are triggered based on time and not based on position, there are less vessel speed limitations compared to conventional marine seismic data acquisition. Also, because the source wavefield is spread out in time, the wavefields emitted from source elements in different cross-line positions can be designed such that the emitted wavefield is spatially white in this direction. This means that source elements in multiple cross-line positions can be operated simultaneously, potentially improving the cross-line sampling and/or the acquisition efficiency.  相似文献   
15.
16.
The thermo-mechanical properties of planetary surface and subsurface layers control to a high extent in which way a body interacts with its environment, in particular how it responds to solar irradiation and how it interacts with a potentially existing atmosphere. Furthermore, if the natural temperature profile over a certain depth can be measured in situ, this gives important information about the heat flux from the interior and thus about the thermal evolution of the body. Therefore, in most of the recent and planned planetary lander missions experiment packages for determining thermo-mechanical properties are part of the payload. Examples are the experiment MUPUS on Rosetta's comet lander Philae, the TECP instrument aboard NASA's Mars polar lander Phoenix, and the mole-type instrument HP3 currently developed for use on upcoming lunar and Mars missions. In this review we describe several methods applied for measuring thermal conductivity and heat flux and discuss the particular difficulties faced when these properties have to be measured in a low pressure and low temperature environment. We point out the abilities and disadvantages of the different instruments and outline the evaluation procedures necessary to extract reliable thermal conductivity and heat flux data from in situ measurements.  相似文献   
17.
Oceans in the icy Galilean satellites of Jupiter?   总被引:1,自引:0,他引:1  
Tilman Spohn  Gerald Schubert 《Icarus》2003,161(2):456-467
Equilibrium models of heat transfer by heat conduction and thermal convection show that the three satellites of Jupiter—Europa, Ganymede, and Callisto—may have internal oceans underneath ice shells tens of kilometers to more than a hundred kilometers thick. A wide range of rheology and heat transfer parameter values and present-day heat production rates have been considered. The rheology was cast in terms of a reference viscosity ν0 calculated at the melting temperature and the rate of change A of viscosity with inverse homologous temperature. The temperature dependence of the thermal conductivity k of ice I has been taken into account by calculating the average conductivity along the temperature profile. Heating rates are based on a chondritic radiogenic heating rate of 4.5 pW kg−1 but have been varied around this value over a wide range. The phase diagrams of H2O (ice I) and H2O + 5 wt% NH3 ice have been considered. The ice I models are worst-case scenarios for the existence of a subsurface liquid water ocean because ice I has the highest possible melting temperature and the highest thermal conductivity of candidate ices and the assumption of equilibrium ignores the contribution to ice shell heating from deep interior cooling. In the context of ice I models, we find that Europa is the satellite most likely to have a subsurface liquid ocean. Even with radiogenic heating alone the ocean is tens of kilometers thick in the nominal model. If tidal heating is invoked, the ocean will be much thicker and the ice shell will be a few tens of kilometers thick. Ganymede and Callisto have frozen their oceans in the nominal ice I models, but since these models represent the worst-case scenario, it is conceivable that these satellites also have oceans at the present time. The most important factor working against the existence of subsurface oceans is contamination of the outer ice shell by rock. Rock increases the density and the pressure gradient and shifts the triple point of ice I to shallower depths where the temperature is likely to be lower then the triple point temperature. According to present knowledge of ice phase diagrams, ammonia produces one of the largest reductions of the melting temperature. If we assume a bulk concentration of 5 wt% ammonia we find that all the satellites have substantial oceans. For a model of Europa heated only by radiogenic decay, the ice shell will be a few tens of kilometers thinner than in the ice I case. The underlying rock mantle will limit the depth of the ocean to 80-100 km. For Ganymede and Callisto, the ice I shell on top of the H2O-NH3 ocean will be around 60- to 80-km thick and the oceans may be 200- to 350-km deep. Previous models have suggested that efficient convection in the ice will freeze any existing ocean. The present conclusions are different mainly because they are based on a parameterization of convective heat transport in fluids with strongly temperature dependent viscosity rather than a parameterization derived from constant-viscosity convection models. The present parameterization introduces a conductive stagnant lid at the expense of the thickness of the convecting sublayer, if the latter exists at all. The stagnant lid causes the temperature in the sublayer to be warmer than in a comparable constant-viscosity convecting layer. We have further modified the parameterization to account for the strong increase in homologous temperature, and therefore decrease in viscosity, with depth along an adiabat. This modification causes even thicker stagnant lids and further elevated temperatures in the well-mixed sublayer. It is the stagnant lid and the comparatively large temperature in the sublayer that frustrates ocean freezing.  相似文献   
18.
Abstract

The combined use of Global Positioning System (GPS) differential positioning as well as ERS‐1 altimeter data is considered in implementing geodetic vertical datums and their unification. The article describes concepts, techniques, practical realization, and associated questions and problems. Particular aspects in view of small sea surface perturbations in offshore areas in determining sea surface components (variable and steady state) are discussed. The combinations of tide gauge data with altimetry and (mainly) GPS positioning for geodetic purposes are discussed in detail. Special attention is devoted to the associated reference systems as well as to the combination of dynamic (level and nonlevel surfaces) with geometric quantities. The discussion is based on a specific ERS‐1 project supported by the National Science Foundation. Implications and practical impact of the project are outlined.  相似文献   
19.
Oscillatory or competitive nucleation about a binary (or perhaps pseudo binary) eutectic and ensuing crystal growth and settling is a commonly suggested means of producing layering in magmatic systems. A quantitative model is presented of this, outwardly, relatively simple process of crystal nucleation, growth, and settling in an otherwise initially crystal-free magma. Avrami-style kinetics of crystallization in an always wellmixed body, buried in conductive wall rock, are coupled to a Stokes-like formulation of crystal settling in magma whose viscosity depends on temperature and crystallinity. Two dimensionless numbers (Se, the settling number and Av, the Avrami or kinetic number) govern all the results. Av and Se measure the relative importance of crystallization time and settling time, respectively, relative to the overall cooling time. For any value of Av, which increases strongly with the maximum nucleation and growth rates and cooling time, layering is possible only over a range or window of values of Se. Both above and below this window a single layer (crystalline below, vitric above) forms, and within this window the number of layers increases systematically with increasing Av and Se. Grain size within any single layer generally coarsens upward. Because the characteristic settling and cooling times both depend on body thickness, the lower limit of the settling window is also dependent on sheet thickness. Within the confines of this model and for nucleation and growth rates set by those observed in natural systems, layering is unlikely in sheet-like magmas thinner than about 100 m. When the body is not always well-mixed and crystallization is within inward-propagating solidification fronts, it is expected that this minimum body thickness will increase.  相似文献   
20.
Part of the lander payload for the comet rendezvous mission Rosetta is the thermal probe multi-purpose sensors for surface and subsurface science (MUPUS). In this paper, we discuss the relationship of the expected MUPUS data to structural and textural parameters of the near-surface layers of the cometary nucleus. Such properties could be crucial parameters concerning the formation and evolution of the nucleus. Thus, we calculate the thermal conductivity of a porous material in terms of microstructural parameters, using a geometrical model with a solid matrix, a surrounding pore space and a distinct contact area between different particles. We include the possibility that a significant amount of heat may be transported by pore filling vapour in addition to heat conducted via the matrix. Furthermore, we consider that the heat is transmitted through only a fraction of the grains and these are organized into a chain-like structure. These chains—and not the single grains—should be regarded as the basic unit of structure. Applying our model to measured thermal conductivities of porous water ice, we interpret the material in terms of microparameters and estimate the effective size of the contact area and the effective pore radius. The results are in good agreement with our knowledge of the prepared samples. Contrary, we can also show that popular models used in cometary research do not fit with laboratory data at all.  相似文献   
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