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Do large craters on Mars represent sites that contain aqueous and hydrothermal deposits that provide clues to astrobiological processes? Are these materials available for sampling in large craters? Several lines of evidence strongly support the exploration of large impact craters to study deposits important for astrobiology. The great depth of impact craters, up to several kilometers relative to the surrounding terrain, can allow the breaching of local aquifers, providing a source of water for lakes and hydrothermal systems. Craters can also be filled with water from outflow channels and valley networks to form large lakes with accompanying sedimentation. Impact melt and uplifted basement heat sources in craters > 50 km in diameter should be sufficient to drive substantial hydrothermal activity and keep crater lakes from freezing for thousands of years, even under cold climatic conditions. Fluid flow in hydrothermal systems is focused at the edges of large planar impact melt sheets, suggesting that the edge of the melt sheets will have experienced substantial hydrothermal alteration and mineral deposition. Hydrothermal deposits, fine-grained lacustrine sediments, and playa evaporite deposits may preserve evidence for biogeochemical processes that occurred in the aquifers and craters. Therefore, large craters may represent giant Petri dishes for culturing preexisting life on Mars and promoting biogeochemical processes. Landing sites must be identified in craters where access to the buried lacustrine sediments and impact melt deposits is provided by processes such as erosion from outflow channels, faulting, aeolian erosion, or excavation by later superimposed cratering events. Very recent gully formation and small impacts within craters may allow surface sampling of organic materials exposed only recently to the harsh oxidizing surface environment. 相似文献
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A numerical investigation of the melt flow and heat and mass transfer is carried out at the crystal growth under zero gravity, when the melt detachment from ampoule walls, crystal vibration, and various magnetic fields are active. Specific features of the melt flow are demonstrated depending on the size of a detachment zone adjacent to the crystallization boundary. The velocity of the averaged flow generated by crystal vibration is determined as a function of the vibration intensity. It is shown that the crystal vibration cannot compensate a thermal capillary flow (caused by detachment of the melt from the ampoule wall) and reduce the macrosegregation of impurities. It is shown that the application of steady and rotating magnetic fields are inefficient for all ampoule methods of crystal growth under microgravity conditions. 相似文献
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《Acta Astronautica》2001,48(2-3):93-100
Numerical simulations were performed to optimize the conditions and parameters for directional solidification of Te-doped GaSb in reduced gravity ranging from 10−3 to 10−5g0. Our key goal was to quantify the velocity and concentration fields with and without a baffle present in the melt. The effect of the distance of the baffle from the solid–liquid interface was investigated. When the baffle is placed 0.5 cm from the solid–liquid interface, acceleration of 10−3g0 does not cause significant interference with segregation. Furthermore, the flow between the baffle and the interface (low Reynolds number “creeping” flow) does not depend on fluid properties (viscosity). 相似文献
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BiSbTe3-mixed crystals have been grown at normal and reduced gravity (during the MIR'97-mission) using a
-configuration of the TITUS facility. The distribution of the components in the melt, and so the homogeneity of the growing crystal, is strongly influenced by the flow in the melt even in the case of weak convection. The flow configuration in the melt especially in front of the solid-liquid phase boundary can be investigated by means of a segregation analysis of the system components and an additional Pb-dopant. The BiSbTe3-system is because of its hydro-dynamic properties a typical representative of semiconductor melts (low
number, high
number) but there are also some special properties relating to the segregationally caused enrichment of the lighter tellurium at the phase boundary and the resulting solutal destabilities. Experimental experiences from segregation analysis have shown that the mass transport in the melt at normal gravity is mainly influenced by convective mixing determined by thermally and solutally caused buoyancy forces. Numerical simulations have been performed for the real experimentally used configurations. These simulations have shown that a strong coupling of thermal and solutal effects exists and have given axial as well as radial segregation profiles being in excellent agreement with the experimental results for the vertical normal gravity grown crystals. For micro gravity conditions a reduction of the flow velocity of more than two orders of magnitude (depending on the micro gravity level and the direction of the residual acceleration) resulting in diffusion controlled component segregation has been predicted.The results of the two micro gravity grown crystals, especially the axial and radial segregation profiles as a sensitive indicator for the flow configuration in front of the phase boundary will be given and discussed in the paper. They will be compared with the results of numerical simulations of the melt flow for the real processing parameters measured during the TITUS growth processes and with experimental as well as numerical results for vertical normal gravity grown reference Samples. 相似文献
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《Acta Astronautica》1999,44(7-12):635-643
BiSbTe3-mixed crystals have been grown at normal and reduced gravity (during the MIR'97-mission) using a Bridgman-configuration of the TITUS facility. The distribution of the components in the melt, and so the homogeneity of the growing crystal, is strongly influenced by the flow in the melt even in the case of weak convection. The flow configuration in the melt especially in front of the solid-liquid phase boundary can be investigated by means of a segregation analysis of the system components and an additional Pb-dopant. The BiSbTe3-system is because of its hydro-dynamic properties a typical representative of semiconductor melts (low Prandtl number, high Schmidt number) but there are also some special properties relating to the segregationally caused enrichment of the lighter tellurium at the phase boundary and the resulting solutal destabilities. Experimental experiences from segregation analysis have shown that the mass transport in the melt at normal gravity is mainly influenced by convective mixing determined by thermally and solutally caused buoyancy forces. Numerical simulations have been performed for the real experimentally used configurations. These simulations have shown that a strong coupling of thermal and solutal effects exists and have given axial as well as radial segregation profiles being in excellent agreement with the experimental results for the vertical normal gravity grown crystals. For micro gravity conditions a reduction of the flow velocity of more than two orders of magnitude (depending on the micro gravity level and the direction of the residual acceleration) resulting in diffusion controlled component segregation has been predicted.The results of the two micro gravity grown crystals, especially the axial and radial segregation profiles as a sensitive indicator for the flow configuration in front of the phase boundary will be given and discussed in the paper. They will be compared with the results of numerical simulations of the melt flow for the real processing parameters measured during the TITUS growth processes and with experimental as well as numerical results for vertical normal gravity grown reference Samples. 相似文献
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The effect of constant and time-dependent accelerations (vibrations) on the melt flow and heat and mass transfer in the process of crystal growth by the method of directional crystallization (Bridgman method) onboard spacecraft is numerically investigated. The mathematical formulation of the problem and the technique to solve it numerically are given. The time-averaged flow arising under the action of vibrations in a nonisothermal fluid is investigated. With the help of a rational choice of dimensionless similitude parameters, a generalized dependence on the intensity of melt flow is obtained for the radial segregation of dopants. This dependence is invariant with respect to the type of motive power and thermal boundary conditions in the region of very small velocities of melt flow (creeping flow), which are characteristic for microgravity conditions. The allowable levels of constant accelerations, as well as the frequency dependences of tolerable vibrations, are obtained for five typical semiconductor materials: Ge(Ga), GaAs(Te), InSb(Te), Si(P), and Si(B). It is shown that the radial segregation of dopant is much more sensitive to microaccelerations than the axial one. In the region of small velocities, the latter is determined by the duration of the transition regime, which depends on certain physical properties of the melt. New problems that resulted from the investigations performed are discussed. 相似文献
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Organic macromolecules ("complex tholins") were synthesized from a 0.95 N(2)/0.05 CH(4) atmosphere in a high-voltage AC flow discharge reactor. When placed in liquid water, specific water soluble compounds in the macromolecules demonstrated Arrhenius type first order kinetics between 273 and 313 K and produced oxygenated organic species with activation energies in the range of approximately 60+/-10 kJ mol(-1). These reactions displayed half lives between 0.3 and 17 days at 273 K. Oxygen incorporation into such materials--a necessary step toward the formation of biological molecules--is therefore fast compared to processes that occur on geologic timescales, which include the freezing of impact melt pools and possible cryovolcanic sites on Saturn's organic-rich moon Titan. 相似文献
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火箭发射时其燃烧尾焰的冲击干扰效应对发射稳定性和发射架、导流槽等地面设施有重要影响。文章采用压力隐式算子分裂算法,通过求解Navier-Stokes方程,对氢氧液体火箭发动机燃烧室内燃烧过程与尾焰流场进行了一体化数值计算,得到了火箭发射后尾焰与地面撞击产生的冲击流场。结果表明:尾焰流场计算模型、方法与结果合理;尾焰冲击干扰效应会大幅度提高地面附近的压力和温度;火箭尾焰撞击地面后,高温区出现在离地面一定距离的高温层内,此时地面附近为低速区;尾焰对其正下面的地面区域产生冲击最大,主要干扰区域集中于半径为15 m的圆形区域。 相似文献
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Heat transfer of thermocapillary convection in a two-layered fluid system under the influence of magnetic field 总被引:1,自引:0,他引:1
Daniele Ludovisi Soyoung S. Cha Narayanan Ramachandran William M. Worek 《Acta Astronautica》2009,64(11-12):1066-1079
Heat transfer of a two-layer fluid system has been of great importance in a variety of industrial applications. For example, the phenomena of immiscible fluids can be found in materials processing and heat exchangers. Typically in solidification from a melt, the convective motion is the dominant factor that affects the uniformity of material properties. In the layered flow, thermocapillary forces can come into an important play, which was first emphasized by a previous investigator in 1958. Under extraterrestrial environments without gravity, thermocapillary effects can be a more dominant factor, which alters material properties in processing. Control and optimization of heat transfer in an immiscible fluid system need complete understanding of the flow phenomena that can be induced by surface tension at a fluid interface. The present work is focused on understanding of the magnetic field effects on thermocapillary convection, in order to optimize material processing. That is, it involves the study of the complicated phenomena to alter the flow motion in crystal growth. In this effort, the Marangoni convection in a cavity with differentially heated sidewalls is investigated with and without the influence of a magnetic field. As a first step, numerical analyzes are performed, by thoroughly investigating influences of all pertinent physical parameters. Experiments are then conducted, with preliminary results, for comparison with the numerical analyzes. 相似文献
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Two phase gas–liquid flow in pipes is widely spread in space applications: bubble flows appear in cryogenic components transport through fuel/oxidant supply lines. Another important application is based on the fact that in liquid flows with small bubbles a close contact between the two phases occurs resulting in high rates of transfer between them. The compactness of a system makes it ideally suited to serve as a space-based two-phase bio-reactor which forms an important unit in environmental control and life support system deployed onboard. A numerical method was developed for solving a nonlinear problem of thermal interaction between a spherical gas bubble and surrounding liquid. The system of equations for describing this interaction was formulated. It includes ordinary and nonlinear partial differential equations. The problem was solved using finite-difference technique by dividing the system into spherical layers inside the bubble and employing the new variable which “freezes” the moving boundary of the bubble. A numerical solution is obtained for the problem of radial bubble motion induced by a sudden pressure change in the liquid—a situation which corresponds to the behavior of bubbles beyond a shock wave front when the latter enters a bubble curtain. 相似文献
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一种新型深空探测样品封装技术 总被引:1,自引:0,他引:1
对样品进行真空封装是深空探测取样及返回任务中的一项核心技术,它能够成功维持样品成分原态。文章提出了一种能够符合我国深空探测任务的样品封装技术,其封装机构由复合式开合机构及火工锁紧机构组成,是一种极低漏率、高可靠的多层金属真空密封容器。通过对这种复合式开合机构和火工锁紧机构的特性方程的推导,对多层金属真空密封的材料匹配等机理的分析,确认这种新型的样品封装技术的原理是正确和可行的。最后对原理样机的性能进行了验证,结果表明:封装机构的体积小、功耗低、可靠性高,样机的密封性能良好,设计具有创新性及工程实用性。 相似文献
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Wasson JT 《Astrobiology》2003,3(1):163-179
Large aerial bursts similar to the 1908 Tunguska bolide but much larger in magnitude have surely been responsible for many catastrophic events in the history of the Earth. Because aerial bursts produce shallow (or even negligible) craters, their existence is difficult to document in the geological record. Even aerial bursts as small as Tunguska deposit enough energy to melt approximately 1mm of dry soil. Silica-rich glass formed in such melts has the potential to survive in the soil for many Ma, thus a potential indicator of large aerial bursts is glass that was formed as thick regions within silicate melt sheets. The layered tektites from Southeast Asia and the Libyan desert glass may have formed by a combination of sedimentation and downslope flow of silicate melt heated by radiation from large aerial bursts. The alternative, formation of layered tektites as crater ejecta, cannot account for observations such as uniformly high 10Be contents, the orientation of the magnetic remanence field, and the absence of splash-form (e.g., teardrop or dumbbell) tektites in regions where layered tektites are common. The largest asteroids or comets make craters no matter what their strength. Recent reviews suggest that, for events in the energy range up to 10(19)-10(20) J (about two orders of magnitude larger than the Meteor Crater impact), aerial bursts are more likely than cratering events, and the layered tektites of Southeast Asia imply the existence of aerial bursts one to two orders of magnitude larger still. 相似文献
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V.V. Iliukhin V.P. Shalimov S.J. Budurov P.D. Kovachev S.A. Toncheva 《Acta Astronautica》1984,11(9):585-592
Thermodynamics relations have been considered under which gas inclusions may form in melts (closed systems), due to both volatility of components and the presence of dissolved gases. It has been shown that in the melts with sufficient volatility of components, gas inclusions may form under equilibrium conditions if the ampoule is wetted well by the melt, whereas in case of low wettability they form under non-equilibrium state only. The foaming in the tellurium-selenium system has been analysed (“Pirin”, KP-4 experiment).In the melts containing dissolved gases, gas inclusions form under conditions of deviation from equilibrium and it is the main mechanism of outgassing. It has been demonstrated that the melt foaming method using hydrid compounds may be applied to the materials exhibiting a hysteresis of temperature dependence of diluted hydrogen concentration. Production of foam aluminium under zero gravity state has been considered as an example (“Pirin”, SP-3 experiment). 相似文献
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This study deals with numerical simulations of the Maxus sounding rocket experiment on oscillatory Marangoni convection in liquid bridges. The problem is investigated through direct numerical solution of the non-linear, time-dependent, three-dimensional Navier–Stokes equations. In particular, a liquid bridge of silicon oil 2[cs] with a length L=20 [mm] and a diameter D=20 (mm) is considered. A temperature difference ΔT=30 [K] is imposed between the supporting disks, by heating the top disk and cooling the bottom one with different rates of ramping. The results show that the oscillatory flow starts as an ‘axially running wave', but after a transient time the instability is described by the dynamic model of a ‘standing wave', with an azimuthal spatial distribution corresponding to m=1 (where m is the critical wave number). After the transition, the disturbances become larger and the azimuthal velocity plays a more important role and the oscillatory field is characterized by a travelling wave. The characteristic times for the onset of the different flow regimes are computed for different rates of ramping. 相似文献
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A possibility of attaining steady flow of detonation products with specific energy much larger than the specific chemical energy of explosive is demonstrated in the case when a cylindrical charge of explosive is fitted with an evacuated cavity. Simple estimates and results of numerical analysis of the process are presented. Steady process may be considered to occur under the following assumptions: (1) effects arising due to jet interaction with cavity walls are negligible; (2) the detonation process is steady. In the case of limited explosive lengths these assumptions have been shown to be correct.When the cavity is filled with gas or liquid, a variety of steady and non-steady flow regimes is possible, depending on the density of the filling medium. One well-known case is that of flow with irregular reflection of shock waves at the cavity axis accompanied with the formation of Mach intersections. Another interesting flow regime is observed to occur in the case of low density filling medium (liquid hydrogen, for example). In this case the filling medium is driven by a “detonation piston” at constant velocity, equal to the velocity of detonation, forming a uniform growing column of hot shock-compressed matter, specific energy of which exceeds by one order of magnitude the specific energy of the explosive. Obviously, the walls of the vessel containing hydrogen must be able to withstand radial loads for a sufficiently long time (20 μ sec).The relative merits of these methods in comparison to others in high speed gas-dynamics is discussed. 相似文献
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对电磁驱动液态金属热控系统进行了分析,与水工质回路的传热特性比较表明:在进行高功率密度器件散热方面,液态金属换热性能明显优于水等常规工质,在较小的流速下,可以保持较低的器件表面温度;更为重要的是,它可以采用无运动部件电磁泵驱动,具有可靠性高、振动噪声小、结构简单紧凑、功耗小、可控性强等优点。该系统不仅可解决空间高热流密度器件散热难题,而且在微小卫星、月球/行星表面探测器等主动热控制方面也有重要的应用前景。 相似文献