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排序方式: 共有372条查询结果,搜索用时 171 毫秒
341.
颗粒体系是远离平衡态的复杂耗散体系,在外力驱动下,能表现出类似于固体、液体或气体的特性.在稀疏的颗粒气体中,由于耗散,体系中常常会形成颗粒在局部的凝聚.这种凝聚行为具有自发对称性破缺的特征,使得颗粒气体中自动形成高密度区域和低密度区域共存的非均匀状态,类似于气体中液滴的形成.本文利用零重力条件下的三维颗粒气体理论模型进行计算和分析,揭示了颗粒气体这一相变行为的不稳定性根源及类气液相变本质,给出了颗粒气体中这种相分离发生的临界条件,并通过分子动力学模拟进行检验,结果从定性上和定量上都能够很好地吻合,为进一步的空间实验提供了理论依据和相关实验参数. 相似文献
342.
Microbial populations in Antarctic permafrost: biodiversity, state, age, and implication for astrobiology 总被引:1,自引:0,他引:1
Gilichinsky DA Wilson GS Friedmann EI McKay CP Sletten RS Rivkina EM Vishnivetskaya TA Erokhina LG Ivanushkina NE Kochkina GA Shcherbakova VA Soina VS Spirina EV Vorobyova EA Fyodorov-Davydov DG Hallet B Ozerskaya SM Sorokovikov VA Laurinavichyus KS Shatilovich AV Chanton JP Ostroumov VE Tiedje JM 《Astrobiology》2007,7(2):275-311
Antarctic permafrost soils have not received as much geocryological and biological study as has been devoted to the ice sheet, though the permafrost is more stable and older and inhabited by more microbes. This makes these soils potentially more informative and a more significant microbial repository than ice sheets. Due to the stability of the subsurface physicochemical regime, Antarctic permafrost is not an extreme environment but a balanced natural one. Up to 10(4) viable cells/g, whose age presumably corresponds to the longevity of the permanently frozen state of the sediments, have been isolated from Antarctic permafrost. Along with the microbes, metabolic by-products are preserved. This presumed natural cryopreservation makes it possible to observe what may be the oldest microbial communities on Earth. Here, we describe the Antarctic permafrost habitat and biodiversity and provide a model for martian ecosystems. 相似文献
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Sherwood Lollar B Voglesonger K Lin LH Lacrampe-Couloume G Telling J Abrajano TA Onstott TC Pratt LM 《Astrobiology》2007,7(6):971-986
Dissolved H(2) concentrations up to the mM range and H(2) levels up to 9-58% by volume in the free gas phase are reported for groundwaters at sites in the Precambrian shields of Canada and Finland. Along with previously reported dissolved H(2) concentrations up to 7.4 mM for groundwaters from the Witwatersrand Basin, South Africa, these findings indicate that deep Precambrian Shield fracture waters contain some of the highest levels of dissolved H(2) ever reported and represent a potentially important energy-rich environment for subsurface microbial life. The delta (2)H isotope signatures of H(2) gas from Canada, Finland, and South Africa are consistent with a range of H(2)-producing water-rock reactions, depending on the geologic setting, which include both serpentinization and radiolysis. In Canada and Finland, several of the sites are in Archean greenstone belts characterized by ultramafic rocks that have under-gone serpentinization and may be ancient analogues for serpentinite-hosted gases recently reported at the Lost City Hydrothermal Field and other hydrothermal seafloor deposits. The hydrogeologically isolated nature of these fracture-controlled groundwater systems provides a mechanism whereby the products of water-rock interaction accumulate over geologic timescales, which produces correlations between high H(2) levels, abiogenic hydrocarbon signatures, and the high salinities and highly altered delta (18)O and delta (2)H values of these groundwaters. A conceptual model is presented that demonstrates how periodic opening of fractures and resultant mixing control the distribution and supply of H(2) and support a microbial community of H(2)-utilizing sulfate reducers and methanogens. 相似文献
344.
This paper presents simulated results of the ionospheric behavior during few geomagnetic storms,which were occurred in the different seasons. The numerical model for ionosphere-plasmasphere coupling was used to interpret the observed variation of ionosphere structure. Reasons why the positive storms are dominant in the winter whereas the negative ones are dominant in the summer season present the special interest for the mid-latitude ionosphere. A theoretical analysis of the processes controlling the ionospheric response to the geomagnetic storms has showed a good agreement between the simulated results and measurements, as well as the crucial role of the neutral composition variations to fit the calculated and the observed ionospheric parameters. 相似文献
345.
煤油在超声速气流中非定常横向喷射的实验观察 总被引:4,自引:0,他引:4
本文目的是研究煤油在超声速气流中非定常横向喷射问题,并与静止和亚声速气流中的结果进行对比。改造普通油嘴并采用单次高压燃油喷射系统,得到不同延时的流场阴影照片。结果表明:在超声速气流中,射流柱破碎是由迎风面的表面波引起的,破碎点位于表面波的波谷。雾化过程过分四个区域。射流激波由近射流柱的迎风面,并在人壁反射,非定常喷射的波系演化复杂。煤油在亚声速气流中喷射,不存在射流激波,破碎点沿射流柱上移,穿透深 相似文献
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348.
Gypsum filled "pipe" features were discovered in the proglacial area of the Borup Fiord Pass supraglacial sulfur spring. Stable isotope data suggest that gypsum is formed through oxidation of sulfides and are consistent with models of sulfuric acid speleogenesis. These results suggest that gypsum pipes are paleo-spring discharge channels analogous to those that feed the modern sulfur spring at Borup Fiord. A conceptual model is proposed whereby retreat of the glacial front and associated growth of permafrost in ground exposed now to low arctic temperatures leads to "freezing-in" of the spring system and abandonment of old channels in favor of more open flow systems in the subglacial region. Results provide a model for glacially driven groundwater systems that may form in association with Mars' polar icecaps and potential geological signatures for paleo-groundwater discharge. 相似文献
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波音早前收购了位于南卡罗来纳州的787机身组装工厂,从而控制了麻烦不断的波音787供应链的关键环节。但分析家认为,该计划并不表明波音公司会放弃把飞机主要部件外包给一级供应商的策略,因为外包策略能提高工作效率且成本更低。 相似文献