共查询到20条相似文献,搜索用时 15 毫秒
1.
Stuart J. Weidenschilling 《Space Science Reviews》2000,92(1-2):295-310
Planetesimals formed in the solar nebula by collisional coagulation. Dust aggregates settled toward the central plane, the larger ones growing by sweeping up smaller ones. A thin, dense layer of particles formed; shear-generated turbulence and differential motions induced by gas drag inhibited gravitational instability. Growth proceeded by collisions, producing planetesimals on a timescale of a few thousand years in the terrestrial zone. For bodies smaller than about a kilometer, motions were dominated by gas drag, and impact velocities decreased with size. At larger sizes gravitational interactions became significant, and velocities increased due to mutual perturbations. Larger bodies then grew more rapidly, this ``runaway' led to formation of tens to hundreds of lunar- to Mars-sized planetary embryos in the zone of terrestrial planets. The final accretion of these bodies into a few planets involved large impacts, and occurred on a timescale of 107 to 108 years. This scenario gives a reasonably consistent picture of the origin of the terrestrial planets, but does not account for the anomalously low eccentricities of the Earth and Venus. This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
2.
Ramon Brasser 《Space Science Reviews》2013,174(1-4):11-25
In this review paper I address the current knowledge of the formation of Mars, focusing on its primary constituents, its formation time scale and its small mass compared to Earth and Venus. I argue that the small mass of Mars requires the terrestrial planets to have formed from a narrow annulus of material, rather than a disc extending to Jupiter. The truncation of the outer edge of the disc was most likely the result of giant planet migration, which kept Mars’ mass small. From cosmochemical constraints it is argued that Mars formed in a couple of million years and is essentially a planetary embryo that never grew to a full-fledged planet. This is in agreement with the latest dynamical models. Most of Mars’ building blocks consists of material that formed in the 2 AU to 3 AU region, and is thus more water-rich than that accreted by Earth and Venus. The putative Mars could have consisted of 0.1 % to 0.2 % by mass of water. 相似文献
3.
J. Smak 《Space Science Reviews》1989,50(1-2):107-116
Basic theoretical concepts concerning formation and properties of disks in close binary systems are reviewed and compared with observations of Algols. 相似文献
4.
J. Smak 《Space Science Reviews》1989,49(3-4):107-116
Basic theoretical concepts concerning formation and properties of disks in close binary systems are reviewed and compared with observations of Algols. 相似文献
5.
A. Lazarian 《Space Science Reviews》2014,181(1-4):1-59
Astrophysical fluids are turbulent a fact which changes the dynamics of many key processes, including magnetic reconnection. Fast reconnection of magnetic field in turbulent fluids allows the field to change its topology and connections. As a result, the traditional concept of magnetic fields being frozen into the plasma is no longer applicable. Plasma associated with a given magnetic field line at one instant is distributed along a different set of magnetic field lines at the next instant. This diffusion of plasmas and magnetic field is enabled by reconnection and therefore is termed “reconnection diffusion”. The astrophysical implications of this concept include heat transfer in plasmas, advection of heavy elements in interstellar medium, magnetic field generation etc. However, the most dramatic implications of the concept are related to the star formation process. The reason is that magnetic fields are dynamically important for most of the stages of star formation. The existing theory of star formation has been developed ignoring the possibility of reconnection diffusion. Instead, it appeals to the decoupling of mass and magnetic field arising from neutrals drifting in respect to ions entrained on magnetic field lines, i.e. through the process that is termed “ambipolar diffusion”. The predictions of ambipolar diffusion and reconnection diffusion are very different. For instance, if the ionization of media is high, ambipolar diffusion predicts that the coupling of mass and magnetic field is nearly perfect. At the same time, reconnection diffusion is independent of the ionization but depends on the scale of the turbulent eddies and on the turbulent velocities. In the paper we explain the physics of reconnection diffusion both from macroscopic and microscopic points of view, i.e. appealing to the reconnection of flux tubes and to the diffusion of magnetic field lines. We make use of the Lazarian and Vishniac (Astrophys. J. 517:700, 1999) theory of magnetic reconnection and show that this theory is applicable to the partially ionized gas. We quantify the reconnection diffusion rate both for weak and strong MHD turbulence and address the problem of reconnection diffusion acting together with ambipolar diffusion. In addition, we provide a criterion for correctly representing the magnetic diffusivity in simulations of star formation. We discuss the intimate relation between the processes of reconnection diffusion, field wandering and turbulent mixing of a magnetized media and show that the role of the plasma effects is limited to “breaking up lines” on small scales and does not affect the rate of reconnection diffusion. We address the existing observational results and demonstrate how reconnection diffusion can explain the puzzles presented by observations, in particular, the observed higher magnetization of cloud cores in comparison with the magnetization of envelopes. We also outline a possible set of observational tests of the reconnection diffusion concept and discuss how the application of the new concept changes our understanding of star formation and its numerical modeling. Finally, we outline the differences of the process of reconnection diffusion and the process of accumulation of matter along magnetic field lines that is frequently invoked to explain the results of numerical simulations. 相似文献
6.
王人美是上海滩上一个非常富有传奇色彩的两栖明星,她的歌声以甜美、清脆曾在上海风靡一时,她在电影中所扮演的电影角色以活泼、野性曾在影迷中红极一时而使人青睐不已。但是,王人美在人生的旅途生涯中却充满了悲剧与磨难。 相似文献
7.
The early development of Mars is of enormous interest, not just in its own right, but also because it provides unique insights into the earliest history of the Earth, a planet whose origins have been all but obliterated. Mars is not as depleted in moderately volatile elements as are other terrestrial planets. Judging by the data for Martian meteorites it has Rb/Sr 0.07 and K/U 19,000, both of which are roughly twice as high as the values for the Earth. The mantle of Mars is also twice as rich in Fe as the mantle of the Earth, the Martian core being small (20% by mass). This is thought to be because conditions were more oxidizing during core formation. For the same reason a number of elements that are moderately siderophile on Earth such as P, Mn, Cr and W, are more lithophile on Mars. The very different apparent behavior of high field strength (HFS) elements in Martian magmas compared to terrestrial basalts and eucrites may be related to this higher phosphorus content. The highly siderophile element abundance patterns have been interpreted as reflecting strong partitioning during core formation in a magma ocean environment with little if any late veneer. Oxygen isotope data provide evidence for the relative proportions of chondritic components that were accreted to form Mars. However, the amount of volatile element depletion predicted from these models does not match that observed — Mars would be expected to be more depleted in volatiles than the Earth. The easiest way to reconcile these data is for the Earth to have lost a fraction of its moderately volatile elements during late accretionary events, such as giant impacts. This might also explain the non-chondritic Si/Mg ratio of the silicate portion of the Earth. The lower density of Mars is consistent with this interpretation, as are isotopic data. 87Rb-87Sr, 129I-129Xe, 146Sm-142Nd, 182Hf-182W, 187Re-187Os, 235U-207Pb and 238U-206Pb isotopic data for Martian meteorites all provide evidence that Mars accreted rapidly and at an early stage differentiated into atmosphere, mantle and core. Variations in heavy xenon isotopes have proved complicated to interpret in terms of 244Pu decay and timing because of fractionation thought to be caused by hydrodynamic escape. There are, as yet, no resolvable isotopic heterogeneities identified in Martian meteorites resulting from 92Nb decay to 92Zr, consistent with the paucity of perovskite in the martian interior and its probable absence from any Martian magma ocean. Similarly the longer-lived 176Lu-176Hf system also preserves little record of early differentiation. In contrast W isotope data, Ba/W and time-integrated Re/Os ratios of Martian meteorites provide powerful evidence that the mantle retains remarkably early heterogeneities that are vestiges of core metal segregation processes that occurred within the first 20 Myr of the Solar System. Despite this evidence for rapid accretion and differentiation, there is no evidence that Mars grew more quickly than the Earth at an equivalent size. Mars appears to have just stopped growing earlier because it did not undergo late stage (>20 Myr), impacts on the scale of the Moon-forming Giant Impact that affected the Earth. 相似文献
8.
Frank Verbunt 《Space Science Reviews》1982,32(4):379-404
This review presents an introduction to the theory of accretion disks. After explaining the importance of accretion disks in stellar X-ray sources, it considers observational and theoretical indications of the formation of accretion disks in close binary systems. The simple Shakura and Sunyaev model and its problems are discussed. A survey of other models that try to improve upon this model is given (in Table III), and critically discussed. 相似文献
9.
Two fractionation models are applied to the problem of generating the widely distributed “Q-component” noble gases in meteorites
from the solar-like isotopic and elemental compositions that presumably characterized the early solar accretion disk. Noble
gas fractionation by mass-dependent dissipation of the solar nebula, as suggested by Ozima et al. (1998), is examined in the context of a model developed by Johnstone et al. (1998) for accretion disk photoevaporation driven by intense UV radiation from a neighboring giant star. Hydrodynamic escape
of heavier species entrained in hydrogen outflow from the UV-heated outer regions of the disk can generate substantial noble
gas fractionations, but they do not match the observed Q-component isotopic pattern and moreover require the physically unrealistic
assumption that the fractionated gases are confined to the heated disk boundary zone, without mixing with the interior nebula,
for long periods of time. It seems more likely that hydrodynamic outflow is actually established below this zone, in the body
of the disk. In this case fractionations are governed by Rayleigh distillation of the entire remaining nebula, and are negligible
at the time when disk erosion is halted by the gravitational potential of the young sun embedded in the disk.
A “local” model of noble gas fractionation by hydrodynamic blowoff of transient, methane-rich atmospheres outgassed from the
interiors of large primitive planetesimals (Pepin, 1991) is updated and assessed against current data. Degassed atmospheres
are assumed to contain isotopically solar noble gases except for an additional nucleogenic Xe component that contributes primarily
to the two heaviest isotopes; there is evidence that this same component is present at varying levels in other solar-system
volatile reservoirs, possibly reflecting a compositional change with time in the solar nebula. Single fixed values for the
two free parameters in the blowoff modeling equations can generate fractionated Xe, Kr, Ar and Ne compositions in the residual
atmosphere that closely match observed meteoritic isotopic distributions, and Q-gas elemental ratios are approximated by adsorption
of fractionated gases on planetesimal surface grains using plausible values of relative Henry Law constants. Additional requirements
for adsorption of sufficient absolute amounts of Q-gases on carrier grains, and their subsequent ejection to space, mixing
in the nebula, and dispersal into meteorite bodies, are examined in the context of current models for body sizes and dynamical
evolution in an early mass-rich asteroid belt (Chambers and Wetherill, 2001). Despite its ability to replicate isotopic compositions,
uncertainties about the environments in which the blowoff model can successfully operate suggest that there is, as yet, no
entirely satisfactory understanding of how the Q-component noble gases might have evolved from solar-like precursor compositions.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
10.
AB139是上世纪90年代中期,阿古斯塔公司在进行A129武装直升机"国际型"开发计划时,以为A129"国际型"研发的动部件系统为基础研发的一种6吨级双发通用直升机。当时该机被命名为A139 相似文献
11.
Wladyslaw Altermann 《Space Science Reviews》2008,135(1-4):55-79
This paper reviews and discusses Archean stromatolite occurrences and their modes of growth in the context of sedimentary facies. Modes of sediment accretion and trapping and binding of sedimentary grains, together with the resulting morphology of stromatolites and microbial mats in the Archean are analysed, in order to show existing interaction between the growth patterns, morphology and facies association. Architectural elements of sediment arrangement in Archean stromatolites, together with the dependence of stromatolite distribution and morphology on sedimentary facies changes, clearly argue for a biological origin of stromatolitic lamination preserved in Archean cherts and carbonates. The observed sediment behaviour of laminae accretion and sediment precipitation, trapping and binding cannot be explained by abiogenic carbonate or silica precipitation from saturated solutions. The time-dependent, increasing complexity of stromatolitic structures in the Archean is an additional strong argument for biologic impact on stromatolite formation. Therefore, biogenic stromatolites and microbial mats were undoubtfully present at 3.5 Ga and occupied an increasingly wide range of sedimentary environments during the Archean. 相似文献
12.
The two-loop, adaptive-control strategy employed by Lee and Yu is shown to consist of adjustment of loop gains so that no output feedback occurs. This means that in so far as the adaptive feature is required, both loops could be dispensed with and feedback applied locally around the input modulator. 相似文献
13.
直15,/EC175是中航二集团所属哈飞公司和欧直公司按50%对50%比例研制的新一代6吨级中型双发多用途民用直升机。凭借着宽敞、舒适的乘坐环境、安全、高效益的运行品质,以及良好的人机操控界面,直15/EC175在同量级产品中堪称佼佼者。 相似文献
14.
Anatol Cherepashchuk 《Space Science Reviews》2002,102(1-4):23-35
Basic properties of the unique object SS 433 are described. Observational spectroscopic and photometric manifestations of a precessing accretion disk around a relativistic object in this X-ray binary system are presented. 相似文献
15.
改变了飞机生产线的精益技术正在维修产品上找到新的应用,可大量节省修理时间。一架美国陆军的AH-64阿帕奇直升机经过在伊拉克长达1年的服役,机上的弹孔多是草草打上补丁 相似文献
16.
Non-thermal components are key ingredients for understanding clusters of galaxies. In the hierarchical model of structure
formation, shocks and large-scale turbulence are unavoidable in the cluster formation processes. Understanding the amplification
and evolution of the magnetic field in galaxy clusters is necessary for modelling both the heat transport and the dissipative
processes in the hot intra-cluster plasma. The acceleration, transport and interactions of non-thermal energetic particles
are essential for modelling the observed emissions. Therefore, the inclusion of the non-thermal components will be mandatory
for simulating accurately the global dynamical processes in clusters. In this review, we summarise the results obtained with
the simulations of the formation of galaxy clusters which address the issues of shocks, magnetic field, cosmic ray particles
and turbulence. 相似文献
17.
18.
离散涡方法(DVM)是一种无网格的涡运动算法,适用于解决易产生分离流的非定常问题,将其应用于结冰过程中的流场求解,在有效模拟分离流动的同时能避免冰形尖角对网格质量的影响。但离散涡方法基于不可压 N-S 方程,无法应用于预测可压缩流动下的结冰过程。本文在离散涡方法的基础上添加普朗特—格劳尔特压缩性修正,进行基于离散涡方法的可压缩流动下的数值模拟,并将其应用于翼型结冰预测;对流场分布、结冰冰形和结冰模型计算过程等仿真结果进行对比和分析。结果表明:引入压缩性修正后的离散涡方法能较好地模拟可压缩流动,与实验值相比,基于该方法得到的结冰数值模拟结果符合良好,对结冰数值模拟在工程上的应用提供了一定的参考。 相似文献
19.