共查询到20条相似文献，搜索用时 78 毫秒
1.
Linda T. ELKINS‐TANTON Pascale AUSSILLOUS Jos BICO David QUR John W. M. BUSH 《Meteoritics & planetary science》2003,38(9):1331-1340
Abstract— Splash‐form tektites are generally acknowledged to have the form of bodies of revolution. However, no detailed fluid dynamical investigation of their form and stability has yet been undertaken. Here, we review the dynamics and stability of spinning, translating fluid drops with a view to making inferences concerning the dynamic history of tektites. We conclude that, unless the differential speed between the molten tektite and ambient is substantially less than the terminal velocity, molten tektites can exist as equilibrium bodies of revolution only up to sizes of 3 mm. Larger tektites are necessarily non‐equilibrium forms and so indicate the importance of cooling and solidification during flight. An examination of the shapes of rotating, translating drops indicates that rotating silicate drops in air will assume the shapes of bodies of rotation if their rotational speed is 1% or more of their translational speed. This requirement of only a very small rotational component explains why most splash‐form tektites correspond to bodies of revolution. A laboratory model that consists of rolling or tumbling molten metallic drops reproduces all of the known forms of splashform tektites, including spheres, oblate ellipsoids, dumbbells, teardrops, and tori. The laboratory also highlights important differences between rolling drops and tumbling drops in flight. For example, toroidal drops are much more stable in the former than in the latter situation. 相似文献
2.
New models for the interiors of Io, Ganymede, and Callisto are proposed. The model of Io consists of a thin, high-rigidity outer layer separated from a solid interior by a thin, molten or partially molten shell. The modulus of rigidity of the outer layer must be at least 100 times larger than that of the underlying partially molten shell. These layers have thicknesses of order 100 km or less. The near-surface partially molten layer was most likely produced early in Io's history as a consequence of accretional heating; enhanced tidal heating in the outer rigid layer has kept the underlying region partially molten to the present day. The model of Ganymede consists of an ice outer layer, a shell of undifferentiated, primordial ice-silicate mixture, and a rock core. Accretional heating is responsible for melting the ice in the outer layers of Ganymede's initially homogeneous ice-silicate interior. Most of the rock in this outer layer accumulates in a shell on top of Ganymede's early cold and rigid central region; the water in the outer layer quickly refreezes. Heating of the undifferentiated region by the decay of radioactive elements in the silicate fraction would gradually warm it and reduce its viscosity. The rock layer would become gravitationally unstable and sink through the undifferentiated materials to form a rock core. Callisto's heavily cratered surface strongly suggests that relatively little, if any, ice-rock differentiation has occured in its interior. 相似文献
3.
4.
The suggestion that the Moon's magnetic field is due to adiabatic magnetohydrodynamic convection of a molten core has been made by a number of recent authors. Considerations based on petrology, mass and rotational inertia limit the size of this hypothetical core to a few 100 km at the most. A proposal has been made that this core is either molten iron or iron sulfide. Fortunately, we know the properties of both molten iron and iron-sulfide at lunar core pressures. We can find no way of maintaining circulation in a hypothetical lunar core, as circulation is contingent upon a temperature gradient being greater than the adiabatic gradient, or an internal heat source. 相似文献
5.
Jafar Arkani-Hamed 《Icarus》1975,26(3):313-320
The undulations of the Martian gravitational potential indicate lateral density variations in the Mars interior. A gravitating and solid Martian model deforms under the influence of these variations, producing stress differences of about 125 bars at a depth of about 200 km. Introduction of a partially molten core of 1300 km radius does not affect the stress distribution in the mantle significantly, whereas the assumption of a partially molten asthenosphere umderlying a solid lithosphere of about 300 km increases the stress differences appreciably. A strong linear correlation of the gravitational potential and the surface topography indicates that the extensive volcanism at the Tharsis region is a recent phenomenon. The high stresses associated with this region imply that there has been no extensive molten region within the upper 300 km since the volcanism. 相似文献
6.
Somenath Chakrabarty 《Astrophysics and Space Science》2008,314(1-3):105-112
The expulsion of magnetic flux lines from a growing superconducting core of a quark star has been investigated. The idea of
impurity diffusion in molten alloys and an identical mechanism of baryon number transport from hot quark-gluon-plasma phase
to hadronic phase during quark-hadron phase transition in the early universe, micro-second after big bang has been used. The
possibility of Mullins-Sekerka normal-superconducting interface instability has also been studied. 相似文献
7.
Ruth Ziethe 《Planetary and Space Science》2009,57(3):306-317
The formation of magma oceans on at least the major terrestrial planets is widely assumed even if the full accretion history and early evolution of terrestrial planets is discussed rather controversial. Various processes occur within these magma oceans, among them the settling of small iron drops and their chemical equilibration with the silicate environment. Different models were proposed to explain the differentiation within a magma ocean. In order to model a magma ocean and give constraints about settling time of iron droplets, a constant velocity is used in most models, which is calculated using Stokes’ famous formula. According to the other model parameters this assumption is invalid, since Stokes’ assumption of a creeping flow is violated. However, former models investigated the behaviour of a large number of droplets, which makes it is impossible to solve the flow around each single iron drop. We introduce a model, in which the flow around iron drops in a molten silicate environment is computed using the appropriate hydrodynamic equations. We investigate the terminal velocity of a single drop descending through a magma ocean and additionally study the effects of the presence of other obstacles in the neighbourhood. We determine a new mean velocity, which may serve as an input parameter for the existent models of magma oceans. Although we used a full fluid-dynamical approach, the velocity computed with our model is not very different from the simple Stokes case and thus proves that the former assumptions were legitimate and can be used in future as well as the new velocity presented here. 相似文献
8.
Manihar Singh Koijam 《Astrophysics and Space Science》1988,149(1):149-161
One of the models which have stable limit cycles but are very close to the transition of the type I intermittency is examined in some detail. The work integrals are calculated for nonlinear oscillations with various amplitudes. The model reaches its limit cycle by saturation of the driving forces due to the ionized helium (He+) ionization. By increasing amplitudes damping becomes superior to the driving forces and so the limit cycle is stable. However, with even larger amplitudes the model becomes pulsational unstable indicating a large positive contribution to the work integral at rather deep interior. Strong luminosity drops are observed in this region during contraction phase. It is shown that the drops come from the neutral helium and hydrogen (He and H) ionization zones moved down to the deep interior at contraction phase with increasing amplitudes. A shock wave is generated by the radiation pressure at the ionization zones and propagates outwards at the phase. The zone between the ionization zones and the detached shock front is compressed locally. Thus, subsequent contraction leads the pressure at the zone becomes very high, causing remarkable enhancement of the opacities. Thus the driving becomes to work efficiently. This is a main driving force with finite amplitudes beyond the limit cycle, and makes the model to have an unstable fixed point beyond it. 相似文献
9.
E. M. Nezhinskij 《Celestial Mechanics and Dynamical Astronomy》1985,35(4):383-396
This paper deals with the investigation of central configurations consisting of a point body, a homogeneous sphere and some drops of homogeneous ideal fluid. The existence of such central configurations, as well as the stability of the drops in linear approximation, has been proved by using the virial method (Chandrasekhar, 1969). 相似文献
10.
If a molten, or partially molten, lunar core exists at present, constraints would be placed on the viscosity of the solid
mantle and the distribution of radioactive heat sources. Models in which the heat sources have been concentrated near the
surface would rapidly solidify if the effective viscosity was equal to, or less than, 1022 cm2 s−1. Retention of most of the heat sources throughout the mantle would permit present day solid convection to occur without cooling
the core. 相似文献
11.
Jafar Arkani-Hamed 《Earth, Moon, and Planets》1973,6(1-2):135-163
Assuming that the lateral variations of density in the lunar crust, the crustal density anomalies, are responsible for the
lateral undulations of the lunar gravitational potential, we compute these anomalies for four different lunar models, which
include an entirely solid Moon and three different solid lunar models with partially molten layers located within 600 km depth.
The stress differences created by the density anomalies are determined for these models. It is found that, since the formation
of the mascons, the entirely solid lunar model should have supported stress differences of the order of 70 bars while in the
case of the other models, the solid layer overlying the partially molten one should have supported stress differences of more
than 100 bars. The high stress differences associated with the partially molten models lead us to conclude that these models
are not proper ones, and thus the Moon has always been solid since the formation of the mascons.
Lunar Science Institute Contribution No. 97.
The research in this paper was done while the author was a Visiting Scientist at the Lunar Science Institute, which is operated
by the Universities Space Research Association under Contract No. NSR 09-051-001 with the National Aeronautics and Space Administration. 相似文献
12.
The effect of Hall currents have been studied on the instability of a stratified layer of a self-gravitating finitely conducting plasma of varying density. It is assumed that the plasma is permeated by a variable horizontal magnetic field stratified vertically. The stability analysis has been carried out for longitudinal mode of wave propagation. The solution has been obtained through integral equation approach. The dispersion relation has been derived and solved numerically. It is found that both the Hall currents and finite conductivity have a destabilizing influence on the growth rate of the unstable mode of disturbance. 相似文献
13.
In the present paper we have studied the resonance problem arising for critical mass values following Birkhoff's method of normalisation. It has been shown that for both the critical values the motion will remain unstable whether the critical terms are left in the Hamiltonian or carried to the coordinates in the process of normalisation. 相似文献
14.
Thermal evolutions of the terrestrial planets 总被引：1，自引：0，他引：1
The thermal evolution of the Moon, Mercury, Mars, Venus and hypothetical minor planets is calculated theoretically, taking into account conduction, solid-state convection, and differentiation. An assortment of geological, geochemical, and geophysical data is used to constrain both the present day temperatures and thermal histories of the planets' interiors. Such data imply that the planets were heated during or shortly after formation and that all the terrestrial planets started their differentiations early in their history. Initial temperatures and core formation play the most important roles in the early differentiation. The size of the planet is the primary factor in determining its present day thermal state. A planetary body with radius less than 1000 km is unlikely to reach melting given heat source concentrations similar to terrestrial values and in the absence of intensive early heating such as short half-life radioactive heating and inductive heating.Studies of individual planets are constrained by varying amounts of data. Most data exist for the Earth and Moon. The Moon is a differentiated body with a crust, a thick solid mantle and an interior region which may be partially molten. It is presently cooling rapidly and is relatively inactive tectonically.Mercury most likely has a large core. Thermal calculations indicate it may have a 500 km thick solid lithosphere, and the core may be partially molten if it contains some heat sources. If this is not the case, the planet's interior temperatures are everywhere below the melting curve for iron. The thermal evolution is dominated by core separation and the high conductivity of iron which makes up the bulk of Mercury.Mars, intermediate in size among the terrestrial planets, is assumed to have differentiated an Fe–FeS core. Differentiation and formation of an early crust is evident from Mariner and Viking observations. Theoretical models suggest that melting and differentiation of the mantle silicates has occurred at least up until 1 billion years ago. Present day temperature profiles indicate a relatively thick (250 km) lithosphere with a possible asthenosphere below. The core is molten.Venus is characterized as a planet similar to the Earth in many respects. Core formation probably occurred during the first billion years after the formation. Present day temperatures indicate a partially molten upper mantle overlain by a 100 km thick lithosphere and a molten Fe–Ni core. If temperature models are good indicators, we can expect that today, Venus has tectonic processes similar to the Earth's.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May 1978. 相似文献
15.
The effects of uniform horizontal shear on a stably stratified layer of gas is studied. The system is initially destabilized by a magnetically buoyant flux tube pointing in the cross‐stream direction. The shear amplifies the initial field to Lundquist numbers of about 200–400, but then its value drops to about 100–300, depending on the value of the sub‐adiabatic gradient. The larger values correspond to cases where the stratification is strongly stable and nearly isothermal. At the end of the runs the magnetic field is nearly axisymmetric, i.e. uniform in the streamwise direction. In view of Cowling's theorem the sustainment of the field remains a puzzle and may be due to subtle numerical effects that have not yet been identified in detail. In the final state the strength of the magnetic field decreases with height in such a way that the field is expected to be unstable. Low amplitude oscillations are seen in the vertical velocity even at late times, suggesting that they might be persistent (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
16.
The Kelvin-Helmholtz discontinuity in two superposed viscous conducting fluids has been investigated in the presence of a two-dimensional horizontal uniform magnetic field. The streaming motion is also assumed to be two-dimensional. The stability analysis has been carried out for two highly viscous fluids of uniform densities. It is found that the streaming motion has dual influence on the unstable system, destabilizing for low values of streaming velocity and stabilizing for high values of streaming velocity. The effect of viscosity is, however, found to be stabilizing as the growth rate of the unstable configuration decreases on increasing the viscosity. 相似文献
17.
The rate coefficient for the important interstellar reaction between CN and C2H2 has been calculated as a function of temperature between 10 and 300 K. The potential surface for this reaction has been determined through ab initio quantum chemical techniques; the potential exhibits no barrier in the entrance channel but does show a small exit channel barrier, which lies below the energy of reactants. Phase-space calculations for the reaction dynamics, which take the exit channel barrier into account, show the same unusual temperature dependence as determined by experiment, in which the rate coefficient at first increases as the temperature is reduced below room temperature and then starts to decrease as the temperature drops below 50-100 K. The agreement between theory and experiment provides strong confirmation that the reaction occurs appreciably at cool interstellar temperatures. 相似文献
18.
A post-Galileo view of Io's interior 总被引：2，自引：0，他引：2
We present a self-consistent model for the interior of Io, taking the recent Galileo data into account. In this model, Io has a completely molten core, substantially molten mantle, and a very cold lithosphere. Heat from magmatic activity can mobilize volatile compounds such as SO2 in the lithosphere, and the movement of such cryogenic fluids may be important in the formation of surface features including sapping scarps and paterae. 相似文献
19.
The effects of Hall currents, finite conductivity, and collision with neutrals have been studied on the gravitational instability of a partially-ionized plasma. It is assumed that plasma is permeated by an oblique magnetic field. The dispersion relation has been obtained and numerical calculations have been performed to obtain the dependence of the growth rate of the gravitationally unstable mode on the various physical parameters involved. It is found that Jeans's criterion remains unchanged in the presence of Hall currents, finite conductivity, and collisions. The Hall currents, finite conductivity, and collisions have destabilizing influence on the unstable mode of wave propagation of a gravitational instability of partially-ionized plasma. 相似文献
20.
Wavenumber spectra of the martian atmosphere covering zonal wavenumbers s=1-6 were obtained as a function of latitude and season for the first time from the temperatures measured by the Thermal Emission Spectrometer onboard the Mars Global Surveyor. The stationary component tends to peak at s=2, where the martian topography has large amplitude, and drops rapidly at higher wavenumbers. The transient component in the middle and high latitudes tends to peak at s=1, which is lower than the most unstable wavenumber based on linear theories, and exhibits spectral slopes much flatter than the stationary component. In the equatorial region, the spectra of the transient component are almost flat, indicating that the organization of large-scale structures is less efficient in this region. The spectral shapes are similar between the 0.5 and 2.2 hPa surfaces, except that the slopes are slightly steeper at 0.5 than at 2.2 hPa, probably due to selective vertical transmission at low wavenumbers. The seasonal variation is relatively large in the middle and high latitudes, where the maximum power occurs in winter and the minimum occurs in summer, with an exception that the transient component is maximum in spring in the southern hemisphere. Intensification of s=1 transient waves is observed around the period of the initiation of global dust storms. 相似文献