共查询到20条相似文献，搜索用时 78 毫秒
1.
J.G. Hills 《Icarus》1973,18(3):505-522
The physically reasonable assumption that the seed bodies which initiated the accretion of the individual asteroids, planets, and comets (subsequently these objects are collectively called planetoids) formed by stochastic processes requires a radius distribution function which is unique except for two scaling parameters: the total number of planetoids and their most probable radius. The former depends on the ease of formation of the seed bodies while the second is uniquely determined by the average pre-encounter velocity, V, of the accretable material relative to an individual planetoid. This theoretical radius function can be fit to the initial asteroid radius distribution which Anders (1965) derived from the present-day distribution by allowing for fragmentation collisions among the asteroids since their formation. Normalizing the theoretical function to this empirical distribution reveals that there were about 102 precollision asteroids and that V = (2?4) × 10?2 km/sec which was presumably the turbulent velocity in the Solar Nebula. Knowing V we can determine the scale height of the dust in the Solar Nebula and consequently its space density. The density of accretable material determines the rate of accretion of the planetoids. From this we find, for example, that the Earth formed in about 8 × 106 yr and it attained a maximum temperature through accretion of about 3 × 103°K. From the total mass of the terrestrial planets and the theoretical radius function we find that about 2 × 103 planetoids formed in the vicinity of the terrestrial planets. Except for the asteroids the smaller planetoids have since been accreted by the terrestrial planets. About 15% of the present mass of the terrestrial planets was accumulated by the secondary accretion of these smaller primary planetoids. There are far fewer primary planetoids than craters on the Moon or Mars. The craters were likely produced by the collisional breakup of a few primary planetoids with masses between one-tenth and one lunar mass. This deduction comes from comparing the collision cross sections of the planetoids in this mass range to that of the terrestrial planets. This comparison shows that two to three collisions leading to the breakup of four to six objects likely occurred among these objects before their accretion by the terrestrial planets. The number of these fragments is quite adequate to explain the lunar and Martin craters. Furthermore the mass spectrum of such fragments is a power-law distribution which results in a power-law distribution of crater radii of just the type observed on the Moon and Mars. Applying the same analysis to the planetoids which formed in the vicinity of the giant planets reveals that it is unlikely that any fragmentation collisions took place among them before they were accreted by these planets due to the integrated collision cross section of the giant planets being about three orders of magnitude greater than that of the terrestrial planets. We can thus anticipate a marked scarcity of impact craters on the satellites of these outer planets. This prediction can be tested by future space probes. Our knowledge of the radius function of the comets is consistent with their being primary planetoids. The primary difference between the radius function of the planetoids which formed in the inner part of the solar system and that of the comets results from the fact that the seed bodies which grew into the comets formed far more easily than those which grew into the asteroids and the terrestrial planets. Thus in the outer part of the Solar Nebula the principal solid material (water and ammonia snow) accreted into a huge (～1012+) number of relatively small objects (comets) while in the inner part of the nebula the solid material (hard-to-stick refractory substances) accumulated into only a few (～103) large objects (asteroids and terrestrial planets). Uranus and Neptune presumably formed by the secondary accretion of the comets. 相似文献
2.
John A. O'Keefe III 《Earth, Moon, and Planets》1974,9(1-2):219-225
Supporting evidence for the fission hypothesis for the origin of the Moon is offered. The maximum allowable amount of free iron now present in the Moon would not suffice to extract the siderophiles from the lunar silicates with the observed efficiency. Hence extraction must have been done with a larger amount of iron, as in the mantle of the Earth, of which the Moon was once a part, according to the fission hypothesis. The fission hypothesis gives a good resolution of the tektite paradox. Tektites are chemically much like products of the mantle of the Earth; but no physically possible way has been found to explain their production from the Earth itself. Perhaps they are a product of late, deep-seated lunar volcanism. If so, the Moon must have inside it some material with a strong resemblance to the Earth's mantle. Two dynamical objections to fission are shown to be surmountable under certain apparently plausible conditions. 相似文献
3.
A. G. W. Cameron 《Earth, Moon, and Planets》1973,7(3-4):377-383
The basic geochemical model of the structure of the Moon proposed by Anderson, in which the Moon is formed by differentiation of the calcium, aluminium, titanium-rich inclusions in the Allende meteorite, is accepted, and the conditions for formation of this Moon within the solar nebula models of Cameron and Pine are discussed. The basic material condenses while iron remains in the gaseous phase, which places the formation of the Moon slightly inside the orbit of Mercury. Some condensed metallic iron is likely to enter the Moon in this position, and since the Moon is assembled at a very high temperature, it is likely to have been fully molten, so that the iron can remove the iridium from the silicate material and carry it down to form a small core. Interactions between the Moon and Mercury lead to the present rather eccentric Mercury orbit and to a much more eccentric orbit for the Moon, reaching past the orbit of the Earth, establishing conditions which are necessary for capture of the Moon by the Earth. In this orbit the Moon, no longer fully molten, will sweep up additional material containing iron oxide. This history accounts in principle for the two major ways in which the bulk composition of the Moon differs from that of the Allende inclusions.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April 1973. 相似文献
4.
A model of planetary formation in a binary system with a small relative mass of primary is computed on the assumption of a mass transfer from the less massive component to the more massive one with no mass and angular momentum carried away from the system under consideration. At the last stage of mass transfer the condensed Moon-like objects (planetoids) are ejected through the inner Lagrange point of the primary Roche lobe with the outflow of gaseous matter.The whole system is considered in the plane of binary star rotation. Newtonian equations of motion are integrated with the initial conditions for the planetoids referred to as the coordinates and velocity of the inner Lagrangian point at the moments of planetoid ejections, all the pairwise gravitational interactions being included in computations but without a gas-drag. The mass transfer ceases at the primary relative mass 10–3 which corresponds to the present Sun-Jupiter system. The total mass of planetoids approximates that of the terrestrial planets. Those are formed through coagulation of the planetoids with the effective radius of capture cross-section as an input parameter in the computer simulation. When the minimum separation between the pair of bodies becomes less than this radius they coalesce into a single body with their masses and momenta summed. If the effective radius value is under a certain limit the computer simulation yields the planetary system like that of terrestrial planets of the present Sun system.Numerical computations reveal the division of the planetoids into 4 groups along their distances from the Sun. Further, each group forms a single planet or a planet and a less massive body at the nearest orbits. The parameters of simulated planet orbits are close to the present ones and the interplanetary spacings are in accord with the Titius-Bode law. 相似文献
5.
An algorithm is developed to find Weak Stability Boundary transfer trajectories to Moon in high fidelity force model using forward propagation. The trajectory starts from an Earth Parking Orbit (circular or elliptical). The algorithm varies the control parameters at Earth Parking Orbit and on the way to Moon to arrive at a ballistic capture trajectory at Moon. Forward propagation helps to satisfy launch vehicle’s maximum payload constraints. Using this algorithm, a number of test cases are evaluated and detailed analysis of capture orbits is presented. 相似文献
6.
John A. O'Keefe 《Astrophysics and Space Science》1972,16(2):201-211
A planet the size of the Earth or the Moon is much like a blast furnace; it produces slag-like rock floating on a mass of liquid metal. In the Earth, the mantle and crust are the slag, and the core is the liquid iron.In the Moon, there is clear chemical evidence that liquid iron was separated from the mass, but the Moon has no detectable iron core. This points to some kind of joint origin, which put the metallic iron in the Earth's core. For instance, the Moon might have been a detached part of the rocky matter of the Earth, as suggested by G. H. Darwin in the 1880's. But is is also clear, as Ringwood has pointed out, the there has been an enormous loss of volatiles from both Earth and Moon, but especially from the Moon. It may be that the Moon formed from a sediment-ring of small bodies detached somehow from the outer parts of the Earth, as Öpik has suggested.If tektites come from the Moon, then Darwin's suggestion is probably right; if they come from the Earth, then the Öpik-Ringwood sediment ring may be the origin.Paper presented at the AAAS Symposium on the Early History of the Earth and Moon in Philadelphia on 28 December 1971. 相似文献
7.
S.J. Weidenschilling 《Icarus》1978,35(1):99-111
Compared with the other terrestrial planets, Mercury has anomalously low mass and high iron content. Equilibrium condensation and inhomogeneous accretional models are not compatible with these properties, unless the solar nebula's thermal structure and history meet stringent conditions. Also, such models predict a composition which does not allow a presently molten core. It appears that most of the solid matter which originally condensed in Mercury's zone has been removed. The planet's composition may be explained if the removal process was only slightly more effective for silicates than for iron. It is proposed that planetesimal orbits in the inner solar nebula decayed because of gas drag. This process is a natural consequence of the non-Keplerian rotation of a centrally condensed nebula. A simple quantitative model shows good agreement with the observed mass distribution of the terrestrial planets. The rate of orbital decay is slower for larger and/or denser bodies, because of their smaller area-to-mass ratios. With plausible assumptions as to planetesimal sizes and compositions, this process can produce fractionation of the sense required to produce an iron-rich planet. Cosmogonical implications are discussed. 相似文献
8.
B.A. Conway 《Icarus》1982,51(3):610-622
A frequency-dependent model of tidal friction is used in the determination of the time rate of change of the lunar orbital elements and the angular velocity of the Earth. The variational equations consider eccentricity, the solar tide on the Earth, Earth oblateness, and higher-order terms in the Earth's tidal potential. A linearized solution of the equations governing the precission of the Earth's rotational angular momentum and the lunar ascending node is found. This allows the analytical averaging of the variational equations over the period of relative precession which, though large, is necessarily small in comparison to the time step of the numerical integrator that yields the system history over geological time. Results for this history are presented and are identified as consistent with origin of the Moon by capture. This model may be applied to any planet-satellite system where evolution under tidal friction is of interest. 相似文献
9.
Tovy Grjebine 《Earth, Moon, and Planets》1980,22(3):367-382
Paleontological data and celestial mechanics suggest that the Moon may have stayed in a geosynchronous corotation around the Earth as a geostationary satellite. Excess energy may have slowly been released as heat, transferred as movement around the Sund or lost with matter ejected into space.The radial segregation process which was responsible for the formation of the Earth's iron core also brought water and lithophile elements dissolved in the water towards the surface. These elements were deposited in the area facing the Moon for several reasons, and a single continent was formed. Its level continuously matched the sea level, so the continent was formed under shallow water. When the geosynchronous corotation of the Moon became impossible, the tides become important, the Moon receded and the Earth slowed down and became more and more spherical; the variation of its oblateness from about 8% to 0.3% was incompatible with the shape of the continent, that broke into pieces.Almost all the data were have on the Earth's age, the composition of the continents, sea water and the atmosphere fit this approach as does lunar data.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy. 相似文献
10.
When Jupiter was on the order of three to ten Earth masses in size, there undoubtedly was a considerably larger mass of condensed matter in its zone, since Jupiter would have perturbed most of it to other parts of the solar system. Monte Carlo studies indicate a significant portion would have crossed the Earth's orbit. If the Earth and Moon had not yet fully formed, the probability of Earth-zone planetesimals being hit by this Jupiter-scattered material was high. Further Monte Carlo models of these collisions and their products indicate a significant portion of matter was heated to melting, even if less than 5% of the relative kinetic energy went into heat. The models include capture probabilities by an embryo Earth and a protolunar swarm. Because heat energy is correlated with comminution energy, and because the capture probability of the swarm is mass-dependent while the embryo's is not, the protolunar material suffered much higher heating on the average than did the proto-Earth material. 相似文献
11.
Fission from the Earth's mantle explains why the density of the Moon is similar to that of the Earth's mantle.If following the fission origin of the Moon, the Earth-Moon distance increases progressively, the Moon can recollect chemicals evaporated by the Earth but not volatile enough to be lost as gases.In this way, the surface of the Moon can be enriched in refractory elements as most of the authors have proposed.At 3 Earth radii the long geosynchronous phase allows the formation of a solid crust which will record the Earth's magnetic field and the equilibrium hydrostatic from at that distance.When geosynchronism is broken the Moon will recede; its shape will no longer fit the hydrostatic form. The crust will either break or will exercise pressure on the lower layers. Meteor craters will allow lava to come to the surface. Such flows will be very large where the shape of the crust does not fit at all the geosynchronous form. Large lava flows will appear this way on the near side where the shape has changed the most. The new lava flows no longer record the magnetic field of the Earth because with the end of the synchronous position the field is alternative for the Moon; only the remanent field can influence the new lava.Three out of five samples dated at 3.6 b.y. suggest nevertheless that the field decreased slowly without becoming alternative. This means that the geosynchronous phase may have lasted longer and put the Moon on a more distant orbit, as Alfvén and Arrhenius suggested.The interpretation of lunar magnetism as influenced by the Earth cannot discard any interpretation or suggestion of its own lunar magnetic process. It is quite possible that both mechanisms have worked as some samples show.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademic Nazionale del Lincei in Rome, Italy. 相似文献
12.
T.A. Heppenheimer 《Icarus》1975,24(2):172-180
The problem of the origin of Jupiter's outer satellites is treated within the framework of the theory of capture through collinear libration points. Lower bounds for the satellites' semimajor axes are found from a corrected rederivation of Bailey's capture theory. Upper bounds are found from a new derivation of the stability limit for satellites, based on Floquet stability theory.It is shown that if the bodies had near-zero relative velocity when passing the libration point, direct orbits would lie outside retrograde orbits, which is not the case for Jupiter. It is found that the dimensions and distributions of the direct group are well explained by libration-point capture with solar mass, which is interpreted as indicating capture soon after Jupiter's formation. But ad hoc assumptions are required for this capture model to explain the retrograde group. It is concluded that the direct and retrograde groups may have had different mechanisms of origin. 相似文献
13.
Voyager images have revealed that most of Amalthea's surface is very dark and very red, with a few isolated bright spots having a distinct greenish spectrum. These unique color characteristics probably result from the unusual environment of the satellite. It is proposed that charged particles from the Jovian magnetosphere, contaminants such as sulfur from Io, and high-velocity micrometeoritic matter combine to darken, redden, and alter Amalthea's surface. The effects of sulfur and sulfur allotrope contamination are shown to redden a variety of bulk compositions: (a) carbonaceous material, (b) refractory minerals, (c) iron and iron sulfides, and (d) moderate temperature silicates. Carbonaceous-sulfur systems provide good, but not unique, spectral matches to the dark areas. The bright, greenish spots probably identify locations in which atypical alteration processes occur. These may include variations in the amount of contaminant sulfur in micrometeoritic glasses or in the relative abundances of certain sulfur allotropes. A major conclusion of this work is that available spectral-reflectance data contain little information about the bulk composition of the satellite. Spectrophotometry over an extended spectral range may be useful in specifying the composition of the surface more uniquely, but a determination of the satellite's mean density may be the only way of discriminating among possible bulk compositions. 相似文献
14.
The problem of the origin of the Moon has led to various hypotheses: simultaneous accretion, fission, capture, etc. These theories were based primarily on global mechanical considerations. New geological data (Turcotteet al., 1974; Kahn and Pompea, 1978) have led to fresh approaches and new versions of these theories.As suggested by Wise (1969) and O'Keefe (1972), the initial Earth may have taken unstable forms when radial segregation sped up the rotation. The Moon may have been created as the small part of the pyroid of Poincaré.Fission theory was mainly discarded, in the past, on the basis of energy considerations. We are now arriving at the conclusion that these considerations are void if the fission was followed by a very long period of geostationary rotation of the Moon at a distance of about 3 Earth radius (i.e., out of the Roche limit). Indeed the large amount of energy of the initial system could have been released slowly and therefore evacuated by losses of material and radiation.The accretion of the Earth and the radial segregation of heavy chemicals toward the center has led to a differential rotation of the different layers with a faster rotation at the center. During the geostationary period the Moon was synchronous with respect to the surface layer. That Earth-Moon system has both a correct angular momentum and a large stability provided that the viscosity of intermediate layers was small enough, which is in concordance with its high temperature.Even with a very hot system, a superficial cold layer appears because of its low conductivity and the radiation equilibrium with outer space. This implies a slow loss of energy: the geosynchronous Moon receded extremely slowly.During the geostationary period lithophile elements were extracted with water by the radial segregation and were deposited in the area facing the Moon. One massive continent was formed, as suggested by Grjebine (1978).As the continent became thicker and sank into the mantle, convection currents appeared and speeded up the cooling of the Earth. The viscosity increased and the synchronization between the Moon and the surface of the Earth became more difficult to maintain. When synchronism was broken important lunar tides transferred energy and momentum from the Earth to the Moon which receded toward its present position and the modification of its equilibrium shape explains the formation of lunar maria in the near side.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy. 相似文献
15.
We present a treatment of libration-point capture in the restricted three-body problem. Examples of capture are given, and a long-term numerical integration is presented, to illustrate major features of orbits arising from capture. A theory of lifetimes is given, providing order-of-magnitude (though rather conservative) estimates of the time a body remains captured. A general capture criterion, giving bounds on admissible values of the postcapture semimajor axis, for given values of eccentricity and inclination. This criterion is used to demonstrate that, in general, direct postcapture orbits lie outside retrograde ones. We also emphasize the importance of mass-change, of one or both primaries, in producing capture. This phenomenon is shown to give rise to a new type of capture, “pull-down capture,” which produces retrograde orbits. The effects of nebular drag also are noted.These results suggest the improbability of a capture origin for Jupiter's outer satellites within the last 4+ billion years, or since the solar system reached its present dynamical configuration. Computations indicate, however, that either mass-change or nebular drag could have been effective in producing capture. The outer satellite groups are shown to resemble Hirayama families physically, thus supporting a hypothesis of capture followed by collisional fragmentation. 相似文献
16.
Since one does not know the photometric functions of various parts of Io, one cannot convert the observed geometric albedo of the satellite to a parameter more directly measurable in the laboratory. One must therefore convert laboratory reflectances to geometric albedos before quantitative comparisons between Io's surface and a laboratory sample are made. This procedure involves determining the wavelength dependence of the sample's photometric function. For substances such as sulfur, whose reflectance varies strongly with wavelength, it is incorrect to assume that the photometric function, and hence the ratio (laboratory reflectance/geometric albedo) is independent of wavelength. To illustrate this point, measurements of the color dependence of this ratio for sulfur are presented for the specific case in which the measured laboratory reflectance is the sample's normal reflectance. In general, unless the laboratory reflectance is precisely the geometric albedo, a wavelength-dependent correction factor must be determined before the laboratory sample can be compared quantitatively with Io's surface. 相似文献
17.
Harrington and Van Flandern (1979, Icarus39, 131–136) suggests that the irregular features of the Neptunian satellite system and Pluto's escape were caused by an encounter with a massive external body. They rule out the alternative mechanism based on the capture of Triton (which seems more plausible because it does not appeal to any unobserved object) on the basis of an incorrect deduction from McCord's (1966, Astron. J.71, 585–590) analysis on the tidal decay of Triton's orbit. As a matter of fact, many recent results show that satellite captures are possible, and in the case of Triton several arguments support this interpretation. 相似文献
18.
Numerical calculation of a simple accretion model including the effects of tidal friction indicate that coformation is tenable only if the planet's Q is less than about 103. The parameter which most strongly affects the final mass ratio of the pair is the time at which the secondary embryo is introduced. Our model yields the proper Moon-Earth mass ratio if the Moon embryo is introduced when the Earth is only about of its final mass. The lunar orbit remains at about 10 Earth radii throughout most of the growth.This model of satellite formation overcomes two difficulties of the “circumterrestrial cloud” model of Ruskol (1960, 1963, 1972): (1) The difficulty of accumulating a mass as great as the entire Moon before gravitational instability reduces the cloud to a small number of moonlets is removed. (2) The differences between terrestrial and outer planet satellite systems is easily understood in terms of the differences in Q between these planets. The high Q of the outer planets does not allow a satellite embryo to survive a significant portion of the accretion process, thus only small bodies which formed very late in the accumulation of the planet remain as satellites. The low Q of the terrestrial planets allows satellite embryos of these planets to survive during accretion, thus massive satellites such as the Earth's Moon are expected. The present lack of such satellites of the other terrestrial planets may be the result of tidal evolution, either infall following primary despinning (Burns, 1973) or escape due to increase in orbit eccentricity. 相似文献
19.
L. Trafton 《Icarus》1984,58(2):312-324
Triton's seasons differ materially from those of Pluto owing to four important differences in the governing physics: First, the obliquity of Triton is significantly less than Pluto's obliquity. Second, Triton's inclined orbit precesses rapidly about Neptune so that a complicated seasonal variation in the latitude of the Sun occurs for Triton. Third, Neptune's orbit is much more circular than Pluto's orbit so that the sunlight intercepted by Triton's disk does not vary seasonally. Finally, Triton's atmosphere cannot be saturated at the lower latitudes so that the mass of the atmosphere is controlled by the temperature of the high-latitude ices or liquids (polar caps), as for CO2 on Mars. The consequences of Triton's entire surface being covered with volatile substances have been examined. It is found that the circularity of Neptune's orbit then implies that Triton would have hardly any seasonal variation at all in surface temperature or atmospheric bulk, in spite of the complicated precessional effects of Triton's orbit. The only seasonal effect would be the migration of surface ices and liquids. This scenario is ruled out because it implies a column CH4 abundance much higher than that observed and because it quickly depletes the lower latitudes of volatiles. It is concluded that Triton's most volatile surface substances are probably relegated to latitudes higher than 35° and probably form polar caps. The temperature of the polar caps should be nearly equal, even during midwinter/midsummer when the insolation of the summer pole is greatest. If the summer pole completely sublimates during one of the “major” summers, Triton's atmosphere may begin to freeze out over the winter caps. It is therefore expected that Triton's atmosphere undergoes large and complex seasonal variations. Triton is currently approaching a “maximum southern summer”, and over the remainder of this century, a dramatic increase in CH4 abundance above the current upper limit of 1 m-Am may be witnessed. 相似文献
20.
A.W. Harris 《Icarus》1975,24(2):190-192
Jeffreys (1947) estimated the size of fragments resulting from breakup of a satellite inside the Roche limit, obtaining a result of ～100 km. This result does not allow for the further breakup of the fragments due to collisions among themselves, which should reduce the maximum size to ?3 km for rock, or ?1 km for ice. This result affects not only Jeffrey's speculations as to the origin of Saturn's rings, but also recent speculations on the origin of the moon by capture and the possible tidal destruction of satellites of Mercury or Venus. 相似文献