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1.
Large amounts of particles ejected from the nucleus surface are present in the vicinity of the cometary nuclei when comets are near the Sun (at heliocentric distances ≤2 AU). The largest dust grains ejected may constitute a hazard for spatial vehicles. We tried to obtain the bounded orbits of those particles and to investigate their stability along several orbital periods. The model includes the solar and the cometary gravitational forces and the solar radiation pressure force. The nucleus is assumed to be spherical. The dust grains are also assumed to be spherical, and radially ejected. We include the effects of centrifugal forces owing to the comet rotation. An expression for the most heavy particles that can be lifted is proposed. Using the usual values adopted for the case of Halley’s comet, the largest grains that can be lifted have a diameter about 5 cm, and the term due to the rotation is negligible. However, that term increases the obtained value for the maximum diameter of the lifted grain in a significant amount when the rotation period is of the order of a few hours.  相似文献   

2.
The shape and intensity distribution of tails for several large comets are estimated on the basis of grain properties in the solar radiation field. The following results are obtained: (1) The ratio of the maximum radiation pressure force to the gravitaional force acting on dust grains in cometary tails is found to be less than 2.5. This means that grains such as graphite particles in the size range 0.02–0.2 μm do not exist in them, because such particles would allow forces greater than 2.5 (2) Tail substances supplied near the time of perihelion passage for the Sun-grazing comet Ikeya-Seki (1965 VIII) and Comet Seki-Lines (1962 III) were composed of particular grains which had values of radiation pressure ratio less than 1.0. Therefore, it is concluded that the material was composed of silicate grains only, since iron grains had sublimated and there were no graphite particles.  相似文献   

3.
We use our newly developed Dust Monte-Carlo (DMC) simulation technique [Crifo, J.F., Lukianov, G.A., Rodionov, A.V., Zakharov, V.V., 2005. Icarus 176, 192-219] to study the dynamics of dust grains in the vicinity of some of the benchmark aspherical, homogeneous cometary nuclei and of the benchmark spherical, inhomogeneous nuclei studied by us precedingly. We use the interim unrealistic simplifying assumptions of grain sphericity, negligible nucleus rotation rate, and negligible tidal force, but take accurately into account the nucleus gravitational force, gas coma aerodynamic force, and solar radiation pressure force, and consider the full mass range of ejectable spherical grains. The resulting complicated grain motions are described in detail, as well as the resulting complicated and often counter-intuitive dust coma structure. The results are used to answer several important questions: (1) When computing coma dust distributions, (a) is it acceptable to take into consideration only one or two of the above mentioned forces (as currently done)? (b) to which accuracy must these forces be known, in particular is it acceptable to represent the gravity of an aspherical nucleus by a spherically symmetric gravity? (c) how do the more efficient but less general Dust Multi-Fluid (DMF) computations compare with the DMC results? (2) Are there simple structural relationships between the dust coma of a nucleus at small heliocentric distance rh, and that of the same nucleus at large rh? (3) Are there similarities between the gas coma structures and the associated dust coma structures? (4) Are there dust coma signatures revealing non-ambiguously a spherical nucleus inhomogeneity or an homogeneous nucleus asphericity? (5) What are the implications of the apparently quite general process of grain fall-backs for the evolution of the nucleus surface, and for the survival of a landed probe?  相似文献   

4.
Assuming that the spin and magnetic axis of Jupiter are strictly parallel and that the grain charge remains constant we have derived two integrals of the 3D equations of motion of charged dust grains moving within the co-rotating regions of the Jovian magnetosphere taking into account both planetary gravitation and magnetospheric rotation. We then apply this model to study the fate of fine dust injected into the Jovian magnetosphere as a result of the tidal disruption of comet Shoemaker-Levy 9 during its first encounter with Jupiter in July 1992. This analysis, which uses the integrals of the equation of motion rather than the equation of motion itself as was done by Horanyi (1994), does not allow us to calculate the orbits or the orbital evolution of the grains. But it does allow us to construct the spatial regions to which the grains are confined, at least initially before evolutionary effects take over. We have chosen three points along the path of the disintegrating comet for the injection of dust and used two values for the uncertain floating potential of the dust in the inner Jovian magnetosphere. Grains can have three different fates, depending on their size, their acquired potential and their point of injection. While the smallest grains are quickly lost by collision with the planet at high latitudes independent of the sign of their charge, those in an intermediate but narrow size range, injected near the equatorial plane can be trapped in a region close to it, this being true for both positive and negative grains. While somewhat larger positive grains may be initially ejected outward by the co-rotational electric force, similar negative grains, pulled inward by this force collide with the planet at low latitudes. In all cases the largest grains, which are dominated by planetary gravity, initially escape from the inner magnetosphere by following in the path of the comet.Using a detailed time dependent numerical calculation of the jovicentric orbits of the charged dust debris of the disintegrating comet, that allows for variation in the grain potential, while also allowing for perturbations of the grain orbits due to solar radiation pressure and solar gravity Horanyi (1994) found that grains in the size range (1.5m <a < 2.5m) which initially make large excursions from the planet, will eventually form a ring in the radial range 4.5R J <r < 6R J . Our present analytical calculation cannot make such a prediction about the evolutionary fate of the dust debris. It can, however, estimate the size of the grains that are initially confined to regions near the points of injection, before evolutionary effects become important.  相似文献   

5.
We consider the estimates of the main forces acting on dust particles near a cometary nucleus. On the basis of these estimates, the motion of dust particles of different structure and mass is analyzed. We consider the following forces: (1) the cometary nucleus gravity, (2) the solar radiation pressure, and (3) the drag on dust particles by a flow of gas produced in the sublimation of cometary ice. These forces are important for modeling the motion of dust particles relative to the cometary nucleus and may substantially influence the dust transfer over its surface. In the simulations, solid silicate spheres and homogeneous ballistic aggregates are used as model particles. Moreover, we propose a technique to build hierarchic aggregates—a new model of quasi-spherical porous particles. A hierarchic type of aggregates makes it possible to model rather large dust particles, up to a millimeter in size and larger, while no important requirements for computer resources are imposed. We have shown that the properties of such particles differ from those of classical porous ballistic aggregates, which are usually considered in the cometary physics problems, and considering the microscopic structure of particles is of crucial significance for the analysis of the observational data. With the described models, we study the dust dynamics near the nucleus of comet 67P/Churyumov–Gerasimenko at an early stage of the Rosetta probe observations when the comet was approximately at 3.2 AU from the Sun. The interrelations between the main forces acting on dust aggregates at difference distances from the nucleus have been obtained. The dependence of the velocity of dust aggregates on their mass has been found. The numerical modeling results and the data of spaceborne observations with the Grain Impact Analyzer and Dust Accumulator (GIADA) and the Cometary Secondary Ion Mass Analyzer (COSIMA) onboard the Rosetta probe are compared at a quantitative level.  相似文献   

6.
R. Hellmich  H.U. Keller 《Icarus》1981,47(3):325-332
The problem of visibility of a cometary nucleus discussed in general terms for single scattering by dust grains. The ratio of radiatio scattered in the dust column above the surface and that reflected from the nucleus determines the visibility of features on the nuclear surface. A contrast parameter characterizing the ration of radiation foming from the nuclear surface and that of the nuclear vicinity describes the visibility of the full nucleus against the dust fore- and background. These quantities and the intensity distribution of scattered solar radiation across the nucleus and its vicinity are calculated for the case of comet Halley at a heliocentric distance of 0.9 AU after perihelion (Giotto encounter). The scattering calculations are based on an isotropic dust distribution derived from hydrodynamics gas-dust interactions resulting in a steep densiity increase right above the surface. For Newburn's nominal model of comet Halley, an optical depth of about 0.5 impairs the visibility of the nucleus somewhat.  相似文献   

7.
This paper describes the first computations of dust distributions in the vicinity of an active cometary nucleus, using a multidimensional Direct Simulation Monte Carlo Method (DSMC). The physical model is simplistic: spherical grains of a broad range of sizes are liberated by H2O sublimation from a selection of nonrotating sunlit spherical nuclei, and submitted to the nucleus gravity, the gas drag, and the solar radiation pressure. The results are compared to those obtained by the previously described Dust Multi-Fluid Method (DMF) and demonstrate an excellent agreement in the regions where the DMF is usable. Most importantly, the DSMC allows the discovery of hitherto unsuspected dust coma properties in those cases which cannot be treated by the DMF. This leads to a thorough reconsideration of the properties of the near-nucleus dust dynamics. In particular, the results show that (1) none of the three forces considered here can be neglected a priori, in particular not the radiation pressure; (2) hitherto unsuspected new families of grain trajectories exist, for instance trajectories leading from the nightside surface to the dayside coma; (3) a wealth of balistic-like trajectories leading from one point of the surface to another point exist; on the dayside, such trajectories lead to the formation of “mini-volcanoes.” The present model and results are discussed carefully. It is shown that (1) the neglected forces (inertia associated with a nucleus rotation, solar tidal force) are, in general, not negligible everywhere, and (2) when allowing for these additional forces, a time-dependent model will, in general, have to be used. The future steps of development of the model are outlined.  相似文献   

8.
A brief discussion of the infrared observations from 4 to 20 micrometers of seven comets is presented. The observed infrared emission from comets depends primarily on their heliocentric distance. A model based on grain populations composed of a mixture of silicate and amorphous carbon particles in the mass ratio of about 40 to 1, with a power-law size distribution similar to that inferred for comet Halley, is applied to the observations. The model provides a good match to the observed heliocentric variation of both the 10 micrometers feature and the overall thermal emission from comets West and Halley. Matches to the observations of comet IRAS-Araki-Alcock and the antitail of comet Kohoutek require slightly larger grains. While the model does not match the exact profile and position of the 3.4 micrometers feature discovered in comet Halley, it does produce a qualitative fit to the observed variation of the feature's strength as a function of heliocentric distance. The calculations predict that the continuum under the 3.4 micrometers feature is due primarily to thermal emission from the comet dust when the comet is close to the Sun and to scattered solar radiation at large heliocentric distances, as is observed. A brief discussion of the determination of cometary grain temperatures from the observed infrared emission is presented. It is found that the observed shape of the emission curve from about 4 to 8 micrometers provides the best spectral region for estimating the cometary grain temperature distribution.  相似文献   

9.
We present the study of dust environment of dynamically new Comet C/2003 WT42 (LINEAR) based on spectroscopic and photometric observations. The comet was observed before and after the perihelion passage at heliocentric distances from 5.2 to 9.5 AU. Although the comet moved beyond the zone where water ice sublimation could be significant, its bright coma and extended dust tail evidenced the high level of physical activity. Afρ values exceeded 3000 cm likely reaching its maximum before the perihelion passage. At the same time, the spectrum of the comet did not reveal molecular emission features above the reflected continuum. Reddening of the continuum derived from the cometary spectrum is nonlinear along the dispersion with the steeper slop in the blue region. The pair of the blue and red continuum images was analyzed to estimate a color of the comet. The mean normalized reflectivity gradient derived from the innermost part of the cometary coma equals to 8% per 1000 Å that is typical for Oort cloud objects. However, the color map shows that the reddening of the cometary dust varies over the coma increasing to 15% per 1000 Å along the tail axis. The photometric images were fitted with a Monte Carlo model to construct the theoretical brightness distribution of the cometary coma and tail and to investigate the development of the cometary activity along the orbit. As the dust particles of distant comets are expected to be icy, we propose here the model, which describes the tail formation taking into account sublimation of grains along their orbits. The chemical composition and structure of these particles are assumed to correspond with Greenberg’s interstellar dust model of comet dust. All images were fitted with the close values of the model parameters. According to the results of the modeling, the physical activity of the comet is mainly determined by two active areas with outflows into the wide cones. The obliquity of the rotation axis of the nucleus equals to 20° relative to the comet’s orbital plane. The grains occupying the coma and tail are rather large amounting to 1 mm in size, with the exponential size distribution of a−4.5. The outflow velocities of the dust particles vary from a few centimeters to tens of meters per second depending on their sizes. Our observations and the model findings evidence that the activity of the nucleus decreased sharply to a low-level phase at the end of April–beginning of May 2007. About 190 days later, in the first half of November 2007 the nucleus stopped any activity, however, the remnant tail did not disappear for more than 1.5 years at least.  相似文献   

10.
Abstract— The COMET program is a program for the collection of micron to submicron interplanetary dust particles in low Earth orbits. Since collection takes place as the Earth crosses a given meteor stream, the particles are mainly of cometary origin. The grain remnants, located at their impact positions on high purity metallic collectors, are analysed in the laboratory for chemical and isotopical identification. The COMET-1 experiment took place in 1985 October, during encounter with the Draconid meteor stream, related to the Giacobini-Zinner comet. The fluence of extraterrestrial grains that had impacted our detectors was ～10x higher than the value of the mean meteroid fluence at ～1AU, which suggests that most of the grains originated from the Giacobini-Zinner comet. One of the most important results of their chemical analysis was that ～90% of them are enriched in low Z elements (C and O have undoubedly been identified). They could contain a CHON phase similar to that observed in the close environment of Halley's nucleus. The first imagery of the grain remnants by field emission scanning electron microscopy suggests that they are very low density aggregates still present at the impacting positions which, in most cases, are very different from the impact craters observed for the same mean relative velocity for full grains of the same size. These results show that the COMET program has constituted an important step towards the analysis of cometary material and the understanding of the evolution of the early Solar System.  相似文献   

11.
Abstract– Low‐iron, manganese‐enriched (LIME) olivine grains are found in cometary samples returned by the Stardust mission from comet 81P/Wild 2. Similar grains are found in primitive meteoritic clasts and unequilibrated meteorite matrix. LIME olivine is thermodynamically stable in a vapor of solar composition at high temperature at total pressures of a millibar to a microbar, but enrichment of solar composition vapor in a dust of chondritic composition causes the FeO/MnO ratio of olivine to increase. The compositions of LIME olivines in primitive materials indicate oxygen fugacities close to those of a very reducing vapor of solar composition. The compositional zoning of LIME olivines in amoeboid olivine aggregates is consistent with equilibration with nebular vapor in the stability field of olivine, without re‐equilibration at lower temperatures. A similar history is likely for LIME olivines found in comet samples and in interplanetary dust particles. LIME olivine is not likely to persist in nebular conditions in which silicate liquids are stable.  相似文献   

12.
Nucleation is a non-equilibrium process: the products of this process are seldom the most thermodynamically stable condensates but are instead those which form fastest. It should therefore not be surprising that grains formed in a circumstellar outflow will undergo some degree of metamorphism if they are annealed or are exposed to a chemically active reagent. Metamorphism of refractory particles continues in the interstellar medium (ISM) where the driving forces are sputtering by cosmic ray particles, annealing by high energy photons and grain destruction in supernova generated shocks. Studies of the depletion of the elements from the gas phase of the interstellar medium tell us that if grain destruction occurs with high efficiency in the ISM, then there must be some mechanism by which grains can be formed in the ISM. Various workers have shown that refractory mantles could form on refractory cores by radiation processing of organic ices. A similar process may operate to produce refractory inorganic mantles on grain cores which survived the supernova shocks. Most grains in a cloud which collapses to form a star will be destroyed; many of the surviving grains will be severely processed. Grains in the outermost regions of the nebula may survive relatively unchanged by thermal processing or hydration. It is these grains which we hope to find in comets. However, only those grains encased in ice at low temperature can be considered pristine since a considerable degree of hydrous alteration might occur in a cometary regolith if the comet enters the inner solar system. Some discussion of the physical, chemical and isotopic properties of a refractory grain at each stage of its life cycle will be attempted based on the limited laboratory data available to date. Suggestions will be made concerning types of experimental data which are needed in order to better understand the processing history of cosmic dust.  相似文献   

13.
《Planetary and Space Science》1999,47(6-7):855-872
From the current understanding we know that comet nuclei have heterogeneous compositions and complex structures. It is believed that cometary activity is the result of a combination of physical processes in the nucleus, like sublimation and recondensation of volatile ices, dust grains release, phase transition of water ice, depletion of the most volatile components in the outer layers and interior differentiation.The evolution of the comet depends on the sublimation of ices and the release of different gases and dust grains: the formation of a dust crust, the surface erosion and the development of the coma are related to the gas fluxes escaping from the nucleus. New observations, laboratory experiments and numerical simulations suggest that the gas and dust emissions are locally generated, in the so-called active regions. This localized activity is probably superimposed to the global nucleus activity. The differences between active and inactive regions can be attributed to differences in texture and refractory material content of the different areas.In this paper we present the results of numerical models of cometary nucleus evolution, developed in order to understand which are the processes leading to the formation of active and non-active regions on the cometary surface. The used numerical code solves the equations of heat transport and gas diffusion within a porous nucleus composed of different ices—such as water (the dominant constituent), CO2, CO- and of dust grains embedded in the ice matrix.By varying the set of physical parameters describing the initial properties of comet P/Wirtanen, the different behaviour of the icy and dusty areas can be followed.Comet P/Wirtanen is the target of the international ROSETTA mission, the cornerstone ESA mission to a cometary nucleus. The successful design of ROSETTA requires some knowledge of comet status and activity: surface temperatures, amount of active and inactive surface areas, gas production rate and dust flux.  相似文献   

14.
An important cause of the activation and development of active processes on the surface of a cometary nucleus is direct solar radiation illuminating a part of the surface that is not shielded by dust. The intensity of solar radiation near the surface of a cometary nucleus depends on the thickness of the dust cloud above the active area. If the size of the dust cloud noticeably changes, the intensity considerably depends on time. In the present paper, we consider the nonlinear equation of radiative transfer in a dust cloud growing towards the incident wave front with a constant velocity. The change in the intensity of direct solar radiation along the dust jet originating from the active surface area of a cometary nucleus has been found. For the sake of comparison, the linear equation of radiative transfer was solved in the framework of this task. It turns out that the linear approach to the solution of the considered problem suggests a noticeable loss in the amount of direct radiation participating in the dust-jet formation. This loss is comparable with the intensity of solar radiation incident to the active area of a cometary nucleus after scattering in the cometary atmosphere.  相似文献   

15.
The extended period of mass extinctions around the K/T boundary correlating with extraterrestrial amino acids in the sediment record constitutes strong evidence of a cometary cause. The input of extraterrestrial matter over 105 yr supports the hypothesis of a giant comet, fragmented into subcomets on close encounter with Jupiter, and subsequently perturbed into Earth-crossing orbits. Copious amounts of dust were emitted via this and possibly successive fragmenting encounters, and via normal cometary evaporation. The dynamics of dust from the disintegrating comet fragments favours retention in Earth-crossing orbits of the sub-micron fraction of organic composition. The shroud of dust accreted in the Earth's upper atmosphere varied with time and imposed climatic stresses that caused species extinctions over 105 yr. While the iridium peak in the sediments coincides with the Chicxulub crater impactor, other iridium detail suggests that some of the impactor material was reinjected into space and in part re-accreted by Earth from the interplanetary orbits.  相似文献   

16.
T. Gold 《Icarus》1975,25(3):489-491
Grains, an abundant constituent of the former solar system, will have had a high probability of being driven into orbits resonant with major bodies already formed. This arises because of the presence of gas drag and Poynting-Robertson drag on small grains, providing the dissipation necessary to concentrate matter into special orbits. Since the mean density in resonant orbits can be built up by such a process without limit, these may become the favored orbits for gravitational contraction to gather material into major bodies. Satellite formation processes may therefore depend upon the buildup of resonant lanes of dust grains around the parent body. Saturn's rings are possibly one example of such lanes, though an unsuitable one for the final step of satellite formation on account of their being too close to Saturn.  相似文献   

17.
The extended period of mass extinctions around the K/T boundary correlating with extraterrestrial amino acids in the sediment record constitutes strong evidence of a cometary cause. The input of extraterrestrial matter over 105 yr supports the hypothesis of a giant comet, fragmented into subcomets on close encounter with Jupiter, and subsequently perturbed into Earth-crossing orbits. Copious amounts of dust were emitted via this and possibly successive fragmenting encounters, and via normal cometary evaporation. The dynamics of dust from the disintegrating comet fragments favours retention in Earth-crossing orbits of the sub-micron fraction of organic composition. The shroud of dust accreted in the Earth's upper atmosphere varied with time and imposed climatic stresses that caused species extinctions over 105 yr. While the iridium peak in the sediments coincides with the Chicxulub crater impactor, other iridium detail suggests that some of the impactor material was reinjected into space and in part re-accreted by Earth from the interplanetary orbits.School of Mathematics, University of Wales College of Cardiff.  相似文献   

18.
S.V.M. Clube  W.M. Napier 《Icarus》1985,62(3):384-388
The observed properties of the long-period comet system, and its periodic disturbance by galactic forces manifesting as terrestrial impact episodes, may be indicative of a comet capture/escape cycle as the Solar System orbits the Galaxy. A mean number density of comets in molecular clouds of ～10?1±1 AU?3 is implied. This is sufficient to deplete metals from the gaseous component of the interstellar medium, as observed, but leads to the problem of how stars are formed nevertheless with solar metal abundances. Formation of comets prior to stars in dense systems of near-zero energy may be indicated, and isotope signatures in cometary particles may be diagnostic of conditions in young spiral arm material.  相似文献   

19.
We present families of symmetric and asymmetric periodic orbits at the 1/1 resonance, for a planetary system consisting of a star and two small bodies, in comparison to the star, moving in the same plane under their mutual gravitational attraction. The stable 1/1 resonant periodic orbits belong to a family which has a planetary branch, with the two planets moving in nearly Keplerian orbits with non zero eccentricities and a satellite branch, where the gravitational interaction between the two planets dominates the attraction from the star and the two planets form a close binary which revolves around the star. The stability regions around periodic orbits along the family are studied. Next, we study the dynamical evolution in time of a planetary system with two planets which is initially trapped in a stable 1/1 resonant periodic motion, when a drag force is included in the system. We prove that if we start with a 1/1 resonant planetary system with large eccentricities, the system migrates, due to the drag force, along the family of periodic orbits and is finally trapped in a satellite orbit. This, in principle, provides a mechanism for the generation of a satellite system: we start with a planetary system and the final stage is a system where the two small bodies form a close binary whose center of mass revolves around the star.  相似文献   

20.
Because of their short cosmic ray exposure ages, chondritic meteorites are more likely to have been broken off from parent bodies in Earth-crossing orbits than from parent bodies in the asteroid belt. The radii of the objects now in the vicinity of the Earth (Apollo and Amor objects) are too small to be unfragmented asteroids of the theory for the origin of gas-rich meteorites of Anders. Because of the abundant evidence for very heavy shock and reheating among L- and H-chondrites, I conclude that the asteroidal origin for the ordinary chondrites is still the most likely. A cometary origin for the CI chondrites is examined. Regolith and megaregolith do not necessarily have to be formed by impacts on the cometary nucleus. The short-period comet Encke receives about 1/10 the solar-wind flux of a belt asteroid at 2.5 AU in its present orbit. The thickness of the megaregolith (C1 chondrites) is estimated between 0.1 and 0.3 km. Stirring of the megaregolith without substantial loss of dust from the comet might occur when the comet is transitional between “active” and “dead.” The consolidation of C1- “dust” into rock is somewhat problematic, but if liquid water and water vapor have played a role, then a crust rich in solar gases might form in the outer regions of a comet. A testable alternative explanation is suggested, namely that the solar gases in the C1 chondrites do not come from the Sun.  相似文献   

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