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1.
彗星是太阳系遗留的原始星子,研究彗星彗核的演化对理解太阳系其他天体的形成和演化历史具有重要意义.在太阳的辐射作用下,彗星携带的挥发性成分会发生升华,并带动尘埃运动,造成彗核物质的损失.因此,彗核的升华活动对其表面形貌甚至整体形状演化都会产生影响.从IAU (International Astronomical Union) MPC (Minor Planet Center)获取轨道数据,并考虑了彗核的自转以及进动,利用MONET (Mass lossdriven shape evolution model)形状演化模型对短周期彗星做数值模拟,计算得到了短周期彗星1P/Halley、9P/Tempel 1、 19P/Borrelly、 67P/C-G (Churyumov-Gerasimenko)、 81P/Wild 2和103P/Hartley 2在一个轨道周期内的太阳辐射能量以及表面侵蚀深度的分布,结合其动力学参数讨论了自转、进动和公转等特性对其表面水冰升华分布的影响以及造成南北侵蚀差异的可能性. 相似文献
2.
In the paper two chosen features of the comet 103P/Hartley 2 are studied. The first one are ‘cometary geysers’ which have been recorded by the camera on Deep Impact spacecraft. The numerical calculations related with this phenomenon have been carried out for large number of values of probable cometary characteristics. Our calculations confirm the assumption what also has been observed by NASA's scientists that the jets of carbon dioxide from the geysers are able to lift large chunks of water ice from the comet. The second discussed feature of the comet 103P/Hartley 2 is the lack of impact holes on the surface of its nucleus. The expected rate of impact holes on the surface of the nucleus of 103P/Hartley 2 is discussed. These holes could be the product of impacts between this comet and other small bodies orbiting in the main asteroid belt. The probability of such impacts, the total number of expected perceptible holes and changes in the luminosity of the comet caused by collisions are examined. We conclude that indeed the number of visible holes on its surface should be negligible (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
3.
The orbital dynamics of comet Shoemaker-Levy 9 was considered. The size of the nucleus before its disintegration was estimated using positional observations of individual fragments of the comet and taking into account the time when the fragment falls on Jupiter were observed. The center of inertia of the parent body was assumed to coincide with the center of fragment H at the moment of disintegration. The diameter of the parent body was estimated under different assumptions about the moments of disintegration. Thus, if the nucleus of the comet was disintegrated at the moment when fragment H was passing the perijovion, the diameter of the nucleus is equal to 1.39 km; if the nucleus was disintegrated two hours before the perijovion passage, the estimate of the nucleus diameter must be increased up to 9.03 km. The period of revolution of the comet nucleus was estimated assuming that the comet rotated about the axis perpendicular to its orbital plane. The calculations allowed us to assume that fragmentation of the nucleus began approximately an hour before the comet passed the perijovion and the diameter of the parent nucleus was about 4 km. 相似文献
4.
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
5.
Here we estimate the regions of stability, fragmentation, and destruction for cometary bodies versus rotational breakup in the radius-rotational period plane. By testing different plausible physical models of the cometary nucleus equation of state, we show that the plane is divided into 3 segments: the allowed, damaged, and forbidden regions. We then compare the location of well-observed comets with respect to the separation lines. The range of constituent material parameters from the literature for cometary nuclei are used to show that all the observed comets lie in the allowed region, except for Comet C/1995 O1 (Hale-Bopp), which resides in the damaged region (where the body is fractured and only held together gravitationally). We speculate that the extremely high activity demonstrated by Comet Hale-Bopp during the 1997 apparition may have been due to its highly fractured state. Comet Hyakutake, observed to emit fragments at perigee in 1996, may be near the boundary of the damaged region. Comet C/1999 S4 (LINEAR) was solidly in the rotationally allowed region, making its disintegration in July 2000 due to centrifugal forces unlikely. In contrast to the comets, the centaurs do not cluster in the allowed region, with the majority falling instead into the rotationally damaged and forbidden regions. The centaurs are only stable against breakup assuming much stronger solid water ice properties, strongly suggesting that on the whole, these bodies have different bulk physical properties than cometary nuclei. 相似文献
6.
Yu Jiang Juergen Schmidt Hexi Baoyin Hengnian Li Junfeng Li 《Earth, Moon, and Planets》2017,120(3):147-168
In this paper we analyze the dynamical behavior of large dust grains in the vicinity of a cometary nucleus. To this end we consider the gravitational field of the irregularly shaped body, as well as its electric and magnetic fields. Without considering the effect of gas friction and solar radiation, we find that there exist grains which are static relative to the cometary nucleus; the positions of these grains are the stable equilibria. There also exist grains in the stable periodic orbits close to the cometary nucleus. The grains in the stable equilibria or the stable periodic orbits won’t escape or impact on the surface of the cometary nucleus. The results are applicable for large charge dusts with small area-mass ratio which are near the cometary nucleus and far from the Solar. It is found that the resonant periodic orbit can be stable, and there exist stable non-resonant periodic orbits, stable resonant periodic orbits and unstable resonant periodic orbits in the potential field of cometary nuclei. The comet gravity force, solar gravity force, electric force, magnetic force, solar radiation pressure, as well as the gas drag force are all considered to analyze the order of magnitude of these forces acting on the grains with different parameters. Let the distance of the dust grain relative to the mass centre of the cometary nucleus, the charge and the mass of the dust grain vary, respectively, fix other parameters, we calculated the strengths of different forces. The motion of the dust grain depends on the area-mass ratio, the charge, and the distance relative to the comet’s mass center. For a large dust grain (> 1 mm) close to the cometary nucleus which has a small value of area-mass ratio, the comet gravity is the largest force acting on the dust grain. For a small dust grain (< 1 mm) close to the cometary nucleus with large value of area-mass ratio, both the solar radiation pressure and the comet gravity are two major forces. If the a small dust grain which is close to the cometary nucleus have the large value of charge, the magnetic force, the solar radiation pressure, and the electric force are all major forces. When the large dust grain is far away from the cometary nucleus, the solar gravity and solar radiation pressure are both major forces. 相似文献
7.
There are many aspects of observational evidence that cometary nuclei have irregular or nonspherical shape. The triaxial figure of the Halley's Comet nucleus is a well known fact. Therefore, the nucleus shape plays a significant role in consideration of the formation and evolution of comets and several attempts have been made to explain their nonsphericity. These studies were mainly based on the random-walk schemes for the aggregation processes. Although some results indeed lead to irregularities and deviation from sphericity, the spherical or irregular shape seem to be prevailing results. On the other hand the triaxial figure can be formed by the tidal and rotational forces. Thus, the assumption that the shape of the cometary nucleus due to some of these effects is in principle acceptable. In here assumed scenario already evolved cometary nucleus is situated as a satellite in the gravitation field of a planetary-like body. Since the rigidity of the nucleus is low, it may be easily transferred in the state of a synchronous satellite and in its shape could be imprinted the dynamical effects from this epoch. Here presented results indicate, that such a possibility should be seriously considered. The theory of this process is applied to the nucleus of comet Halley. It is shown, that the nucleus might be synchronously orbiting around a planetary-like hypothetical body with a period of 0.7 days. The minimal bulk tensile strength of the cometary material of about 102 N m–2 is estimated. 相似文献
8.
Splitting events affect cometary nuclei to a different level of severity ranging from complete disruption of the nucleus (e.g., C/1999 S4 LINEAR) to separation of major fragments (e.g., 73P/Schwassmann-Wachmann 3) and spill-offs of smaller boulders (e.g., C/2001 A2 LINEAR).Fragmentation of comets produces secondary products over a wide range of sizes (from cometesimals to sub-micron dust). It is detectable through the presence of fragments (with own comae and tails) in the coma of the parent nucleus, through outbursts in its activity and through arc-lets (“coma wings”)associated with fragments. The secondaries have different life times and show different non-gravitational forces. Nucleus splitting is also considered to generate whole families of comets (Kreutz group) or — if gravitational bound — multiple nuclei (e.g., C/1995 O1 Hale-Bopp). It may explain the striae phenomena seen in dust tails of bright comets (C/1995 O1 Hale-Bopp) and the detection of chains of impact craters onother bodies in the solar system. As process of significant mass loss it is relevant for the scenario of nucleus extinction, at the same time it also plays a role for the number statistics of existing (observable) comets and for the size distribution of comet nuclei. Various model scenarios for nucleus splitting are proposed: tidal disruption, rotational splitting, break-up due to internal gas pressure, fragmentation due to collision with other bodies. Only in one case, Comet D/1993 F1Shoemaker-Levy 9, the physical process of fragmentation could be undoubtedly identified. In any case, comet splitting provides important insights inthe internal structure, surface layering and chemistry of comet nuclei. 相似文献
9.
关于太阳系中光压对各种天体运动的影响问题 总被引：2，自引：0，他引：2
关于太阳辐射（简称光压）对各种天体（大行星、小行星、慧星以及各类航天器）运动的影响,本文从较完整的力学模型着手,作了详尽的分析,其结论是：由于太阳辐射相对较弱,而辐射压的作用大小还取决于承受天体的有效面质比,对太阳系中各自然天体（包括慧星的一些物理现象）运动的影响并不重要,而对各类航天器运动的影响却是不可忽视的。中文给出的力学模型和相应的研究方法,在一定条件下同样适用于强辐射的天体系统。 相似文献
10.
Q. L. Hou E. M. Kolesnikov L. W. Xie M. F. Zhou M. Sun N. V. Kolesnikova 《Planetary and Space Science》2000,48(15):110-1455
Ten Sphagnum fuscum peat samples collected from different depths of a core including the layer affected by the 1908 Tunguska explosion in the Tunguska area of Central Siberia, Russia, were analyzed by ICP-MS to determine the concentrations of Pd, Rh, Ru, Co, REE, Y, Sr, and Sc. The analytical results indicate that the Pd and Rh concentrations in the event- and lower layers were 14.0–19.9, and 1.23–1.56 ppb, respectively, about 3–9 times and 3 times higher than the background values in the normal layers. In addition, the patterns of CI-chondrite-normalized REE in the event layers were much flatter than in the normal layers, and differed from those in the nearby traps. Hence, it can be inferred from the characteristics of the elemental geochemistry that the explosion was probably associated with extraterrestrial material, and which, most probably, was a small comet core the dust fraction of which was chemically similar to carbonaceous chondrites (CI). In terms of the Pd and REE excess fluxes in the explosion area, it can be estimated that the celestial body that exploded over Tunguska in 1908 weighed more than 106 t, corresponding to a radius of >60 m. If the celestial body was a comet, then its total mass was more than 2×107 t, and it had >160 m radius, and released an energy of >107 t TNT. 相似文献
11.
K. G. Miesserov 《Earth, Moon, and Planets》1982,27(1):13-25
The formation of the solar system is considered from the physico-chemical point of view. The main role in the process is ascribed to heavy metals and to the surface tension that had arisen as a result of appearance of a liquid layer of fused substance in the equatorial region of the protostar. The formation of the liquid layer was caused by the transfer of fused substance droplets under the action of centrifugal forces in the direction of the protostar surface. Due to the surface tension the prevalence of the centrifugal forces over the gravitational ones was able to reach the value when the density differentiation of the substance began to take place under the effect of the centrifugal forces, and accumulation of heavy metals proceeded in outermost equatorial region of the protostar. As a result the disk has been formed and a liquid ring was separated from the protostar. Later explosions on the young Sun sent parts of the hardened ring which possessed the first cosmic velocity to different distances away from the Sun. In such a way planets, their satellites, asteroids, meteorites and comets were formed. The physical characteristics of planets, the parameters of their orbits, and the data on the structure of meteorites are consistent with ideas developed in the paper. 相似文献
12.
V. M. Reshetnyk Yu. V. Skorov P. Lacerda P. Hartogh L. Rezac 《Solar System Research》2018,52(3):266-281
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
13.
The entry and subsequent breakup of the ~17–20 m diameter Chelyabinsk meteoroid deposited approximately 500 kT of TNT equivalent energy to the atmosphere, causing extensive damage that underscored the hazard from small asteroid impacts. The breakup of the meteoroid was characterized by intense fragmentation that dispersed most of the original mass. In models of the entry process, the apparent mechanical strength of the meteoroid during fragmentation, ~1–5 MPa, is two orders of magnitude lower than the mechanical strength of the surviving meteorites, ~330 MPa. We implement a two-material computer code that allows us to fully simulate the exchange of energy and momentum between the entering meteoroid and the interacting atmospheric air. Our simulations reveal a previously unrecognized process in which the penetration of high-pressure air into the body of the meteoroid greatly enhances the deformation and facilitates the breakup of meteoroids similar to the size of Chelyabinsk. We discuss the mechanism of air penetration that accounts for the bulk fragmentation of an entering meteoroid under conditions similar to those at Chelyabinsk, to explain the surprisingly low values of the apparent strength of the meteoroid during breakup. 相似文献
14.
We present a simple, semianalytic model of the vaporization of H2O and HDO ice from a comet nucleus. We use this model to show that the flux of HDO relative to H2O can be much higher, at times, than would be expected from the D/H ratio in the nuclear ice itself. This effect varies with position in the comet's orbit. It is negligible sufficiently near the Sun but could lead to erroneous interpretations of the primordial D/H ratio in cometary ice if measurements are made in other parts of the cometary orbit. 相似文献
15.
Frans J. M. Rietmeijer 《Meteoritics & planetary science》2019,54(2):259-266
So far there is no conclusive evidence for water in the nucleus of 81P/comet Wild 2. Recently magnetite in collected Wild 2 samples was cited as proxy evidence for parent body aqueous alteration in this comet (Hicks et al. 2017 ). A potentional source for water of hydration would be layer silicates but unfortunately there is no record, neither texturally nor chemically, for hydrated layer silicates that survived hypervelocity impact in the Wild 2 samples. This paper reports large vesicles in the matrix of allocation C2044,2,41,2,5 from a volatile‐rich type B/C Stardust track. These vesicles were probably caused by boiling water that were generated when hydrated Wild 2 silicates impacted the near‐surface silica aerogel layer. Potential water sources were partially and fully hydrated GEMS (glass with embedded metal and sulfides) and CI carbonaceous chondrite materials among the earliest dusts that experienced hydration and icy‐body formation and long‐range transport and mixing with materials from across the solar system. 相似文献
16.
Surface morphology and related issues for nuclei of three comets: Halley, Borrelly and Wild 2, are considered in the paper. Joint consideration of publications and results of our analysis of the comets’ images led to conclusions, partly new, partly repeating conclusions published by other researchers. It was found that typical for all three nuclei is the presence of rather flat areas: floors of craters and other depressions, mesas and terraces. This implies that flattening surfaces or planation is a process typical for the comet nuclei. Planation seems to work through the sublimation-driven slope collapse and retreat. This requires effective sublimation so this process should work only when a comet is close to the Sun and if on the nucleus there are starting slopes, steep and high enough to support the “long-distance” avalanching of the collapsing material. If the surface had no starting slopes, then instead of planation, the formation of pitted-and-hilly surfaces should occur. An example of this could be the mottled terrain of the Borelly nucleus. Both ways of the sublimational evolution on the nucleus surface should lead to accumulation of cometary regolith. The thickness of the degassed regolith is not known, but it is obvious that in surface depressions, including the flat-floor ones, it should be larger compared with nondepression areas. This may have implications for the in situ study of comets by the Deep Impact and Rosetta missions.Our morphological analysis puts constraints on the applicability of the popular “rubble-pile comet nucleus” hypothesis (Weissman, 1986. Are cometery nuclei primordial rubble piles? Nature 320, 242-244.). We believe that the rubble pile hypothesis can be applicable to the blocky Halley nucleus. The Borelly and Wild 2 nuclei also could be rubble piles. But in these cases the “rubbles” have to be either smaller than 30-50 m (a requirement to keep lineament geometry close to ideal), or larger than 1-2 km (a requirement to form the rather extended smooth, flat surfaces of mesa tops and crater floors). Another option is that the Borelly and Wild 2 nuclei are not rubble piles.In relation to surface morphology we suggest that three end-member types of the comet nuclei may exist: (1) impact cratered “pristine” bodies, (2) non-cratered fragments of catastrophic disruption, and (3) highly Sun-ablated bodies. In this threefold classification, the Wild 2 nucleus is partially ablated primarily cratered body. Borrelly is significantly ablated and could be either primarily cratered or not-cratered fragment. Halley is certainly partially ablated but with the available images it is difficult to say if remnants of impact craters do exist on it.Recently published observations and early results of analysis of the Tempel 1 nucleus images taken by Deep Impact mission are in agreement with our conclusions on the processes responsible for the Halley, Borrelly and Wild 2 nuclei morphologies. In particular, we have now more grounds to suggest that decrease in crater numbers and increase of the role of smooth flat surfaces in the sequence Wild 2?Tempel 1?Borelli reflects a progress in the sublimational degradation of the nucleus surface during comet passages close to the Sun. 相似文献
17.
This work is dedicated to the application to 67P/Churyumov-Gerasimenko of a new quasi-3D approach for non-spherically shaped comet nuclei with the aim to interpret the current activity of the comet in terms of initial characteristics and to predict shape and internal stratification evolution of the nucleus. The model is applied to differently shaped nuclei taking into account the characteristics of Comet 67P/Churyumov-Gerasimenko deduced from observations. We focus our attention on the combined effects that shapes and obliquity have on the comet surface and sub-surface evolution. We discuss the results in terms of activity, local dust mantle formation and disruption, erosion of the surface and internal stratigraphy.The results show that differently shaped nuclei can have different internal structures leading to different activity patterns and behaviors. Our calculations have shown that local variations in the dust and gas fluxes can be induced by the nucleus shape. The distribution of “active” areas on Comet 67P/Churyumov-Gerasimenko is different because of different shapes, reflecting the illumination conditions on the surface. These shapes can influence the structure of the inner coma, but the coma far away from the nucleus is only marginally affected by the nucleus shape. However, different comet behaviors can arise from differently shaped comet nuclei, especially in terms of local activity, surface and sub-surface characteristics and properties. The water flux local distribution is the most influenced by the shape as it is directly linked to the illumination. Irregular shapes have large shadowing effects that can result in activity patterns on the comet surface.The effects of different pole directions are discussed to see the relations with the nucleus activity and internal structure. It is shown that the orientation of the rotation axis plays a strong role on the surface evolution of 67P/Churyumov-Gerasimenko, determining seasonal effects on the fluxes. The activity of the comet changes greatly with the nucleus obliquity leading to pre-post-perihelion differences in the activity and seasonal effects. The effects of the dust deposition and crust formation on the cometary activity have also been simulated and are discussed with respect to 67P/Churyumov-Gerasimenko observations. The dust mantling is also strongly obliquity dependent, with different surface distributions of the dust-covered regions according to the different comet pole orientations. Finally, we show that our model can reproduce the fluxes behavior near perihelion in terms of amplitude and asymmetry, and we estimate 20% of the illuminated surface to be active. 相似文献
18.
R. Schulz 《Solar System Research》2009,43(4):343-352
One of the two planetary cornerstone missions of the European Space Agency is the Rosetta mission to comet 67P/Churyumov-Gerasimenko. Rosetta is a rendezvous mission with a comet nucleus, which combines an Orbiter with a Lander. It will monitor the evolution of the comet nucleus and the coma as a function of increasing and decreasing solar flux input along the comet’s pre- and post-perihelion orbit. Different instrumentations will be used in parallel, from multi-wavelength spectrometry to in-situ measurements of coma and nucleus composition and physical properties. Rosetta will go in orbit around the nucleus of its target comet 67P/Churyumov-Gerasimenko, when it is still far from the Sun and accompany the comet along its way to perihelion and beyond. In addition the Rosetta Lander Philae will land on the nucleus surface, before the comet is too active to permit such a landing (i.e. at around r = 3 AU) and examine the surface and subsurface composition of the comet nucleus as well as its physical properties. 相似文献
19.
H. Boehnhardt 《Earth, Moon, and Planets》1989,46(3):221-226
Clusters and packets of dust grains, detected by the VEGA spacecrafts in the coma of comet Halley, are interpreted either as due to microjets arising directly from the nucleus or as due to fragmentation processes of larger conglomerates. After a summary of the characteristics of the particle swarms observed and of the conclusion from their interpretation as microjets, some aspects from the fragmentation scenario of this phenomenon are outlined. From the spatial widths and the total number of grains in a particle swarm the mass of the parent conglomerate can be estimated. For the clusters and packets parent bodies of about 1 to several 100 g can be expected. Their travel time in the coma after release from the nucleus range from 10 to about 100 days before the spacecraft encounters. Finally, possible fragmentation processes are discussed and some comments for further investigations, both in theory and for in-situ data, are given. 相似文献
20.
P. Gronkowski 《Monthly notices of the Royal Astronomical Society》2009,397(2):883-889
The catastrophic thermodynamic destruction of large cometary heterogeneous grains lying on the surface of a comet nucleus is examined. The core–mantle grain-structure model is assumed. Grain fragmentation as an explanation of sudden changes in cometary brightness is proposed. The approach presented to the problem of cometary outbursts is a development of a previous author's paper. The proposed mechanism is based on the idea of thermodynamical destruction of heterogeneous cometary grains. Numerical simulations have been carried out for a wide range of values of physical characteristics of cometary material. The results obtained are consistent with observational data. The main conclusion of this paper is that thermodynamical fragmentation of large grains can explain variations in brightness and also outbursts of comets. 相似文献