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1.
《Planetary and Space Science》1999,47(6-7):787-795
The infrared emission of various comets can be matched within the framework that all comets are made of aggregated interstellar dust. This is demonstrated by comparing results on Halley (a periodic comet), Borrelly (a Jupiter family short period comet), Hale-Bopp (a long period comet), and extra-solar comets in the β Pictoris disk. Attempts have been made to generalize the chemical composition of comet nuclei based on the observation of cometary dust and volatiles and the interstellar dust model. Finally, we deduce some of the expected dust and surface properties of comet Wirtanen from the interstellar dust model as applied to other comets.  相似文献   

2.
One minute counts obtained during the meteor outburst of α-Monocerotids on November 22, 1995, are analyzed in order to examine the possibility of filamentary structure in the stream profile. None is found. It is argued that far-comet type outbursts are due to the Earth's passage through the dust trail of a long period comet, thus offering a direct means of studying such comet dust trails. Hence, the meteor stream activity curve is the first accurate cross section of dust densities through a comet dust trail.  相似文献   

3.
D.K. Yeomans 《Icarus》1981,47(3):492-499
The distribution of dust surrounding periodic comet Tempel-Tuttle has been mapped by analyzing the associated Leonid meteor shower data over the 902–1969 interval. The majority of dust ejected from the parent comet evolves to a position lagging the comet and outside the comet's orbit. The outgassing and dust ejection required to explain the parent comet's deviation from pure gravitational motion would preferentially place dust in a position leading the comet and inside the comet's orbit. Hence it appears that radiation pressure and planetary perturbations, rather than ejection processes, control the dynamic evolution of the Leonid particles. Significant Leonid meteor showers are possible roughly 2500 days before or after the parent comet reaches perihelion but only if the comet passes closer than 0.025 AU inside or 0.010 AU outside the Earth's orbit. Although the conditions in 1998–1999 are optimum for a significant Leonid meteor shower, the event is not certain because the dust particle distribution near the comet is far from uniform. As a by-product of this study, the orbit of comet Tempel-Tuttle has been redetermined for the 1366–1966 observed interval.  相似文献   

4.
Areview about K-H instability related to comet tail is presented.The effect of dust graing on KHI is theoretically analysed and relations of critical shear to excit KHI with the properties of dust grains both for sheared ion flow and for sheared dust flow re put out.Their possible application in comet tail is suggested.  相似文献   

5.
The photographic observations of comet Hale-Bopp with the 26-inch Pulkovo Observatory refractor in March–April, 1998, revealed three hemispherical gas-dust envelopes and one spiral jet in the comet head. We determined the angular distances of these envelopes from the comet nucleus and estimated their velocities. The masses and sizes of dust grains were estimated. We conclude that submicron-sized grains dominate in the envelopes. We also estimated the time scale of the comet nuclear activity, which manifests itself in dust ejection, the initial velocity of the ejected dust grains, and the ratio of the radiation-pressure force exerted on dust grains to the force of their gravitational attraction to the Sun. Our observations yielded an estimate for the radius of the comet nucleus, ～30 km.  相似文献   

6.
We calculate the position of dust trails from comet 8P/Tuttle, in an effort to explain unusual Ursid meteor shower outbursts that were seen when the comet was near aphelion. Comet 8P/Tuttle is a Halley-type comet in a 13.6-year orbit, passing just outside of Earth's orbit. We find that the meteoroids tend to be trapped in the 12:14 mean motion resonance with Jupiter, while the comet librates in a slightly shorter period orbit around the 13:15 resonance. It takes 6 centuries to decrease the perihelion of the meteoroid orbits enough to intersect Earth's orbit, during which time the meteoroids and comet separate in mean anomaly by 6 years, thus explaining the 6-year lag between the comet's return and Ursid outbursts. The resonances also prevent dispersion along the comet orbit and limit viewing to only one year in each return. We identified past dust trail encounters with dust trails from 1392 (Dec. 1945) and 1378 (Dec. 1986) and predicted another outburst on 2000 December 22 at around 7:29 and 8:35 UT, respectively, from dust trails dating to the 1405 and 1392 returns. This event was observed from California using video and photographic techniques. At the same time, five Global-MS-Net stations in Finland, Japan, and Belgium counted meteors using forward meteor scatter. The outburst peaked at 8:06±07 UT, December 22, at zenith hourly rate ∼90 per hour, and the Ursid rates were above half peak intensity during 4.2 h. We find that most Ursid orbits do scatter around the anticipated positions, confirming the link with comet 8P/Tuttle and the epoch of ejection. The 1405 and 1392 dust trails appear to have contributed similar amounts to the activity profile. Some orbits provide a hint of much older debris being present as well. This work is the strongest evidence yet for the relevance of mean motion resonances in Halley-type comet dust trail evolution.  相似文献   

7.
An analysis is presented for the photometric data on comet C/2013 A1 (Siding Spring) from observations at a large heliocentric distance (~4.1 AU). Comet C/2013 A1 (Siding Spring) displays intense activity despite the relatively large heliocentric distance. The morphology of the comet’s coma is analyzed. The following parameters are measured: the color indices V-R, the normalized spectral gradient of the reflectivity of the comet’s dust S', and the dust production rate Afρ. A numerical simulation is performed for the evolution of the comet’s orbit after a close encounter with Mars. The most probable values are obtained for the Keplerian orbital elements of the comet over a hundred-year period. The comet’s orbit remains nearly parabolic after passing the orbits of all the Solar System planets.  相似文献   

8.
Owing to sublimation of ice, comet nuclei eject dust particles when they are near to the sun. Those particles assume velocities and then vary their orbits to ones similar to that of the comet. The most notable difference between the orbit of the parent comet and those of the particles is their semi-major axes. This difference (Δ a ) has been widely used in modern meteor shower predictions. Observational evidence of the distribution showed that it is a function of Δ a , and the age of the dust trail. However, the relation is not well known. In this paper, a simplified relation between Δ a , the mass index ( s ) and the age of the dust trail is presented, taking the instance of a recent Leonid meteor shower.  相似文献   

9.
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   

10.
We consider the infrared brightness of a flattened comet belt beyond the orbit of Neptune using a disk-like model with a power-law density distribution of comets. We compare this spectrum with the emission from a model zodiacal dust cloud in the ecliptic and with published IRAS data and present some consequences of dust in the comet belt.  相似文献   

11.
Earth, Moon, and Planets - Comet Hale-Bopp has shown an extremely elevated level of dust activity during its apparition. We examine the dust activity of the comet, particularly the dust jets which...  相似文献   

12.
Synthetic images of the dust tail are presented for a comet which has a rotating nucleus with one predominant dust source fixed to it. The images have been generated using a new computer model which, unlike similar models, allows for the study of dust tails caused by a rotating nucleus with an anisotropic distribution of sources.The dust tail is studied in the post-perihelion phase of a parabolic comet with a perihelion distance of 0.5 AU. One finds that in the case of a rotating nucleus with anisotropic emission characteristics streamers caused solely by the dynamics of the dust particles are forming in the dust tail even if there is no dependence between the solar irradiation angle of the source and the amount of dust emitted. If the dust emission depends on the solar irradiation angle of the dust source, then the brightest tail regions do not necessarily coincide with the synchrones for the times of maximum dust emission.As a consequence, a thorough analysis of streamer patterns in a cometary dust tail requires assumptions on the rotational state and the dust source distribution of the nucleus. Otherwise, it seems not possible to discern between streamers which are caused dynamically by nucleus rotation and others which reflect variations in the emission activity.  相似文献   

13.
We review the composition of Jupiter-family comet (JFC) dust as inferred from infrared spectroscopy. We find that JFCs have silicate emission features with fluxes roughly 20-25% over the dust continuum (emission strength 1.20-1.25), similar to the weakest silicate features in Oort Cloud (OC) comets. We discuss the grain properties that alter the silicate emission feature (composition, size, and structure/shape), and emphasize that thermal emission from the comet nucleus can have significant influence on the derived silicate emission strength. Recent evidence suggests that grain porosity is the is different between JFCs and OC comets, but more observations and models of silicates in JFCs are needed to determine if a consistent set of grain parameters can explain their weak silicate emission features. Models of 8 m telescope and Spitzer Space Telescope observations have shown that JFCs have crystalline silicates with abundances similar to or less than those found in OC comets, although the crystalline silicate mineralogy of comets 9P/Tempel and C/1995 O1 (Hale-Bopp) differ from each other in Mg and Fe content. The heterogeneity of comet nuclei can also be assessed with mid-infrared spectroscopy, and we review the evidence for heterogeneous dust properties in the nucleus of comet 9P/Tempel. Models of dust formation, mixing in the solar nebula, and comet formation must be able to explain the observed range of Mg and Fe content and the heterogeneity of comet 9P/Tempel, although more work is needed in order to understand to what extent do comets 9P/Tempel and Hale-Bopp represent comets as a whole.  相似文献   

14.
The activity of a meteor shower is thought to be proportional to the activities through time of the parent comet. Recent applications of the dust trail theory provide us not only with a new method to forecast the occurrences and intensities of shower activities, but it is also offers a new approach to explore the history of past activities of the parent comet by retro-tracking its associated meteor showers. We introduce the result of an effort for relating meteor shower activities to the parent comet activities for which we chose the October Draconids and comet 21P/Giacobini-Zinner in this paper.  相似文献   

15.
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   

16.
The NASA Stardust mission has provided for laboratory study an extensive data set of cometary dust of known provenance (from comet 81P/Wild 2) yielding detailed insights into the composition of the comet. Combined with the results of data from other missions to short-period Jupiter family comets (JFC), this has greatly deepened the understanding of such objects. If depressions on the surface of comet 81P/Wild 2 are all taken as evidence of impact cratering, their number suggests a long occupancy in the outer region of the Solar System. The dust from comet 81P/Wild 2 has been shown to be heavily deficient in pre-Solar grains and rich in materials formed at high temperatures in the inner Solar System. Although it is too early to know if this is typical of JFC, it does argue for rapid and thorough mixing of materials in the disk on timescales related to comet formation, and may also suggest outward migration of small icy bodies after their formation. Thus, instead of providing mainly new knowledge of the pre-Solar materials expected to be rich in comets, Stardust and comet 81P/Wild 2 have instead focussed attention on large-scale transport processes during the critical period when cometary parent bodies were forming in the early Solar System.  相似文献   

17.
《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   

18.
A theoretical model of the nucleus thermal evolutionand differentiation is used to simulatethe evolution along the orbit of the gasand dust emissions of comet Hale–Bopp. The model was already applied to this comet (Capria et al., 2000b): At that time only the results of the observations obtained shortly after the perihelion were available. Now much more data have been published and we present more refined and complete results about the production rates of gasand dust along the orbit and the internal stratigraphy. The results of our model on long distance activity and its explanation are also presented.  相似文献   

19.
The paper presents an analysis of the actual brightness change of comet 73P/Schwassmann-Wachmann, which took place in 1995. The consequence of a cometary outburst is the destruction of a fragment of its surface. This causes the emission of comet material from both the surface and from exposed subsurface layers. Therefore, the calculations take into account the scattering cross-sections that come from ice and dust particles. It was assumed that the dust particles are silicates which are characterized by high irregularity of their structure. This assumption is a consequence of the analysis of the results provided by the Rosetta mission to the comet 67P/Churyumov-Gerasimenko. The main factor determining the amplitude of a cometary outburst is the mass ejected as well as the loss of ice that holds the individual nucleus structures together. Consequently, this phenomenon can significantly contribute to the destruction and even decay of the cometary nucleus.  相似文献   

20.
A stellar appulse of comet 1983d was observed in 1983 May 08.956 using a video photometry technique. Although the miss distance did not exceed 420 ± 50 km no significant absorption was detected. Another close appulse, of comet 1983j this time, was measured with a photoelectric photometer on 1983 September 14.036. A small feature of the lightcurve, only 1.8% deep, could be attributed to dust surrounding the nucleus within a 350-km radius. The deduced dust ejection rate is 6 × 106 g/sec.  相似文献   

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