A New Simple Model of Comets-Like Activity of Centaurs

Outbursts and variations of brightness are well known manifestations of the physical activity of the comets. Most cometary outbursts are recorded not very far from the Sun, where sublimation of water ice plays a major role in the activity of this celestial bodies. However, comets sometimes show physical activity far from the Sun, where the rate of water ice sublimation is small. Also a special kind of small bodies, i.e. centaurs sometimes show strong physical activity far from the Sun. The paper is based on the idea that the nuclei of centaurs may contain numerous cavities that are filled with gas under pressure and debris of cometary material. Numerical simulations were carried out for realistically assumed values of a wide range of physical parameters of centaurs. The obtained results are consistent with the observations of the physical activity of these celestial bodies.     

Comet Splitting – Observations and Model Scenarios

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.     

Are the cometary outbursts caused by impacts with interplanetary vagabonds probable?

The possibility of impacts and their results in relation to the cometary outbursts between comets and other small bodies in the solar system has been investigated. Taking into consideration certain physical features of cometary nuclei and impacting bodies, the probability of impacts of small bodies moving in the main asteroid belt with hypothetical comets which represent three types: Jupiter family comets, Halley family comets and long period comets has been computed. The probability of impacts between comets and meteoroids at large heliocentric distances has also been estimated. Potential consequences of these events in relation to outbursts of the cometary brightness have been discussed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

All comets are born equal: infrared emission by dust as a key to comet nucleus composition

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.     

Modeling the Comet Nucleus Interior

Numerical simulation of the structure and evolution of a comet nucleus is reviewed both from the mathematical and from the physical point of view. Various mathematical procedures and approximations are discussed, and different attempts to model the physical characteristics of cometary material, such as thermal conductivity, or permeability to gas flow, are described. The evolution and activity of comets is shown to depend on different classes of parameters: Defining parameters, such as size and orbit, structural parameters, such as porosity and composition, and initial parameters, such as temperature and live radio isotope content. The latter are related to the formation of comets. Despite the large number of parameters, general conclusions, or common features, appear to emerge from the numerous model calculations — for different comets — performed to date. Thus, the stratified structure of comet nuclei, volatile depletion, and the role of crystallization of ice in cometary outbursts are discussed. Finally, an evolution model applied to comet C/1995 O1 Hale-Bopp — using different assumptions — is described and analysed in the light of observations.     

Cometary dust properties retrieved from polarization observations: Application to C/1995 O1 Hale-Bopp and 1P/Halley

The analysis of the polarized light scattered by cometary dust particles provides information on the physical properties of the solid component of cometary comae for C/1995 O1 Hale-Bopp and 1P/Halley. A model of light scattering by a size distribution of aggregates of up to 256 submicron-sized grains (spherical or spheroidal) mixed with single spheroidal particles has been developed, with its parameters adjusted to fit the phase angle and wavelength dependence of the polarization observations. The particles are built of two materials: a non-absorbing silicates-type material and a more absorbing organic-type material. The model reproduces accurately the inversion angle and the positive branch of the polarization phase curves from the visible to the near-infrared spectral domains. A negative branch of the polarization phase curves appears in our model, although the negative branch is not deep enough to reproduce accurately the observations. Significant differences are shown between the two comets, with dominance of small grains in the coma of Comet C/1995 O1 Hale-Bopp, well fitted by a distribution of the volume-equivalent diameter, a, following a^−3.0 with a lower cutoff around 0.20 μm and an upper cutoff of at least 40 μm. For 1P/Halley, the size distribution follows a^−2.8 with a lower cutoff around 0.26 μm and an upper cutoff of about 38 μm. The relative amount of organic-type particles is larger for 1P/Halley while the amount of aggregates, significant for both comets, is larger for C/1995 O1 Hale-Bopp.     

Cometary outbursts – the post-Deep Impact outlook on collisions as possible causes

The possibility of impacts between comets belonging to the Jupiter Family and other small bodies orbiting in the main asteroid belt, and the consequences in relation to cometary activity are discussed. The probability of such events and the jumps in cometary brightness caused by impacts are examined. The results are compared with the results of the Deep Impact mission to Comet 9P/Tempel 1. The main conclusion of this paper is in agreement with previous findings, namely that an impact mechanism cannot be the main cause of the outburst activity of comets.     

青岛观象台海尔—波普慧星观测

中国科学院紫金山天文台青岛观象台从1996年4月开始到1995年5月1日结束,在138个观测夜,共拍摄Hale-Bopp彗星底片508张。发现:Hale-Bopp彗星在1996年9月24日的一次爆发,1996年10月26日,11月13日的两次可能爆发。在1996年9月23日的底片上,Hale-Bopp呈现巨大的喷流。在1997年2月8日至5月1日期间,Hale-Bopp彗星彗头内存在大量壳层结构。     

Note on the structure of comet nuclei

The recent developments in cometary studies suggest rather low mean densities and weak structures for the nuclei. They appear to be accumulations of fairly discrete units loosely bound together, as deduced from the observations of Comet Shoemaker–Levy 9 during its encounter with Jupiter. The compressive strengths deduced from comet splitting by Öpik and Sekanina are extremely low. These values are confirmed by theory developed here, assuming that Comet P/Holmes had a companion that collided with it in 1892. There follows a short discussion that suggests that the mean densities of comets should increase with comet dimensions. The place of origin of short-period comets may relate to these properties.     

A Monte Carlo Code to Compute Energy Fluxes in Cometary Nuclei

A Monte Carlo model designed to compute both the input and output radiation fields from spherical-shell cometary atmospheres has been developed. The code is an improved version of that by H. Salo (1988, Icarus76, 253-269); it includes the computation of the full Stokes vector and can compute both the input fluxes impinging on the nucleus surface and the output radiation. This will have specific applications for the near-nucleus photometry, polarimetry, and imaging data collection planned in the near future from space probes. After carrying out some validation tests of the code, we consider here the effects of including the full 4×4 scattering matrix in the calculations of the radiative flux impinging on cometary nuclei. As input to the code we used realistic phase matrices derived by fitting the observed behavior of the linear polarization as a function of phase angle. The observed single scattering linear polarization phase curves of comets are fairly well represented by a mixture of magnesium-rich olivine particles and small carbonaceous particles. The input matrix of the code is thus given by the phase matrix for olivine as obtained in the laboratory plus a variable scattering fraction phase matrix for absorbing carbonaceous particles. These fractions are 3.5% for Comet Halley and 6% for Comet Hale-Bopp, the comet with the highest percentage of all those observed.The errors in the total input flux impinging on the nucleus surface caused by neglecting polarization are found to be within 10% for the full range of solar zenith angles. Additional tests on the resulting linear polarization of the light emerging from cometary nuclei in near-nucleus observation conditions at a variety of coma optical thicknesses show that the polarization phase curves do not experience any significant changes for optical thicknesses τ?0.25 and Halley-like surface albedo, except near 90° phase angle.     

Observational Constraints On Surface Characteristics Of Comet Nuclei

Direct observations of the nuclear surfaces of comets have been difficult; however a growing number of studies are overcoming observational challenges and yielding new information on cometary surfaces. In this review, we focus on recent determinations of the albedos, reflectances, and thermal inertias of comet nuclei. There is not much diversity in the geometric albedo of the comet nuclei observed so far (a range of 0.025 to 0.06). There is a greater diversity of albedos among the Centaurs, and the sample of properly observed TNOs(2) is still too small. Based on their albedos and Tisser and invariants, Fernández et al. (2001) estimate that about 5% of the near-Earth asteroids have a cometary origin, and place an upper limit of 10%. The agreement between this estimate and two other independent methods provide the strongest constraint to date on the fraction of objects that comets contribute to the population of near-Earth asteroids. There is a diversity of visible colors among comets, extinct comet candidates, Centaurs and TNOs. Comet nuclei are clearly not as red as the reddest Centaurs and TNOs. What Jewitt (2002) calls ultra-red matter seems to be absent from the surfaces of comet nuclei. Rotationally resolved observations of both colors and albedos are needed to disentangle the effects of rotational variability from other intrinsic qualities. New constraints on thermal inertia of comets are consistent with previous independent estimates. The thermal inertia estimates for Centaurs 2060 Chiron and 8405 Asbolus are significantly lower than predicted by thermal models, and also lower than the few upper limits or constraints known for active, ordinary nuclei.     

Physical-mechanical properties of a cometary nucleus

Physical-mechanical properties of cometary nuclei matter are described in detail. As compared to other Solar System bodies, cometary nuclei are characterized by low strength properties. The ultimate tensile strength of cometary matter and cometary nuclei on the whole is about 2 kPa. An analysis performed based on a rheological model of a self-gravitating triaxial solid body showed that cometary nuclei less than 50–60 km (this actually being all known comets) are characterized by a constant ultimate tensile strength which is determined only by the matter composition and structure. The effective ultimate tensile strength for bodies larger than 50–60 km is determined by the body mass and figure parameters and increases according to the quadratic law depending on the body dimensions and mass. Such an increase of the effective strength can explain the absence or deficit of cometary nuclei more than 60 km in size, since it can significantly affect the parameters of the parent body destruction and the formation of a secondary population. The dependence of the mechanism and character of destruction on the parameters of the figure for Kuiper objects more than 50–60 km is size can yield a deficit of the population of the bodies whose figure parameters are a/c > 1.75 with respect to the bodies with a/c < 1.75 figure parameters.     

Comparison of the composition of the Tempel 1 ejecta to the dust in Comet C/Hale-Bopp 1995 O1 and YSO HD 100546

Spitzer Infrared Spectrograph observations of the Deep Impact experiment in July 2005 have created a new paradigm for understanding the infrared spectroscopy of primitive solar nebular (PSN) material—the ejecta spectrum is the most detailed ever observed in cometary material. Here we take the composition model for the material excavated from Comet 9P/Tempel 1's interior and successfully apply it to Infrared Space Observatory spectra of material emitted from Comet C/1995 O1 (Hale-Bopp) and the circumstellar material found around the young stellar object HD 100546. Comparison of our results with analyses of the cometary material returned by the Stardust spacecraft from Comet 81P/Wild 2, the in situ Halley flyby measurements, and the Deep Impact data return provides a fundamental cross-check for the spectral decomposition models presented here. We find similar emission signatures due to silicates, carbonates, phyllosilicates, water ice, amorphous carbon, and sulfides in the two ISO-observed systems but there are significant differences as well. Compared to Tempel 1, no Fe-rich olivines and few crystalline pyroxenes are found in Hale-Bopp and HD 100546. The YSO also lacks amorphous olivine, while being super-rich in amorphous pyroxene. All three systems show substantial emission due to polycyclic aromatic hydrocarbons. The silicate and PAH material in Hale-Bopp is clearly less processed than in Tempel 1, indicating an earlier age of formation for Hale-Bopp. The observed material around HD 100546 is located ∼13 AU from the central source, and demonstrates an unusual composition due to either a very different, non-solar starting mix of silicates or due to disk material processing during formation of the interior disk cavity and planet(s) in the system.     

Collision with meteoroids as one of the possible mechanisms for the splitting of cometary nuclei

Results are presented of a statistical analysis of dynamic parameters for 114 comets with split nuclei. A list of the objects includes actually split comets, fragments of cometary pairs, lost comets with designation D, and comets with large-scale atmospheric features. Some aspects of the hypothesis that splitting is caused by collisions of cometary nuclei with meteoroid swarms are investigated. To verify the hypothesis, an analysis is conducted of the positions of split comets’ orbits relative to 58 meteor streams from Cook’s catalogue. The calculations give the number (N) of orbital nodes of split comets relative to the plane of each swarm within a distance of 0.001, 0.005, 0.01, 0.05, and 0.1 AU from each swarm. A special algorithm is proposed for determining the degree of redundancy of N by finding the expected value and dispersion for the number of the nodes. The comparison of N with the expected value, together with the consideration of the dispersion, reveals a redundancy of N in 29 cases. Therefore, collisions of comets with meteoroid swarms can be considered as one of the possible causes of comet splitting. A similar testing is conducted for the asteroid belt and Kuiper belt as potential sources of a vast number of sporadic meteoroids. Based on the results of the calculations, the former may be considered as the most effective region of splitting of periodic comets.     

Lessons of Comet Hale-Bopp for Coma Chemistry: Observations and Theory

Many new cometary molecules — both parents and daughters — were detected in the exceptionally productive comet C/1995 O1 (Hale-Bopp).The space distribution of several of these species could be investigated from radio interferometry or from long-slit spectroscopy in the infrared. The distinction between parent species — directly sublimated from nucleus ices — and secondary species — resulting from chemical processing in the coma or produced by a secondary source — is not always clear. It is important to assess whether or not observed minor species (HCOOCH₃, HCOOH...) could be synthesized by chemical reactions favoured by the high density of the coma of comet Hale-Bopp. Chemical modelling by Rodgers and Charnley suggests that this is notthe case. CO and H₂CO are abundant cometary species which partly come from distributed sources. The nature of these sources is still a mystery. A special case, now well documented, is that of HNC, for which the abundance evolution with heliocentric distance could be observed in comet Hale-Bopp and which was observed in several much less productive comets.     

Relative motions of fragments of the split comets.: I. A new approach

A new approach is formulated for the study of motions of the split comets. It is based on the assumption that two fragments of a comet separate at a rate that is determined primarily by a slight difference between their effective solar attractions rather than by the impulse imparted on them at the time of splitting. The net dynamical effect is interpreted as due to differential nongravitational forces, which depend on the size, density, structure, composition, and spin rate of the fragments. Since at least at smaller distances from the Sun these forces vary inversely as roughly the square of heliocentric distance, their dynamical effect resembles that of radiation pressure, so that the formalism developed for the motion of a dust particle in a cometary tail is applicable in principle. The calculations show that this approach provides reasonably good to excellent fits of the observed separations for a great majority of the split comets, and that it fails only in the case of Comet 1957 VI. The correlation between the differential nongravitational forces and the endurance of the fragment is investigated in terms of the physical behavior of the fragments, with the emphasis on the short-lived objects. Some of the unusual phenomena accompanying the split comets are discussed, and comments are also offered on the sequence of splitting for comets with multiple nuclei and on the distribution of the points of splitting in space.     

Maximum non-gravitational acceleration due to out-gassing cometary nuclei

Maximum possible acceleration due to out-gassing from cometary nuclei is calculated for H₂O and CO(N₂) molecules. It is found that the maximum excess velocity at great distance is 0.18 km s^–1 so that excess velocities less than this value are compatible with the non-gravitational acceleration due to non-symmetric out-gassing. On the other hand, Comet 1975q and comet 1955V have excess velocities 0.81 and 0.80 km s^–1 respectively. These comets may be regarded as the candidates for possible interstellar comets.     

Kelvin-Helmholtz instabilities in cometary ion tails

We interpret observations of wave-like phenomena in cometary ion tails in terms of the interaction of the tails with the solar wind through Kelvin-Helmholtz helical instabilities. The calculations are applied to three comets, Comet Kohoutek (1973f), Comet Arend Roland (1957 III) and Comet Morehouse (1908c). Whilst some disagreement is found with earlier work, it is nevertheless shown that, assuming typical parameters, the growth rate of the K-H helical mode should be significant for wavelengths approximately equal to the tail radius.