Large cometary grains – their destruction and changes in the luminosity of comets

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.     

Thermal destruction of cometary materials – application to activity of comets

Destruction mechanisms connected with thermodynamical behaviour of cometary material are reviewed with a special consideration of their effects on activity of comets. Consequences of thermal stresses which occur in the interior of a comet are discussed with reference to changes in the cometary brightness. Moreover, thermal destruction of grains placed in the head of the comet as well as on the surface of the nucleus is considered. It has been shown that the destruction of the cometary material can lead to an essential increase in the activity of the comet. Calculations have been carried out for a large assumed range of cometary parameters. The obtained simulated changes in the brightness of comets are consistent with the ones observed during the real variations and outbursts of brightness. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of the nucleus shape on the maximum size of grains ejected from a comet by gentle sublimation and jet‐like features

This paper deals with obtaining the maximum size of cometary grains ejected from nuclei of different shapes. Two mechanisms in terms of grain ejection from comets are taken into consideration. The first one is dragging of particles by outflowing gas molecules released by gentle sublimation from the comets. The second one is related with gas jets from the cavities in a nucleus by cometary jet‐like phenomena. We focused on ellipsoidal shapes of cometary nuclei but with different flattening. Calculations have been carried out for a large range of cometary parameters. It has been shown that for fixed mass of the nucleus the maximum size of grains is an increasing function of the nucleus flattening. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cometary outbursts – search of probable mechanisms – case of 29P/Schwassmann‐Wachmann 1

Cometary outbursts, sudden increases in luminosity have not been clearly explained so far and their source is still a mystery. Various possible mechanisms as a source of cometary outbursts at large distances from the Sun have been considered. It has been stated that plausible mechanisms are the polymerization of HCN and the amorphous water ice transformation combined with electrostatic destruction of cometary grains in the head of the comet. The calculations have been carried out for a large range of cometary parameters and it has been shown that the proposed scenario of the outburst gives a jump in the comet brightness which is consistent with the real jump observed during the 29P/Schwassmann‐Wachmann 1 outbursts.     

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)     

Collisions of comets and meteoroids: The post Stardust‐NExT discussion

The paper considers results of collisions between comets and meteoroids. We re‐discuss the five different approaches to estimate the sizes of holes created during such collisions. The results of the Deep Impact and the Stardust‐NExT missions to comet 9P/Temple 1 are applied to the estimation of these methods. We use the observed amount of ejected mass, the jump of brightness of the comet 9P/Tempel 1 as well as the diameter of the excavated crater. In the paper the simple way of estimation of impact consequences by use of the conception of the fragmentation energy of comet is also discussed. The numerical calculations were carried out for reasonable assumed values of a large range of cometary characteristics. The main conclusion of this paper confirms a general presumption that the main factor which determines the size of the impact crater on the comet 9P/Tempel 1 is the kinetic energy of impactor and strength or fragmentation energy of cometary material. In the considered case the gravitation of a comet has a minor meaning (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The search for a cometary outbursts mechanism: a comparison of various theories

In the paper the potential sources of energy of cometary outbursts have been reviewed. Considerations focus on four probable sources of the outbursts' energy. These are the polymerization of hydrogen cyanide HCN, impacts with the meteoroids, destruction of cometary grains in the field of strong solar wind and the transformation of amorphous water Ice into the crystalline one. The values of released energy and jumps of cometary brightness caused by these mechanisms have been discussed. A modern approach to the problem of the thermodynamical evolution of the comet nucleus which includes amorphous water ice is considered as the starting point in the discussion presented in the paper. The main characteristics of an outburst of a hypothetical comet belonging to the Jupiter family comet are calculated. The obtained results are in a good agreement with the characteristics observed during the real outbursts of comets. The main conclusion of this paper confirms a general presumption that the cometary outbursts can have different causes. However, the hypothesis concerning the amorphous water ice transformation appears to be the most probable one. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ionization mechanisms of cometary comas

A new approach is considered to the problem of ionization of the inner comas of comets connected with two phenomena: meteor-like process due to cometary molecules — interplanetary meteoroids impacts and explosion-type process due to high-velocity collisions between cometary dust grains and interplanetary meteoroids. It is found that the efficiency of explosive ionization exceeds the efficiency of meteor ionization approximately 100 times. The explosive ionization may be possible mechanism for anomaly ionization of the inner comae of dusty comets like Halley 1986 III with the dust to gas production rate ratio more than 0.1.     

Dynamics of high‐temperature materials delivered by jets to the outer solar nebula

Abstract— The presence of high‐temperature materials in the Stardust collection that are isotopically similar to those seen in chondritic meteorites argues for the outward transport of materials from the hot, inner region of the solar nebula to the region where comets formed. A number of mechanisms have been proposed to be responsible for this transport, with a number of models being developed to show that such outward transport is possible. However, these models have not examined in detail how these grains are transported after they have been delivered to the comet formation region or how they may be distributed in the cometary nuclei that form. Here, the dynamical evolution of crystalline silicates injected onto the surface of the solar nebula as proposed by jet models for radial transport is considered. It is generally found that crystalline grains should be heterogeneously distributed within the population of comets and within individual cometary nuclei. In order to achieve a homogeneous distribution of such grains, turbulence must be effective at mixing the crystalline silicates with native, amorphous grains on fine scales. However, this turbulent mixing would serve to dilute the crystalline silicates as it would redistribute them over large radial distances. These results suggest that it is difficult to infer the bulk properties of Wild 2 from the Stardust samples, and that the abundance of crystalline grains in these samples cannot alone be used to rule out or in favor of any of the different radial transport models that have been proposed.     

The Electrostatic Blow-Off of Cometary Grains and Variations and Outbursts of Brightness of Comets at Large Heliocentric Distances

Interaction between the strong solar wind and the dark side of a cometary nucleus was considered. It was calculated that the potential of the dark side of the cometary nucleus could be numerically large and negative. Assuming that the nucleus of the comet has surface mantle which consists of loose, fine dust-ice particles, it was shown that cometary particles could electrostatically levitate over the nucleus. It was examined how this phenomenon affected the changes in the cometary brightness. Calculations were carried out for realistically assumed values of a large range of cometary parameters. It was shown that the considered mechanism could lead to the variations of cometary brightness, sometimes even to the outbursts of brightness.     

Structure and density of cometary nuclei

Abstract— Understanding the nature of the cometary nucleus remains one of the major problems in solar system science. Whipple's (1950) icy conglomerate model has been very successful at explaining a range of cometary phenomena, including the source of cometary activity and the nongravitational orbital motion of the nuclei. However, the internal structure of the nuclei is still largely unknown. We review herein the evidence for cometary nuclei as fluffy aggregates or primordial rubble piles, as first proposed by Donn et al. (1985) and Weissman (1986). These models assume that cometary nuclei are weakly bonded aggregations of smaller, icy‐conglomerate planetesimals, possibly held together only by self‐gravity. Evidence for this model comes from studies of the accretion and subsequent evolution of material in the solar nebula, from observations of disrupted comets, and in particular comet Shoemaker‐Levy 9, from measurements of the ensemble rotational properties of observed cometary nuclei, and from recent spacecraft missions to comets. Although the evidence for rubble pile nuclei is growing, the eventual answer to this question will likely not come until we can place a spacecraft in orbit around a cometary nucleus and study it in detail over many months to years. ESA's Rosetta mission, now en route to comet 67P/Churyumov‐Gerasimenko, will provide that opportunity.     

On the formation of cometary nuclei in dense globules

Evolution of cometary orbits by planetary perturbations, weakly hyperbolic original orbits of comets calculated by Marsdenet al. (1978) are taken to indicate the interstellar origin of comets, and the possible formation of cometary nuclei in interstellar globules is discussed. The process is sedimentation of dust grains. It is shown that if a globule is at 40 K, its lifetime is sufficiently long to allow the sedimentation.     

The Destruction of Heterogeneous Cometary Grains as the Possible Cause of Variations and Outbursts of Comets' Brightness

It is examined whether the destruction of heterogeneous cometary grains can explain the variation of cometary brightness and comet outburst.Calculations were carried out for water ice grains with carbonoxideinclusions. It was shown that for realistic chosen values of parametersthe comet luminosity can increase by even a few stellar magnitudes.     

Processing of cometary grains at the nucleus surface

Cometary material inevitably undergoes chemical changes before and on leaving the nucleus. In seeking to explain comets as the origin of many IDPs (interplanetary dust particles), an understanding of potential surface chemistry is vital. Grains are formed and transformed at the nucleus surface; much of the cometary volatiles may arise from the organic material. In cometary near-surface permafrost, one expects cryogenic chemistry with crystal growth and isotope. This could be the hydrous environment where IDPs form. Seasonal and geographic variations imply a range of environmental conditions and surface evolution. Interplanetary dust impacts and electrostatic forces also have roles in generating cometary dust. The absence of predicted cometary dust ‘envelopes’ is compatible with the wide range of particle structures and compositions. Study of IDPs would distinguish between this model and alternatives that see comets as aggregates of core-mantle grains built in interstellar clouds.     

Observations Of Scattered Light From Cometary Dust And Their Interpretation

This review begins with a discussion of the techniques needed for observations of scattered light from cometary dust. After an introduction into the basic concepts of the scattering process, observations of the phase curves of brightness, colour and polarization are covered. Images of colour and polarization are presented and the observed relation of colour and polarization in jets and shells is discussed. The interpretation of the measurements is based on the power law size distributions of dust grains observed from space. The power index must lie between 2 and 4 to provide the mass budget and visibility of the dust coma in accordance with the basic facts of cometary physics. Application of mechanical (radiation pressure) theory to cometary images allows us to derive related power law distributions for comets not explored by spacecraft. Grain scattering models are presented and compared with observations. A prediction is made of the spatial distribution of Stokes parameters U and V in the presence of aligned particles. Up to now such patterns have not been observed. Future work should include the exploration of comets at small and possibly very small phase angles and a detailed comparison of polarization and colour images of comets with thermal images and with models based on mechanical theory. This revised version was published online in July 2006 with corrections to the Cover Date.     

Simulation Experiments with Cometary Analogous Material

Comet simulation experiments are discussed, in the context of physical models and the results in cometary physics, gathered especially from the GIOTTO space mission to comet P'Halley. The “status of the today knowledge” about comets, the experiments could start from, is briefly reviewed. The setup of the KOSI (German = Kometen Simulation) - experiments and the techniques to produce cometary analogous material, on the basis of that knowledge are described in general, as for the different KOSI experiments. The limitations of the simulation of physical processes at the surface of real comets in an earth-bound laboratory are discussed, and the possibilities to receive common insights in cometary physics are shown. Methods and procedures are described, and the major results reviewed. As with attempting to reproduce any natural phenomenon in the laboratory, there are short-comings to these experiments, but there are possibly major new insights to be gained. Physical laws only have the same consequences under same experimental or environmental conditions. A number of small-scale comet simulation experiments have been performed, since the early 60ties in many laboratories, but the largest and most ambitious series of comet simulation experiments to date were performed between 1987 and 1993 using the German space agency's (DLR) space hardware testing facilities in Cologne. These experiments were triggered by the scientific community after the comet P'Halley's recurrence in 1986 and the many data gathered by the space missions in this year. Simulation experiments have proved valuable in developing methods for making cometary analogues, and for exploring specific properties of such materials in detail. These experiments provided new insights into the morphology and physical behavior of aggregates formed out of silicate- /water-ice -grains likely to exist in comets. The formation of a dust mantle on the surface, and a system of ice layers below the mantle from the different admixed materials, have been detected after the insolation of the artificial comet. The mechanisms for heat transfer between the comet's surface and its interior, compositional, structural, and isotopic changes that occur near the comet's surface, were described by modeling in accordance with the experimental results. The mechanisms of the ejection of dust and ice grains from the surface, and the importance of gas-drag in propelling grains were investigated by close-up video cameras. This revised version was published online in July 2006 with corrections to the Cover Date.     

Peculiarities of Cometary Light Curves and Outburst Activity

We give an overview of the main results of our works on the revision of cometary light curves and on the search for new patterns and features in the evolution of the integrated brightness of comets as they move in circumsolar space. These works revealed several new, previously unknown phenomena in the integrated-brightness variations and outburst activity of comets. Our results supplement and expand the body of observational data that provides a basis for constructing a model of the cometary nucleus and developing a theory of the cometary evolution.     

Possible mechanism of brightness outbursts of comets

The mechanism of brightness outbursts of comets based on selective absorption of solar ultraviolet radiation by hydrogen atoms in the cometary head is considered. Due to this process, influence of the radiation on parent and daughter molecules in the near-nucleus region of the cometary head is different. As a result, under certain physical conditions in the cometary coma, the electronic-temperature increase may cause an outburst in the brightness of the comet.     

An idealized short-period comet model: Surface insolation,H2O flux,dust flux,and mantle evolution

A model of cometary activity is developed which integrates the feedback processes involving heat, gas, and dust transport, and dust mantle development. The model includes the effects of latitude, rotation, and spin axis orientation. Results are obtained for various grain size distributions, dust-to-ice ratios, and spin axis orientations. Attention is focused on the development, change of structure and distribution of dust mantles and their mutual interaction with ice surface temperature and gas and dust production. In this model the dust mantle controls the mechanism of gas transport not onlu by its effect on the temperature but, more importantly, by its own dynamic stability. Results suggest that an initially homogeneous short-period comet with a “cosmic” dust-to-water ice ratio, typical orbit, rotation rate, and grain size distribution would develop at most only a thin (<1 mm) cyclic mantle at all points on the nucleus. Such a fully developed temporary mantle would exist throughout the diurnal cycle only beyond ～4AU. Thus, cyclic behavior would be expected for such an idealized comet, at least for most of its lifetime. Long-term irreversible mantle development on comets with typical rotation rates was not found except regionally on Encke and also on objects with perihelia ?1.5 AU. Even in these cases, free silicate exists, after a few cycles, only as relatively rare large grains and agglomerates with radii ～1 cm scattered over exposed ice. Full mantle development would require hundreds to thousands of cycles. In the case of an initially homogeneous comet Encke, this slow incipient mantle development is shown to be the direct result of its peculiar axial orientation. High obliquity appears required for long-term mantle development for typical rotation rates and perihelia ?1.5 AU. Heat conduction into the nucleus for an incompletely mantled or bald comet has been found to be very important in maintaining relatively higher ice surface temperatures, and hence fluxes, during those portions of the diurnal and orbital cycles which would otherwise be cooler. It is also shown to be at least one cause of post perihelion brightness asymmetries, especially in lower obliquity comets. Maximum heliocentric distances at which 1-μm dust, sand, pebbles, cobbles, and boulders can be permanently ejected from the subsolar point by H₂O (CO₂) are (in AU): 6.9 (16.8), 5.2 (11.5), 1.8 (3.0), 0.21 (0.34) and 0.07 (0.11), respectively. A detailed anatomy of temperature, gas and dust fluxes vs latitude and longitude for a homogeneous rotating comet with fixed axis is given for comparison with future observations. Most H₂O flux histories deduced from brightness data are found to be in reasonable agreement with the model, allowing for uncertainty in radius and albedo. A clear exception is Encke. It is shown that the large discrepancy between Encke's observed and model predicted fluxes, based on radar cross section, can be used to evaluate the extent of exposed ice (<10%). The model is then used to place an active area so as to explain a reported sharp drop in flux on approach to the Sun at 0.78 AU. An active area or areas, <10% of the comet's surface, centered near 65°N latitude appears indicated. Although cyclic mantles are generally indicated for the set of parameters we used, our results show that a global mantle only 1 to 3 cm thick (depending on the orbit) consisting of a full range of grain sizes can cause irresversible evolution to a noncometary body. We investigated the long-term evolution of such a postulated initially thinly mantled cometary object. It was found that after the first few passes and until the end of its dynamic lifetime the object averaged <3 × 10^?12 g cm^?1 sec^?1 H₂O flux. Therefore, if cometary objects evolve into Apollo asteroids, ice should always be accessible within 10 m of the surface despite numerous close perihelion passages. The possible impact of factors not included in the model, such as initial inhomogeneities, coma scattering of radiation, and global redistribution of ejected silicate around the nucleus, are discussed.     

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.