On the visibility of nuclei of dusty comets

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

The Dust in Comet C/1999 S4 (LINEAR) during Its Disintegration: Narrow-Band Images, Color Maps, and Dynamical Models

Comet C/1999 S4 was observed with the 2m-telescopes of the Bulgarian National Observatory and Pik Terskol Observatory, Northern Caucasus, Russia, at the time of its disintegration. Maps of the dust brightness and color were constructed from images obtained in red and blue continuum windows, free from cometary molecular emissions. We analyze the dust environment of Comet C/1999 S4 (LINEAR) taking into account the observed changes apparent in the brightness images and in plots of Afρ profiles as function of the projected distance ρ from the nucleus. We also make use of the syndyne-synchrone formalism and of a Monte Carlo model based on the Finson-Probstein theory of dusty comets. The brightness and color of individual dust particles, which is needed to derive theoretical brightness and color maps of the cometary dust coma from the Monte Carlo model, is determined from calculations of the light scattering properties of randomly oriented oblate spheroids. In general, the dust of Comet C/1999 S4 (LINEAR) is strongly reddened, with reddening values up to 30%/1000 Å in some locations. Often the reddening is higher in envelopes further away from the nucleus. We observed two outbursts of the comet with brightness peaks on July 14 and just before July 24, 2000, when the final disintegration of the comet started. During both outbursts an excess of small particles was released. Shortly after both outbursts the dust coma “turns blue.” After the first outburst, the whole coma was affected; after the second one only a narrow band of reduced color close to the tail axis was formed. This difference is explained by different terminal ejection speeds, which were much lower than normal in case of the second outburst. In particular in the second, final outburst the excess small particles could originate from fragmentation of “fresh” larger particles.     

Hamiltonian Formulation and Perturbations for Dust Motion Around Cometary Nuclei

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.     

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.     

Infrared spectroscopy of interplanetary dust in the laboratory

Diagnostic infrared spectra of individual nanogram-sized interplanetary dust particles (IDPs) collected in the Earth's stratosphere have been obtained. A mount containing three crushed “chondritic” IDPs shows features near 1000 and 500 cm^?1, suggestive of crystalline pyroxene, and different from those of crystalline olivine, amorphous olivine, or meteoritic clay minerals. The structural diversity of chondritic IDPs and possible effects of atmospheric heating must be considered when comparing this spectrum with astrophysical spectra of interplanetary and cometary dust. Transmission electron microscope (TEM) and infrared observations are also reported on one member of the rare subset of IDPs which resemble hydrated carbonaceous chondrite matrix material. The infrared spectrum of this particle between 4000 and 400 cm^?1 closely matches that of the C2 meteorite Murchison. TEM observations suggest that this class of particles might serve as a thermometer for the process of heating on atmospheric entry.     

The role of the critical ionization velocity phenomena in the production of inner coma cometary plasma

The critical velocity triggering anomalous ionization of the neutral gas by plasma flow is calculated for the model based on the lower hybride instability. It depends strongly on the plasma and gas parameters, denning the instability development of the ionized atoms beam in the counter-streaming plasma.In particular, the possible role of the critical ionization mechanism for Halley's comet is examined. The fulfilment of both Townsend's condition for the selfustained beam plasma discharge and Alfven's condition for the critical velocity mechanism indicates that this mechanism may operate only within 10⁴ km from the cometary nucleus and give an ion production rate close to that observed for Kohoutek's comet.     

Dynamics of Dust Particles of Different Structure: Application to the Modeling of Dust Motion in the Vicinity of the Nucleus of Comet 67P/Churyumov–Gerasimenko

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.     

CoMA — An advanced space experiment forin situ analysis of cometary matter

Exploring one of the most pristine bodies in our solar system — a comet — with a spacecraft will be a great step towards a deeper understanding of our solar system's beginnings. We here present the advanced space experiment CoMA (cometarymatteranalyzer), which will be flown on NASA's cometary rendezvous and asteroid flyby mission CRAF. CoMA is a high resolution time-of-flight secondary ion mass spectrometer. It will analyze m-sized cometary dust grains and cometary gases with an unprecedented mass resolution and will yield data about the elemental, isotopic, and molecular composition.     

On the implausibility of a cometary origin for most Apollo-Amor asteroids

As the rate of replenishment of short-lived Apollo-Amor asteroids from the main belt seems to be insufficient to compensate for their losses, the idea was put forward that most of them are inactive cometary nuclei. However, the observational and theoretical evidence is inconclusive about whether it is possible to transform cometary nuclei into asteroid-like objects. There is good evidence that Apollo-Amor asteroids represent the last parent bodies of most, or even all, classes of meteorites. But meteorites cannot be formed within cometary nuclei having a constitution like Whipple's classical model, and alternative models seem to be unsatisfactory. Therefore, it is concluded that the cometary origin of most Apollo-Amor asteroids is implausible and they are genuine asteroids coming from the main belt. The process of their losses and replenishment must be studied further.     

Thermal modeling of cometary nuclei

A new model of the sublimation of volatile ices from a cometary nucleus has been developed which includes the effects of diurnal heating and cooling, rotation period and pole orientation, and thermal properties of the ice and subsurface layers. The model also includes the contribution from coma opacity, scattering, and thermal emission, where the properties of the coma are derived from the integrated rate of volatile production by the nucleus. The model is applied to the specific case of the 1986 apparition of Halley's comet. It is found that the generation of a cometary dust coma actually increases the total energy reaching the Halley nucleus. This results because of the significantly greater geometrical cross section of the coma as compared with the bare nucleus, and because the coma provides an essentially isotropic source of multiply scattered sunlight and thermal emission over the entire nucleus surface. For Halley, the calculated coma opacity is approximately 0.2 at 1 AU from the Sun, and 1.2 at perihelion (0.587 AU). At 1 AU this has little effect on dayside temperatures (maximum ≈200°K) but raises nightside temperatures (minimum ≈150°K) by about 40°K. At perihelion the higher opacity results in a nearly isothermal nucleus with only small diurnal and latitudinal temperature variations. The general surface temperature is 205°K with a maximum of 209°K at local noon on the equator. Some possible consequences of the results with respect to the generation of nongravitational forces, observed volatile production rates for comets, and cometary lifetimes against sublimation are discussed.     

Axel dust on Saturn and Titan

The scattering and absorption properties of Axel dust were investigated by means of Mie theory. We find that a flat distribution of particle radii between 0 and 0.1 μm, and an imaginary part of the index of refraction which varies as λ^?2.5 produce a good fit to the variation of Titan's geometric albedo with wavelength (λ) provided that τ_ext, the extinction optical depth of Titan's atmosphere at 5000 Å, is about 10. The real part of the complex index is taken to be 2.0. The model assumes that the mixing ratio of Axel dust to gas is uniform above the surface of Titan. The same set of physical properties for Axel dust also produces a good fit to Saturn's albedo if τ_ext = 0.7 at 5000 Å. To match the increase in albedo shortward of 3500 Å, a clear layer (containing about 7 km-am H₂) is required above the Axel dust. Such a layer is also required to explain the limb brightening in the ultraviolet. These models can be used to analyze the observed equivalent widths of the visible methane bands. The analysis yields an abundance of the order of 1000 m-am CH₄ in Titan's atmosphere. The derived CH₄/H₂ mixing ratio for Saturn is about 3.5 × 10^?3 or an enhancement of about 5 over the solar ratio.     

Negative ions in comets

The primary negative ion sources in comets are shown to be: for the inner coma—both polar photodissociation of HCN, electron attachment of OH and collisions with alkalis; in the vicinity of the nucleus—plasma, excavated during interplanetary dust impacts on the nucleus; for both the contaminated solar wind region and sporadic discharges in the non-homogeneous inner coma plasma—dissociative electron attachment and charge inversion during keV positive ion scattering by cometary dust are also significant sources. Negative ion abundance for Halley's Comet has been estimated to be from 10^–6 to 10^–10 of electron densities. However, this ratio may be more due to the formation of clusters A^–(H₂O)_n. Some possible cometary plasma effects, caused by negative ions, have also been discussed.     

Composition of cometary dust from polarization spectra

The wavelength dependence of the polarization (“polarization spectra”) of cometary dust is discussed. It is shown that, in the case of large phase angles, the wavelength dependence of the polarization is mainly controlled by the complex refractive index of the particle material, whereas the spectral dependence of the intensity is also sensitive to the size of the particles. This suggests that observations of “polarization spectra” may determine the composition of cometary dust. An attempt is made to find the composition of the cometary dust material by comparing the observed polarimetric data with laboratory measurements of complex refractive indices of possible cometary constituents. Silicates, graphite, metals, organics, water ice and their mixtures are considered. It is shown that astronomical silicate must be the most abundant constituent of cometary dust in the range of heliocentric distances from 0.8 to 1.8 AU, whereas the volume fraction of pure graphite or pure metals is less then 1%. A substance similar to that of F-type asteroids may be present in comets. There is evidence for an organic material that is being destroyed between heliocentric distances of 0.8–1.8 AU.     

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.     

Energization of positive ions in the cometary foreshock region

The energization of positive ions in front of a cometary bow shock is investigated. Ions produced by ionization of the cometary neutrals interact with the solar wind protons to produce, among other waves, large amplitude oscillations of the ambient magnetic field. Such oscillations are convected towards the comet at the unperturbed solar wind speed far from the shock and at a lower speed closer to the shock (due to the solar wind mass loading) ; hence, they can energize the suprathermal ions by Fermi acceleration. The spatial extension of the acceleration region is of the order of 10⁶ km and the resulting ion energy spectrum is harder than in the Earth's bow shock case. The energization of cometary ions produces an additional deceleration of the solar wind, such that the cometary bow shock of Halley-type comet may be regarded as a “cosmic ray shock”.     

A comparison of modeled and observed intensity profiles for C2, CN, and the blue continuum for Comet Bradfield (1987s)

Intensity profiles were obtained for the C₂ and CN emission and blue continuum of Comet Bradfield (1987s), from observations obtained over a 10 week period starting shortly before perihelion. Model intensity profiles were produced and then fitted to the observed profiles, and used to put constraints on some of the dust and gas parameters. Most of these parameters, including the gas and dust outflow speeds from the cometary nucleus and the molecular lifetimes, were consistent with expected values. The best fitting models incorporate significant dust particle fragmentation and extended emission of CN from dust, both occurring in the inner coma. In addition, although there may have been enhancement of gas and dust emission on the sunward side of the cometary nucleus, it appears that the tailward side maintained a significant level of activity.     

Comet Tempel-Tuttle and the Leonid meteors

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.     

Dust particles in the atmospheres of terrestrial planets and their roles for prebiotic chemistry: An overview

The available literature on sources, chemical composition, and importance of dust particles for the origins of life is analyzed. The most abundant sources of dust on the early terrestrial planets are sedimentation of interplanetary dust, meteoritic/cometary impacts, and volcanic eruptions. Interplanetary dust can originate directly from interstellar space, from evaporation of cometary bodies, from collisional destruction of asteroidal and meteoritic bodies, and nucleation in sunspots. Many rather complex organic species, including those of prebiotic interest, have been identified in the interstellar medium and cometary dust. Some of them are believed to formvia catalytic processes on the surfaces of dust particles. However, the mechanisms are not known, and even simulating experiments are difficult to perform due to insufficient knowledge of physical conditions in the space media and of chemical composition and properties of the dust. Besides the catalytic roles, cometary dust is believed to be the best delivery vehicle for organic matter of space origin to the atmospheres of terrestrial planets. Abundant sources of catalytically active fine dust can be volcanoes. Various organic and biological compounds have been found in terrestrial volcanic gases and ash, which are assumed to formvia the catalytic Fischer-Tropsch reactions. At present the eruptions on the Earth provide a unique opportunity to observein situ the formation of organic matter, and knowledge of the ash composition and local conditions allows to perform simulating experiments.     

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.