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)     

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.     

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.     

Models of P/Wirtanen nucleus: active regions versus non-active regions

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.     

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.     

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)     

The provenance and evolution of comets

The process of comet formation through the hierarchical aggregation of originally submicron-sized interstellar grains to form micron-sized particles and then larger bodies in the protoplanetary disc, culminating in the formation of planetesimals in the disc extending from Jupiter to beyond Neptune, is briefly reviewed. The ‘planetesimal’ theory for the origin of comets implies the existence of distinct cometary reservoirs, with implications for the immediate provenance of observed comets (both long-period and short-period) and their evolution as a result of planetary perturbations and physical decay, for example splitting and sublimation. The principal mode of cometary decay and collisional interaction with the terrestrial planets is through the formation and evolution of streams of cometary debris and hitherto undiscovered ‘families’ of cometary asteroids. Recent dynamical results, in particular the sungrazing and sun-colliding end-state for short-period comet and asteroid orbits, are briefly discussed.     

Large Dust Grains Around Cometary Nuclei

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.     

Extraterrestrial samples from low Earth orbits: techniques for their collection and analysis

In the context of dust samples collections in space, the COMET experiment (Collecte en Orbite de Matière ExtraTerrestre) was proposed for the first time in 1982. The idea of such an experiment was to collect grains with identified cometary parent body, instead of mixing all extraterrestrial contributions present in low Earth orbit. It was thus proposed to install collectors inside hermetic boxes, to have these boxes mounted outside a space station, orbiting the Earth and to have the capability of choosing the time and duration of the collection. Since 1985, the COMET experiment has been exposed three times to space (COMET-1, in October 1985 during the encounter of the Earth with the Draconid meteor stream; the EUROMIR-95 instrument, exposing collectors, during the crossing by the Earth of the Orionid meteor stream associated to comet P/Halley and, in November 1998, during the crossing by the Earth of the Leonid meteor stream associated to comet Temple-Tuttle, COMET-99). Specific collection techniques, and corresponding analytical procedures have been developed. The collected particles are the only ones accessible in the laboratory with a known cometary origin, before the return to Earth (2006) of the Stardust mission, which will collect cometary grains in the tails of comet Wild 2. Such a challenge justifies the tremendous efforts brought into play, and that are summarized here.     

Production,in a stationary state,of solid material in a coma of cometary nuclei under the action of solar radiation

A stationary state of production of solid material in the coma of a comet is sought by assuming a production and dynamics of solid grains liberated during the vaporization of cometary nuclei under the action of solar radiation.     

Silicates in Hale–Bopp: hints from laboratory studies

The recent passage of the Hale–Bopp (C/1995 O1) comet has provided the first opportunity to analyse the infrared spectral properties of a bright comet both from the ground and by the ISO space observatory. Previous works have already been dedicated to study the potential candidates to reproduce the cometary feature at 10 μm observed for different comets. We have applied a similar approach to compare the Hale–Bopp (C/1995 O1) spectra with laboratory data. The best fit has been obtained by using a mixture of crystalline Mg-rich olivine (forsterite), amorphous olivine and amorphous carbon grains. Some constraints on the possible cometary grain types derive from our simulation. Aggregates of submicron particles, composed of amorphous and crystalline olivine and amorphous carbon materials seem to be compatible with the cometary emission. Moreover, the possibility of fitting observational data on a wide IR spectra range, offered by ISO, provides interesting hints about the size distribution of grains responsible for the detected features.     

A ∼25 K temperature of formation for the submicron ice grains which formed comets

Following the observations of ice grains in cometary comae and their size distributions, we reexamined experimentally our previous conclusion that the ice grains which agglomerated to form comet nuclei were formed at ∼25 K. The suggestion of a ∼25 K formation temperature was confirmed experimentally. Moreover, we suggest that these ice grains had to be of submicron size.     

The destruction of cometary grains and changes in the luminosity of comets

One explanation of the sudden changes in the brightness of comets is proposed based on the author's earlier suggestions involving the fragmentation of cometary grains. Within the inner coma, a core‐mantle model of the structure of grains is assumed. The proposed mechanism is a combination of electrostatic stress and thermodynamical fragmentation of the cometary grains water‐ice mantle. It has been shown that the vapour pressure of volatile inclusions placed in the waterice mantle of grains can increase sufficiently to cause their fragmentation. It takes place before grains can completely sublime into the vacuum away. Numerical calculations have been carried out for a large range of values of probable physical characteristics of cometary material. The proposed approach yields increases in cometary brightness consistent with observations of typical cometary outbursts. It is concluded that this approach can provide an explanation of the sudden change in activity of comets for a wide range of heliocentric distances (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A review of radio observations of comets

The current status of cometary radio observations is reviewed. Radio continuum observations made at different wavelengths can be used to model the properties of cometary particles. Continuum observations have been successful for two comets but the interpretation of the data is subject to some disagreement. Radar observations are important for determining the size, angular momentum, direction of motion, and surface properties of the cometary nucleus. One comet, p/Encke, has been successfully observed by radar. The reasons why radio observations can fail are discussed. These include the undue influence of the highly volatile “comet frost” which often coats new comets, small errors in radio ephemerides, the inopportune scheduling of observing periods at less than optimum cometary heliocentric distances and velocities, and poor spectroscopic properties of the molecular transitions chosen for observations. In order to clarify the sometimes confusing observations which have been reported, cometary radio spectroscopy is reviewed in chronological order, comet by comet, starting from the earliest reported searches for polyatomic molecules in the early 1970s through progress in understanding cometary OH and into current searches for glycine, the simplest amino acid. The results of current OH ultraviolet pumping models are briefly discussed and several formalisms for computing molecular production rates arepresented. Radio observational programs which can aid in discriminating between current theories of terrestrial biological evolution are introduced. Both specific and general conclusions are drawn from the available material on cometary radio spectroscopy.     

Infrared emission from comets

A brief discussion of the infrared observations from 4 to 20 micrometers of seven comets is presented. The observed infrared emission from comets depends primarily on their heliocentric distance. A model based on grain populations composed of a mixture of silicate and amorphous carbon particles in the mass ratio of about 40 to 1, with a power-law size distribution similar to that inferred for comet Halley, is applied to the observations. The model provides a good match to the observed heliocentric variation of both the 10 micrometers feature and the overall thermal emission from comets West and Halley. Matches to the observations of comet IRAS-Araki-Alcock and the antitail of comet Kohoutek require slightly larger grains. While the model does not match the exact profile and position of the 3.4 micrometers feature discovered in comet Halley, it does produce a qualitative fit to the observed variation of the feature's strength as a function of heliocentric distance. The calculations predict that the continuum under the 3.4 micrometers feature is due primarily to thermal emission from the comet dust when the comet is close to the Sun and to scattered solar radiation at large heliocentric distances, as is observed. A brief discussion of the determination of cometary grain temperatures from the observed infrared emission is presented. It is found that the observed shape of the emission curve from about 4 to 8 micrometers provides the best spectral region for estimating the cometary grain temperature distribution.     

Physical conditions in the plasma tail of comet C/1987 P1 bradfield

Photometric measurements of photographic images of comet C/1987 P1 Bradfield have been carried out with a flat-bed scanner equipped with a slide module. Lengthwise and transverse photometric profiles of the cometary plasma tail have been obtained. Magnetic field induction and some other physical characteristics of the cometary plasma tail observed in November 1987 have been estimated with the use of the diffusion model for a cometary tail by Shul’man and Nazarchuk (1968). It has been shown that the scanned images of comets can be used for estimating the physical characteristics of cometary tails.     

Physico-chemical phenomena in comets—III: The continuum of comet Burnham (1960 II)

The new model of the cometary head proposed in papers I and II is developed and applied to comet Burnham. It takes into account the likely existence of a halo of large icy particles surrounding the nucleus. These particles are steadily stripped from the nucleus by evaporating gases. Their terminal velocity and their rate of evaporation set the size of the halo. The existence of the icy halo influences in two ways the photometric characteristics of the coma. This paper establishes the photometric shape of the continuum as reflected by the icy grains, and compares it to the observed continuum of comet Burnham. Paper IV will compare the predictions of the model with the photometric profile of the molecular emission bands of C₂, in the same comet.     

“Primitive” galactic dust in the solar system?

Recent experimental results on the evolution of targets, simulating cometary bodies and interplanetary grains, under the action of energetic protons are summarized. The main finding is that carbon-rich volatiles evolve toward a complex organic material which in turn, when bombarded at higher doses, is transformed to a carbon-like material becoming completely different from the original matrix. The applications of these results to comets and interplanetary grains shows that galactic or solar fast protons are able to build up large amounts of organic refractory material down to depths of ∼ 100 m in a comet and of carbon-like materials on grains that are supposed to be cometary debris. This implies that relevant component materials of comets and grains lose their supposed “primitive” nature and “forget” what they were in the galactic cloud from which the Solar System was generated.     

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.     

Short-period Jupiter family comets after Stardust

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.