Probing the internal structure of the nuclei of comets

There is no direct evidence about the internal structure of cometary nuclei, which are mostly hidden by their gas and dust comae, and have not yet been orbited by any spacecraft. Their densities are low, typically of about 400 kg m⁻³ for 9P/Tempel 1 (that was impacted by the Deep Impact probe) and 67P/Churyumov-Gerasimenko (that is the target of the Rosetta mission). Such low densities are in favour of a high macro-porosity, or a high micro-porosity, or both. Observations of disruption or splitting of nuclei indeed suggest that some huge sub-nuclei or some meter-sized fragments could be the building blocks of comets. Analysis, from in-situ measurements and from remote light scattering observations, of the structure of the dust particles, which significantly consist of fluffy aggregates of submicron-sized grains, could be in favour of a fractal structure. However, the presence of huge icy grains in the innermost coma, and of flat layers on the surface of 9P/Tempel 1, are clues to the complexity of these objects, which have suffered drastic erosion phenomena on their elongated orbits. It is expected that the Rosetta mission will provide a fair understanding of the structure of the deep interior of the nucleus of 67P/Churyumov-Gerasimenko, thanks to the on-board CONSERT experiment.     

The visibility of gamma-ray burst afterglows in dusty star-forming regions

Recent observations of the environments of gamma-ray bursts (GRBs) favour massive stars as their progenitors, which are likely to be surrounded by gas and dust. The visibility of the optical and UV emission of a GRB is expected to depend on the characteristics of both the dust and the GRB emission itself. A reasonable distribution of surrounding dust is capable of absorbing all the optical and UV emission of the optical flash and afterglow of a GRB, unless the optical flash has a peak isotropic luminosity L _peak≳10⁴⁹ erg s⁻¹ . This means that dark bursts should exist and these bursts will have to be studied at infrared rather than optical wavelengths. In this paper details will be given about the infrared GRB dust emission. The reprocessed dust emission peaks at a rest-frame wavelength of about 8 μm. Forthcoming space telescopes, in particular the IRAC camera on board the Space Infrared Telescope Facility , could detect this emission out to a redshift of about two. However, an accurate position of the GRB afterglow must be provided for this emission to be identified, because the light curve of the reprocessed dust emission does not vary on time-scales less than several years.     

On the origin of comets

The paper presents a brief history of cometary cosmogony. It discusses critically the eruptive hypothesis, the hypothesis on the relict origin of comets, and the hypothesis on a genetic connection between comets and trans-Plutonian planets. Laplace’s theoretical prediction as to the capture of long-period comets by Jupiter into short-period orbits is confirmed. We conclude that the interstellar hypothesis promising is for the provenance of comets.     

On the aging of comets

This study is based primarily on the calculations of comet orbits over ~ 10⁶ years for 160 short-period comets by Harold F. Levison and Martin J. Duncan from which there are calculated ablation AGES. There are positive statistical correlations (having many deviations) with radial nongravitational forces, comet activity measures, and dust-to-gas ratios in the spectra, in the sense that comets of greater AGES tend to be less active and to show less dust in their spectra than comets of lesser AGES.Harvard-Smithsonian Center for Astrophysics     

On the aging of comets

This study is based primarily on the calculations of comet orbits over ~ 10⁶ years for 160 short-period comets by Harold F. Levison and Martin J. Duncan from which there are calculated “ablation AGES”. There are positive statistical correlations (having many deviations) with radial nongravitational forces, comet activity measures, and dust-to-gas ratios in the spectra, in the sense that comets of greater “AGES” tend to be less active and to show less dust in their spectra than comets of lesser “AGES”.     

On the origin of comets

The fact that comets are rich in volatile material shows that they were formed and kept for a long time in the outer, low-temperature regions of the solar system. In this paper we analyse the structure in the outer edge of the solar nebula and show that no formation zone of comets can exist there. Our view is that the comets evolved from the residual planetesimals in the zone between Jupiter and Neptune.     

Numerical simulation of comet nuclei I. Water-ice comets

A one-dimensional simulation of pure water-ice cometary nuclei is presented, and the effect of the nucleus as a heat reservoir is considered. The phase transition from amorphous to crystalline ice is studied for two cases: (1) where the released latent heat goes entirely into heating adjacent layers and (2) where the released latent heat goes entirely into sublimation. For a Halley-like orbit it was found that for case 1 the phase boundary penetrates about 15 m on the first orbit and does not advance until sublimation brings the surface to some 10 m from the phase boundary. For case 2 the phase boundary penetrates about 1 m below the surface and remains at this depth as the surface sublimates. For an orbit like that of Schwassmann-Wachmann 1 the phase boundary penetrates about 50 m initially for case 1 and about 1 m for case 2. There is no further transformation until the entire comet is heated slowly to near the transition temperature, after which the entire nucleus is converted to crystalline ice. For an Encke-type orbit case 1 gives a nearly continuous transition of the entire nucleus to crystalline ice, while for case 2 the initial penetration is about 8 m and remains at this depth relative to the surface as sublimation decreases the cometary radius. Thus the entire comet is converted to crystalline ice just before it is completely dissipated.     

On the hydromagnetic model of comets

In this paper we have developed several consequences of Alfvén's (1957) hydromagnetic model of comets. It is shown that such a model not only accounts for the observed morphology and time variations of the fine structure in the plasma tail, but also leads, in a natural way, towards explanations of two of the central problems in cometary physics; namely, the short ionization time-scales of the cometary molecules, and the large velocities and accelerations observed far down the tail.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May 1978.     

On the stochasticity of Halley like comets

Using mainly Lyapunov characteristic numbers the stochasticity of Halley like cometary orbits has been investigated in the framework of different models of the restricted three body problem.     

On the monochromatic brightness variations of comets

A calculation of the non-steady development of a multi-species atmosphere of a comet moving in a near-parabolic heliocentric orbit is presented. The monochromatic brightness variations of the characteristic cometary emission bands due to OH, CN and C₂ are then evaluated assuming that the parent molecules of these chemically unstable species are respectively H₂O, HCN and H₂C₂ present in a homogeneous H₂O clathrate nucleus. For small heliocentric distances where a quasi-steady approximation is valid, the brightness variations follow Levin's (1943) law, provided all the destruction mechanisms of the cometary molecules vary as the inverse square of the heliocentric distance. On the other hand, at large heliocentric distances Levin's law breaks down, essentially due to the large time-scales of residence of the emitting species in the cometary atmosphere. This large residence time at large heliocentric distance also produces an asymmetry between the brightness profiles of the inbound and outbound passages, such that the brightness declines less steeply with distance on the outbound passage than on the inbound. Consequently, the monochromatic brightness of OH at 4 AU outbound is about twice as large as the corresponding value inbound. While some comets show such an effect, others show just the opposite effect. These deviations, which show the limitations of our simple homogeneous model, are discussed qualitatively in terms of the plausible time varying physical structure of the cometary nucleus. The variations of the relative monochromatic brightnesses of the various emissions are also discussed, and the need for extending monochromatic brightness measurements to larger heliocentric distances is stressed.     

H II regions and radio visibility of quasar nuclei

The core (injector) and the jet (relativistic plasma outflow) of AGN objects are surrounded by an ionized medium, an H II region observed in emission lines. The synchrotron radiation from the core and the jet is observed through a thin screen that cocoons the structure under consideration. The screen transparency depends on wavelength and distance from the injector. We consider the objects 3C 345 and 1803+784 whose core emission at decimeter wavelengths is absorption by more than 25 dB. The visible bright compact component is the bright nearby portion of the jet that extends outside the dense part of the screen. We explore the possibility of measuring the screen transparency from absorption in Hα recombination lines with different quantum numbers at centimeter wavelengths.     

On stellar occultations by comets

Stellar occultations by comets can be used for studying the dust opacity of the coma in the vicinity of the cometary nucleus. This method provides high spatial resolution. If faint stars (V ≤ 15) are used in the densest regions of the sky, the event probability is typically one every 2 hr. This probability can be enhanced by a factor of 40 if the Space Telescope is used.     

On the dynamic connection of comets with Uranus

We consider the connection with Uranus for: (1) 945 near-parabolic comets (the period P > 200 years, the perihelion distance q > 0.1 AU), (2) 1277 Kreutz comets (P > 200 years, q < 0.01 AU), and (3) 414 short-period comets (P < 200 years). It turns out that none of near-parabolic comets passed through Uranus’s activity sphere, none of the Kreutz comets approach Uranus closer than 11 AU, and only two short-period comets, C/2006 U7 and C/2006 F2, could have a close approach to Uranus during 5000 years.     

On the collision probability for comets with the Sun

We consider the collision probability for comets with the Sun under the suppositions of different velocity distributions and various initial conditions. We solve the problem applying Laplace's method and using Schiaparelli's hyperboloid of visibility. The probabilities obtained in this manner are given separately for elliptic and hyperbolic orbits.     

On the hypothesis of the eruption origin of near-parabolic comets

The hypothesis on the genetic connection of near-parabolic comets with Jupiter, Saturn, and the transPlutonian region (5–3000 AU) proposed by E.M. Drobyshevskii is considered. It has been shown that, on average, 5.6 comets per an area of 10⁶ AU² passed through the transPlutonian region during the whole history of observations. Six-hundred nineteen comets crossed the ecliptic at heliocentric distances ranging from 0 to 2 AU. As has been shown, from the total number of 945 near-parabolic comets, eight comets closely approached Jupiter and five closely approached Saturn. The Kreutz comets, 1277 objects, did not approach Jupiter closer than 3 AU. Their minimal distance to Saturn was 5.5 AU. The minimal distance of the Kreutz comets from the edge of the transPlutonian region was 28.8 AU. The analysis led to the conclusion that the concept on the origin of the near-parabolic comets suggested by Drobyshevskii is groundless.     

Searching for the source of short-period comet nuclei: The direction of the spatial migration of comets

An attempt is made to determine the spatial location of the main source of short-period comet nuclei. Numerical calculations for the orbital evolution of Jupiter family comets, medium-period comets, and Centaurs are used to show that the orbits of small solar system bodies tend to evolve in the direction of increasing semimajor axes. This relates to bodies that can experience encounters with planets and whose orbital evolution is shaped by gravitational perturbations. It is concluded that there is good reason to search for the main source of the nuclei of Jupiter family comets at distances of 6 AU or less from the sun.     

On the dectectability of icy grains in the comae of comets

Evaporation of icy grains over the distance scale of the visible cometary coma sets very specific limits on their temperature. Unless the grains are very pure water ice, the maximum size of an icy grain halo will be limited to a few hundred kilometers at heliocentric distances ?2.5 AU. It is unlikely that the 1.5- or 2-μm ice band could be detected in the scattering by icy grains. Detection of the 3?μm ice band might be possible in comets which display a coma at large heliocentric distances.     

The nuclei of Jupiter family comets: A critical review of our present knowledge

Of the currently over 300 identified Jupiter family comets (JFCs), we have estimated nucleus sizes and shapes for fewer than 70 and have detailed nucleus observations arising from spacecraft fly-bys for just 3: 19P/Borrelly (Deep Space 1), 81P/Wild 2 (Stardust), and 9P/Tempel 1 (Deep Impact). These observations reveal similarities but also significant diversity. In this review, we make a critical assessment of our knowledge of JFC nuclei and suggest a priority list for observations of the nucleus of the JFC, 67P/Churyumov-Gerasimenko, the Rosetta target comet.