Physico-Chemistry of Comets: Models and Laboratory Experiments

Until cometary matter can be studied in-situ or cometary samples are brought back to Earth for analysis, theoretical models and laboratory studies remain a crucial tool for revealing the nature of cometary matter. Constraints on the nature of the primordial material available for incorporation into comets and other solar system material comes from analysis of data from space-based and ground-based observatories. The structure of the nuclear ice component, which may have coexisting amorphous/crystalline phases and include clathrates and other trapped guest molecules, strongly influences the cometary outgassing properties. This paper reviews laboratory work on ice and carbon aceous compounds and discusses their significance for cometary chemistry. Special emphasis will be given to studies on the thermal processing of ices and their implications for the structure changes and subsequent release of volatiles. We also describe the preliminary results of a model of nuclear outgassing, and discuss how such model scan be used to infer the chemical structure of the nuclearices. Furthermore, we confront cometary data with the analysis of carbonaceous meteorites. Recent laboratory results on volatile compounds and the macro molecular structure of carbonaceous meteorites allow us to investigate the link of small bodies in the Solar System. Until ROSETTA will land on comet Wirtanen and study directly the nuclear composition, laboratory measurements of ice and refractory analogs will — together with the analysis of meteorites —significantly improve our knowledge on the origin and structure ofcomets.     

The Chemical Diversity of Comets: Synergies Between Space Exploration and Ground-based Radio Observations

A fundamental question in cometary science is whether the different dynamical classes of comets have different chemical compositions, which would reflect different initial conditions. From the ground or Earth orbit, radio and infrared spectroscopic observations of a now significant sample of comets indeed reveal deep differences in the relative abundances of cometary ices. However, no obvious correlation with dynamical classes is found. Further results come, or are expected, from space exploration. Such investigations, by nature limited to a small number of objects, are unfortunately focussed on short-period comets (mainly Jupiter-family). But these in situ studies provide “ground truth” for remote sensing. We discuss the chemical differences in comets from our database of spectroscopic radio observations, which has been recently enriched by several Jupiter-family and Halley-type comets.     

Observations of Ions in Comets: A Contribution Towards Understanding the Comet-Solar Wind Interaction

Since many years cometary ions have been observed by the authors and their coworkers in order to study the comet-solar wind interaction. Comets with water production rates ranging from 10²⁸(46P/Wirtanen) to 6 10³⁰molecules s⁻¹(C/1995 O1 Hale-Bopp) have been observed. In this paper we briefly introduce the physics of the comet-solar wind interaction. New observations of comet C/1996 Q1 (Tabur) are presented, where for the first time H₂O⁺and CO⁺ions have been recorded exactly simultaneously with a two-channel system. They are compared with previous observations of comets C/1989 X1 (Austin), 46P (Wirtanen) and 109P (Swift-Tuttle). We use a new method of Wegmann et al. (1998), based on the MHD scaling law, to determine the water production of comet Tabur from its H₂O⁺column density map and obtain a value of 3.3 10²⁸water molecules s⁻¹. Nonstationary phenomena like tail rays and so-called tail disconnections are very briefly reviewed. A movie of plasma envelopes observed in the light of OH⁺in comet 1995 O1 (Hale-Bopp) is presented on the attached CD-ROM. This revised version was published online in July 2006 with corrections to the Cover Date.     

Observations of C2 Molecules in the Coma of Comets at Mitaka

In order to monitor the various phenomena of comets, we continuously made the near-nucleus imaging observations of comets at National Astronomical Observatory, Mitaka. Here we report on the results of the observations of two spectacular comets. One is the analysis of photometry of C₂ emission images in the coma of comet Hyakutake (C/1996 B2). By comparing the production rates of C₂ radicals with C₂H₂ molecules, we conclude that the most C₂ radicals were originated from C₂H₂, while some of them might be produced from other parents. The second item is the analysis of the morphology of C₂ emission images in the coma of comet Hale-Bopp(C/1995 O1). By applying a ring masking technique, we detected asymmetrical distribution of the C₂ molecules. The results of these two comets might suggest that a portion of C₂ radicals were formed by the disintegration of the small organic dust grains, like the so-called CHON particles. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chemical Composition Diversity Among 24 Comets Observed At Radio Wavelengths

We present a comparative study on molecular abundances in comets basedon millimetre/submillimetre observations made with the IRAM 30-m,JCMT, CSO and SEST telescopes. This study concerns a sample of 24comets (6 Jupiter-family, 3 Halley-family, 15 long-period) observedfrom 1986 to 2001 and 8 molecular species (HCN, HNC, CH₃CN,CH₃OH, H₂CO, CO, CS, H₂S). HCN was detected in all comets,while at least 2 molecules were detected in 19 comets. From the sub-sample of comets for which contemporary H₂O productionrates are available, we infer that the HCN abundance relative to water variesfrom 0.08% to 0.25%. With respect to other species, HCN is the moleculewhich exhibits the lowest abundance variation from comet to comet. Therefore,production rates relative to that of HCN can be used for a comparative study ofmolecular abundances in the 19 comets. It is found that: CH₃OH/HCN varies from ≤ 9 to 64; CO/HCN varies from ≤ 24 to 180; H₂CO/HCN varies between 1.6 and 10; and H₂S/HCN varies between 1.5 and 7.6. This study does not show any clear correlation between the relative abundancesand the dynamical origins of the comets, or their dust-to-gas ratios.     

Conference summary: Comets

Conference summary: Comets     

High Resolution Spectroscopic Campaigns to Support Spacecraft Observations of Comets with Small Telescopes

Ground based observations of comets obtained in support of spacecraft missions and space telescopes have provided critical supporting context that greatly enhances the value of the combined dataset. Major areas of ground-based contribution include providing unique instrumental capabilities and an increased temporal or global perspective on the system under study. This paper describes a decades long program of supporting cometary observations focused on high resolving power measurements of ions and atomic/molecular radicals in the coma. The instrumentation is described, along with the species under study and the results from a large campaign to study comet C/1995O1 (Hale-Bopp).     

The Shape of the Solar Limb: Models and Observations

In this paper we compare observed, empirical, and modelled solar limb profiles and discuss their potential use to derive physical properties of the solar atmosphere. The PHOENIX, SolMod3D, and COSI radiative transfer codes as well as VAL-C models are used to calculate the solar limb shape under different assumptions. The main properties of each model are shown. The predicted limb shape as a function of wavelength for different features on the solar disk, such as quiet Sun, sunspots, and faculae, is investigated. These models provide overall consistent limb shapes with some discrepancies that are discussed here in terms of differences in solar atmosphere models, opacities, and the algorithms used to derive the solar limb profile. Our analysis confirms that the most common property of all models is limb shapes that are much steeper than what is observed, or predicted by the available empirical models. Furthermore, we have investigated the role of the Fraunhofer lines within the spectral domain used for the solar limb measurements. Our results show that the presence of the Fraunhofer lines significantly displaces the limb inflection point from its position estimated assuming only the photospheric continuum. The PICARD satellite, launched on 15 June 2010, will provide measurements of the limb shape at several wavelengths. This work shows that the precision of these measurements allows for discrimination among the available models.     

Eagle Nebula Pillars: From Models to Observations

Over the past few years, our group has been developing hydrodynamic models to simulate formation of the Eagle Nebula pillars. The true test of any model is, of course, how well it can reproduce the observations. Here, we discuss how we go about testing our models against observations. We describe the process by which we “observe” the model data to create synthetic maps. We show an example of this technique using one of our model runs and compare the resultant synthetic map to the real one.     

Observations of the Oh Radio Lines of Comets at Nançay – Recent Developments

The 18 cm lines of OH have been observed in more than 70 comets withthe Nançay radio telescope since 1974. The observations made from1982 to 1999 are now organized in a data base. The Nançay radiotelescope has been recently upgraded, and observations since 2000 aremade with a sensitivity increased by about a factor of two. Twelvecomets were observed from July 2000 to May 2002.     

Meteor Streams and Comets

Recent progress on the interrelation between meteor streams and comets is reviewed both on dynamical and physical aspects. The topics include the recent concept of the structure of meteor streams, resulted success of the prediction of the meteor storms, and the recent observational situation on the cometary dust grains and meteoroids. Two possible explanations for the origin of the meteoroids together with the implication for the relation between the birthplace of the parent comets and the meteoroids are discussed.     

Migration of Comets

Based on one of the particular cases of twice averaged model Hill problem with the allowance for the oblateness of the central body a quadrature is derived for the determination of the migration time of cometary nuclei from various cometary reservoirs and a (14-th order) algebraic equation for the determination of the initial conditions that allow the escape of the cometary nucleus (which at the initial instant of time moves in an orbit with arbitrary eccentricity (0 < e < 1) and inclination (0° < i < 180°) deeply inside the sphere o f action of the central body) from the sphere of action of the central body or its impact onto the central body. We analyze the shape of the boundaries of the hypothetical cometary reservoirs and the method of searching for regions of high concentration of interstellar particles in the Solar System.     

Isotopic Ratios in Comets: Status and Perspectives

Isotopic abundance ratios are excellently suited to probe the origin of solar system matter. We review the recent measurements of the isotopic ratios of the light elements (D/H, ¹²C/¹³C, ¹⁶O/¹⁸O, ¹⁴N/¹⁵N, ³²S/³⁴S) in cometary dust and gas and discuss briefly their implications. Special emphasis will be put on the determinations and progress performed in the field over the past years thanks to high resolution spectroscopy of cometary comae obtained with the ESO Very Large Telescope. Future perspectives from space missions and ground-based observations with new large and extremely large telescopes operating in the optical, infrared and submillimeter wavelengths will be presented.     

Tidal Breakup of Comets

In the context of the survival of periodic comets of different origins, rotational breakup and tidal disruption could be important, especially of the short period comets injected from the Kuiper belt. This is because long-period comets from the distant Oort cloud tend to be subject to thermal stress and volatile 'explosion' far more severely. A simple calculation using the Öpik method of random planetary close encounters was performed to estimate the probability of tidal disruption of comets and scattered Kuiper belt objects (SKBOs) during their orbital migration. It was found that a large fraction of the short period comets and SKBOs might have been internally fragmented by single or multiple close encounters with the outer planets.     

Comets,Enceladus and panspermia

A growing body of evidence suggests the operation of life and life processes in comets as well in larger icy bodies in the solar system including Enceladus. Attempts to interpret such data without invoking active biology are beginning to look weak and flawed. The emerging new paradigm is that life is a cosmic phenomenon as proposed by Hoyle and Wickramasinghe (Lifecloud: the Origin of Life in the Galaxy, 1978) and first supported by astronomical spectroscopy (Wickramasinghe and Allen, Nature 287:518, 1980; Allen and Wickramasinghe, Nature 294:239, 1981; Wickramasinghe and Allen, Nature 323:44, 1986). Comets are the transporters and amplifiers of microbial life throughout the Universe and are also, according to this point of view, the carriers of viruses that contribute to the continued evolution of life. Comets brought life to Earth 4.2 billion years ago and they continue to do so. Space extrapolations of comets, Enceladus and possibly Pluto supports this point of view. Impacts of asteroids and comets on the Earth as well as on other planetary bodies leads to the ejection of life-bearing dust and rocks and a mixing of microbiota on a planetary scale and on an even wider galactic scale. It appears inevitable that the entire galaxy will be a single connected biosphere.     

Resonance and Capture of Jupiter Comets

A number of Jupiter family comets such as Otermaand Gehrels 3make a rapid transition from heliocentric orbits outside the orbit of Jupiter to heliocentric orbits inside the orbit of Jupiter and vice versa. During this transition, the comet can be captured temporarily by Jupiter for one to several orbits around Jupiter. The interior heliocentric orbit is typically close to the 3:2 resonance while the exterior heliocentric orbit is near the 2:3 resonance. An important feature of the dynamics of these comets is that during the transition, the orbit passes close to the libration points L ₁and L ₂, two of the equilibrium points for the restricted three-body problem for the Sun-Jupiter system. Studying the libration point invariant manifold structures for L ₁and L ₂is a starting point for understanding the capture and resonance transition of these comets. For example, the recently discovered heteroclinic connection between pairs of unstable periodic orbits (one around the L ₁and the other around L ₂) implies a complicated dynamics for comets in a certain energy range. Furthermore, the stable and unstable invariant manifold tubes associated to libration point periodic orbits, of which the heteroclinic connections are a part, are phase space conduits transporting material to and from Jupiter and between the interior and exterior of Jupiter's orbit.