The Joint Discussion (JD14) is intended to focus attention on several apparent inconsistencies found in comet observations when compared with observations of collapsing interstellar clouds, star-forming regions, and models of the solar nebula. The JD is not intended to resolve these problems, for that a one-day discussion is too short. It rather is intended to draw attention to the inconsistencies to be discussed and resolved in future workshops.

The relevance of hot molecular cores (HMCs) to the solar nebula has been strengthened as evidence emerges for a hot core phase in low mass star formation regions. Consequently, interferometric studies of large highly saturated molecules of biological interest (biomolecules) in HMCs have been extended to comets. A key goal is the interferometric detection and mapping of large biomolecules, which could be formed along chemical pathways similar to those for HMC species.

We present 3-5 µm VLT-ISAAC spectroscopy showing the presence of methanol ices in edge-on disks of young embedded stars. Examples include the disks of L1489 IRS in Taurus and CRBR 2422.8-3423 in Ophiuchus, the last of which has the highest column density of solid CO known toward a YSO. Several additional low-mass sources in the Serpens and Chameleon molecular clouds exhibit abundant solid methanol although it is not clear if the ice is associated with a disk or with the envelope. These are the first detections of solid methanol in the disks and circumstellar environments of embedded young low-mass stars providing evidence that complex molecular species previously observed only in the solid state toward high-mass star forming regions are also present near solar-type young stars. The constraints on the formation mechanisms of methanol and the chemical evolution of ices as the material is incorporated into circumstellar disks are discussed.

Since the time that the ortho-to-para ratio (OPR) of cometary water was first Determined from observations of Comet Halley, the real meaning of OPR has not been discussed in detail. Here we review the OPRs of water and ammonia and discuss the possibilities that the OPRs were modified in the coma or the nucleus. Our conclusion indicates that the OPRs were not altered after comet nucleus formation, i.e., the OPRs reflect the temperatures in the solar nebula or the pre-solar molecular cloud.

To better understand the processing of pristine materials in comets, we compare the composition of cometary and interstellar dust. We suggest that the deficit of N in comet dust bears evidence for the processing of the organic refractory mantle of pre-solar interstellar dust, unless it arises from the protosolar disk chemistry. The amorphous silicate core, in contrast, seems to be protected against processing due to the presence of the organic refractory mantle.

Comets, formed in the cold outer parts of the solar system, provide a record of pristine material from the parent interstellar cloud. The investigation of outgassing curves from bright comets has provided a relationship to the abundances of interstellar ices and gas phase molecules. However, being porous and stratified in various layers of different densities and temperatures, the out-gassing characteristics of comets can not always be directly reconciled with the interstellar composition. This is due to the structure of the nuclear ice component, which contains different coexisting ice phases, clathrates, and trapped gases. Ices, silicates and carbonaceous compounds – studied through astronomical observations and by laboratory simulations – serve as reference material to obtain information on cometary bulk material. A major fraction of cosmic carbon in the interstellar medium, comets and meteorites seems to be incorporated into complex aromatic networks, which are difficult to observe and to identify spectroscopically. However, recent measurements of the macromolecular structure and soluble organic species in carbonaceous meteorites provide a powerful tool to investigate the link of small bodies in the solar system.

Laboratory surface science under ultra-high vacuum (UHV) conditions allows us to simulate the growth of ices in astrophysical environments. Using the techniques of temperature programmed desorption (TPD), reflection-absorption infrared spectroscopy (RAIRS) and micro-balance methods, we have studied binary ice systems consisting of water (H2O) and variety of other species including carbon monoxide (CO), at astrophysically relevant conditions of temperature and pressure. We present results that demonstrate that the morphology of water ice has an important influence on the behavior of such systems, by allowing processes such as diffusion and trapping that can not be understood through a knowledge of the binding energies of the species alone. Through an understanding of the implications of water ice morphology on the behavior of ice mixtures in the interstellar environment, additional constraints can be placed on the thermodynamic conditions and ice compositions during comet formation.

Evidence for interstellar material in comets is deduced from IR spectra, in situ measurements of Comet Halley, and chondritic porous interplanetary dust particles (CP IDPs). IR spectra of comets reveal the spectrally active minerals: amorphous carbon, amorphous silicates, and (in some comets) crystalline silicates. Evidence suggests amorphous silicates are of interstellar origin while crystalline silicates are of nebular origin.

10 μm spectra of comets and sub-micron amorphous silicate spherules in CP IDPs have shapes similar to absorption spectra through lines-of-sight in the ISM. Thermal emission models of cometary IR spectra require Fe-bearing amorphous silicates. Fe-bearing amorphous silicates may be Fe-bearing crystalline silicates formed in AGB outflows that are amorphized through He+ ion bombardment in supernova shocks in the ISM.

Crystalline silicates in comets, as revealed by IR spectra, and their apparent absence in the ISM, argues for their nebular origin. The high temperatures (>1000 K) at which crystals form or are annealed occur in the inner nebula or in nebular shocks in the 5 – 10 AU region. Oxygen isotope studies of CP IDPs show only 1% by mass of the silicate crystals are of AGB origin. Together this suggests crystalline silicates in comets are probably primitive grains from the early solar nebula.

The linear polarization of comets depends upon the cometary dust physical properties, as well as upon the phase angle and the wavelength. The observed similarities and discrepancies provide drastic constraints on the physical properties of the dust. A series of measurements of light scattering properties on clouds of particles and of the aggregates they form under microgravity conditions should allow the interpretation of the observations in terms of physical properties of the dust.

Diatomic sulfur was first discovered in comets during the close approach to Earth of comet IRAS-Araki-Alcock (C/1983 HI). The spatial distribution of S2 was restricted to within a few hundred kilometers of the nucleus, consistent with S2 being a parent molecule. This molecule has been detected in comets Hyakutake (C/1996 B2), Lee (C/1999 HI), and recently in Ikeya-Zhang (C/2002 CI), leading to the conclusion that S2 is ubiquitous in comets.

The enrichment of heavy elements on Jupiter appears to require the existence of a new class of icy planetesimal that exhibits solar relative abundances.

Prior to the Galileo probe mission, observations of methane in Jupiter’s atmosphere had revealed that C/H was approximately three times the solar ratio. This enrichment was thought to be the result of the delivery of heavy elements by icy planetesimals, which were assumed to be essentially identical to comets. However, comets are notoriously deficient in nitrogen (e.g., Geiss 1987; Krankowsky 1991) and recent upper limits on argon in three comets (Weaver et al. 2002) indicate that this element is also sub-solar relative to O. Hence it was assumed that Jupiter would exhibit the same deficiency in argon and nitrogen relative to carbon (Pollack and Bodenheimer 1989; Owen & Bar-Nun 1995). Yet the mass spectrometer on the Galileo Probe clearly showed that Ar, Kr, Xe, N, C, and S are all enriched in Jupiter’s atmosphere by the same factor of 3 ± 1 (Niemann et al. 1998; Owen et al. 1999).

To fully understand the formation and the evolution of the Solar Nebula, the physical and chemical characteristics of the “pre-solar” cloud should be known. Here, I review recent findings on the study of starless and pre-stellar cores, pointing out the importance of molecular freeze out in the chemical and dynamical evolution of star forming regions.

This paper presents an interpretation of cometary compositional data in the frame of an evolutionary 2D turbulent model of the Solar Nebula: D/H ratios, crystalline silicates and N2/CO deficiency.

Recent results relevant for the survival and chemical alteration of interstellar material at the nebular accretion shock are summarised.

Molecular evolution is calculated in migrating fluid parcels in a protoplanetary disk considering gas-phase and grain-surface reactions. Radial distribution of molecules is obtained, which depends on ionization rate, temperature and/or desorption rate in the disk. If the temperature is high enough (> 20 K) or non-thermal desorption is efficient enough to sublimate dominant species such as CO in the comet forming regions, the desorbed species are transformed to less volatile species and again incorporated into ice mantles. Chemistry in these outer disks is similar to that in molecular clouds; it produces both oxidized and reduced species with a high D/H ratio.

We describe a theoretical study of protoplanetary disk chemistry. By considering physical conditions similar to that of the protosolar nebula, we attempt to assess the contribution made by material from the cooler nebular regions to cometesimal composition. Calculations are presented which determine the spatial and temporal chemistry of the gas and dust within the 5-40 AU comet-forming region of the nebula. We show that there is little radial variation in the solid-state distribution of some molecules which could potentially be parents of the carbon-chain species observed in comets. We conclude that the apparent variation in abundance of C2 and C3 between long- and short-period comets is the result of chemical processing during their lifetimes and not differences in composition at the time of formation.

We propose a new mechanism of crystallization of cometary silicate. The crystallization is due to heat released by chemical reactions in the organic refractory mantle of Greenberg’s model of cometary dust induced by moderate heating by solar radiation. It is shown that the strength of the crystalline feature observed in the comets can be reproduced by the present mechanism of crystallization.

Crystalline silicate dust particles have been found in some comets, though progenitors of those dust particles are thought to be amorphous. Here, the origin of the crystalline particles was investigated based on the shock-wave heating mechanism. We find that appropriate shock waves can crystallize amorphous dust particles and conditions of these shock waves (shock velocity and pre-shock gas density) are clarified.

The gas density in the solar nebula and the shock velocity that may be induced in comet forming regions by some mechanisms were discussed. It was suggested that comets formed in a region closer than about 20 AU to the Sun can contain the crystalline particles, whereas comets formed in a further region can hardly have them.

Collisional simulations of cometary nuclei (icy dust aggregates) are conducted. Conditions for sticking are determined as a function of the impact velocity and mechanical properties of nuclei. Possible collisional outcomes in three evolutionary stages of icy-dust aggregates are discussed.

It is expected that the Deep Impact mission will hit the Comet Tempel 1 nucleus in July 4, 2005. So, modeling of the impacts onto icy/mineral granular and porous materials, and therefore low-dense media, are very relevant. A model of result of the impact onto icy-mineral-porous target is presented on the basis of many series of impact experiments performed in the laboratories. Experimental results are extrapolated to the Deep Impact scale and farther on, to the planetary scale of collisions, e.g., to impacts of the meteorites on comet nuclei. Crucial for modeling are the parameters of the medium forming the nucleus. The most important of them are the ice to total mass ratio and the porosity. Both are virtually unknown, so they are discussed within the large ranges. Self-references. Icarus, 131, 210-222, 1998; Planet. Space Sci., 48, 1437-1446, 2000; Icarus, 158, 516-531, 2002.