The Infrared Space Observatory (ISO) offered us the opportunity to observe celestial bodies from 2.4 to 196 μm, which is of particular interest for comets. We present here spectroscopic observations with ISO of comets C/1995 O1 (Hale-Bopp) and 103P/Hartley 2. Fluorescence emissions of H2O, CO2 and (for comet Hale-Bopp only) CO and CH4 are observed, yielding the production rates of these species. High-resolution spectra of water permit the study of the rotational and spin temperatures of this molecule. In comet Hale-Bopp, the thermal infrared region of the spectra shows specifically, in addition to continuum-like emission, the signatures of crystalline, Mg-rich olivine (forsterite), and signs of crystalline pyroxenes, amorphous silicates and water ice. The presence of crystalline silicates is also suggested in the Jupiter-family comet P/Hartley 2.

Molecules in cometary nuclei are considered to contain information on the constituents in the proto-solar nebula at the time of solar system formation. The composition of cometary nuclei has been indirectly derived from spectroscopic observations of molecules in comae. Ambitious space missions to cometary nuclei for landing and sample return are planned to be realized in 4–7 years. Until then, spectroscopic observations remain a powerful way to remotely investigate molecules and ions in cometary comae. Recent bright comets such as Hale-Bopp (C/1995 O1) and Hyakutake (C/1996 B2) have provided detections of new molecules in comae. Observational results from high-resolution optical and infrared spectroscopy of Hale-Bopp, Hyakutake and recent comets, are presented. In particular, symmetric molecules, which have zero electric dipole moment and are therefore not observable in the radio range, have been detected in several recent comets by infrared observations. The new data can also be used to derive accurate abundances of known molecular species. We present the time-dependent excitation processes of the bands of these molecules and discuss the influence of the fluorescence calculations on the derivations of molecular abundances in comets.

The discovery by the Galileo Probe Mass Spectrometer that argon is enriched to the same extent as carbon and sulfur on Jupiter requires a revision of models for the formation of this giant planet. Evidently the excess heavy elements were carried to Jupiter in icy planetesimals that formed at temperatures ≤ 30 K. This result indicates that there is no original significance in the present position of Jupiter's orbit.

The recent, concommitant, operation of the Galileo mission to the Jovian system and of the Infrared Space Observatory has provided a wealth of new results on the atmospheric composition of the Giant Planets. These results include (i) first or improved measurements of several elemental and isotope ratios, particularly in Jupiter; (ii) new insights into Jupiter's (and Saturn's) condensible species and tropospheric cloud structure; (iii) a much enriched view of stratospheric hydrocarbon chemistry; and (iv) the discovery of an external source of oxygen into the Giant Planets. These discoveries address key issues such as the origin of Giant Planets, their tropospheric meteorology and stratospheric physico–chemistry and dynamics, and their interaction with the interplanetary medium.

Between the masses of the Jovian planets and the stars, a new population of exotic substellar-mass objects (SMOs) is now being discovered in profusion, that are challenging our notions of what a planet, brown dwarf, or star can be. I focus on some exotic and distinctive aspects of the compositions of SMO atmospheres. In particular, I review the formation of silicate, water, and ammonia clouds, the role of rainout in altering the abundance of the alkali metals, and the suppression by atomic potassium of the optical fluxes of a methane dwarf such as Gliese 229B. Furthermore, I discuss the order in which various molecular, atomic, and condensed species might dominate or influence SMO atmospheres, with an eye to anchoring new spectral classes from Teff ~ 2000 K to ~ 100 K.

Primitive meteorites have preserved material that was present in the presolar nebula and record processes that occurred as evolution proceeded from the earliest solids. The discovery of isotopic anomalies in these samples led to the isolation of presolar grains and allowed the presence of short-lived radionuclides in the early solar system to be inferred. Isotopic anomalies in oxygen may reflect non-linear chemical fractionation rather than a nuclear effect, but the theory is as yet insufficiently developed to be rigorously assessed.

Analyses of individual SiC and refractory oxide presolar grains reveal that a large number of distinct nucleosynthetic sites contributed material to the solar nebula, and much progress has been made in identifying the various environments in which they formed. Isotopic anomalies associated with nanometre-size diamonds are best explained by supernova nucleosynthesis but it is clear that several sub-populations exist.

The extinct nuclides 26Al, 53Mn and 129I have each been used to establish the relative timing of events in the formation of the solar system. Calibrations of the Mn-Cr and I-Xe systems against the Pb-Pb system (based on decay of uranium isotopes) have been proposed, and Al-Mg data can be included through a calibration with the I-Xe scheme. Assuming these calibrations to be valid allows a tentative chronology of the early solar system to be developed, the plausibility of which can be seen as a test of the calibrations. In this chronology, the first solids to form in the solar system were refractory inclusions. Chondrules (rapidly cooled silicate droplets) appear to have formed later than CAIs over a period of a few million years. Parent body processing began early in solar system history and was ongoing as chondrules formed.

Meteorites and interplanetary dust particles (IDPs) are primitive solar system materials which contain preserved nebular condensates, circumstellar dust grains and partially preserved molecular cloud matter. The circumstellar dust grains found in meteorites are direct samples of a variety of stars, and provide detailed constraints on models of stellar nucleosynthesis. Hydrogen and nitrogen isotopic anomalies in organic matter in meteorites and IDPs are thought to originate from chemical processes in a presolar molecular cloud. This material is better preserved, but less well characterized among IDPs.

The molecular cloud material which became the asteroids, meteorites and inner planets was extensively processed in the inner solar nebula. The chemical complexity of the most ancient meteorites attests to the highly nonequilibrium nature of the chemistry in this region, particularly regarding the volatile elements. Theoretical models describing the time evolution of the temperature and density profiles of protoplanetary accretion disks have recently become available. Such models provide a realistic framework within which to study, for instance, the effects of lightning and surface-catalyzed (gas-grain) reactions in the inner nebula. Here, we show that the latter would have been most efficient during the meteorite-forming epoch of the nebula, at the present position of the asteroid belt.

Some thoughts on the state of our subject, as described in the presentations at the Symposium, are given here.

The cgs unit system is most often used in the astrophysical literature. Some astronomical constants and conversion factors are listed below.