Although the existence of Oort’s cometary cloud has been generally accepted, his hypothesis on its origin has been repeatedly called into question, in particular because of the large mass that Jupiter would also have simultaneously ejected out of the solar system. However, the extremely slow growth of particles in regions of small density seems to rule out that comets condensed “in situ” at their present large distances. Also, the accumulation of interstellar grains in satellite disks orbiting around the primitive solar nebula seems an “ad hoc” hypothesis that cannot be proved or disproved. Therefore, the most reasonable hypothesis is that comets were ejected from the region of the giant planets as a natural by-product of their accretion.

The gradual scattering of small bodies by the Jovian planets in the late stage of their accretion is simulated by Monte Carlo calculation. The effects of collisional interaction of the scattered planetesimals with the inner planets and asteroidal belt are estimated. The total mass influx injected into the terrestrial zones from the outer planetary zone could be appreciable; however, if the damping effects due to the mutual inelastic or catastrophic collisions among the small bodies were significant, the total mass influx would be greatly reduced. The origin of the cometary Oort cloud in relation to these scattering processes is also discussed. The calculation indicates that, in addition to Jupiter, Uranus and Neptune were likely to be the major contributors to the population of long-period comets if they originated from such scattering processes.

The growth and chemical evolution of a typical interstellar dust grain are followed starting from average interstellar conditions to the dense cloud and contraction phase. Based on the theory of cometary accretion directly from cold interstellar dust it is shown that the bulk material of a primordial comet would consist mostly of an icy conglomerate of complex organic molecules and frozen radicals in which are imbedded approximately equal volumes (10% each) of small grains in two different sizes - one variety being the order of 0.05 μ and the other ~0.005 μ in size - consisting mostly of silicates

The 12C/13C isotope ratio in the interstellar medium and in stellar atmospheres is discussed and compared to the value found in the solar system and especially in comets. The cometary value (≥ 100) tends to be slightly above the terrestrial value (89) and is definitively higher than that for interstellar molecular clouds (about 30 to 50). This result implies that comets are not of interstellar origin.

Equilibrium thermodynamic calculations of the sequence of condensation of minerals from a cooling gas of solar composition play an important role in explaining the mineralogy and trace element content of different types of inclusions in carbonaceous chondrites. Group IV B iron meteorites and enstatite chondrites may also be direct condensates from the solar nebula. Condensation theory provides a framework within which chemical fractionations between different classes of chondrites may be understood.

Organic compounds in meteorites seem to have formed by Fischer-Tropsch-type, catalytic reactions of CO, H2, and NH3 in the solar nebula, at 360-400 K (and 4-10) x 10-6 atm. The onset of these reactions was triggered by the formation of catalytically active grains of magnetite and serpentine at these temperatures.

Laboratory experiments show that the Fischer-Tropsch reaction gives a large kinetic isotope fractionation of C12/C13, duplicating the hitherto unexplained fractionation in meteorites. All of the principal compound classes in meteorites are produced by this reaction, or a variant involving a brief excursion to higher temperatures. (1) normal, mono-, and dimethylalkanes; (2) arenes and alkylarenes; (3) dimeric isoprenoids from Cg to C14; (4) purines and pyrimidines, such as adenine, guanine, uracil, thymine, xanthine, etc.; (5) amino acids, including tyrosine and histidine; (6) porphyrin-liJce pigments; (7) aromatic polymer with -OH and -COOH groups.

These reactions may also have played a major role in the evolution of life: first, by converting carbon to a sufficiently non-volatile form to permit its accretion by the inner planets; second, by synthesizing organic compounds on the primitive planets whenever CO, H2, NH3, and clay or magnetite particles came together at the right temperature. Similar reactions in other solar nebulae may be the source of interstellar molecules, as first suggested by G. H. Herbig. Ten of the twelve polyatomic interstellar molecules have in fact been see in these synthesis or in meteorites.

The thermodynamics of condensation from a cooling neutral gas of solar composition, have now been explored in great detail. Theories based on equilibrium condensation have had some striking successes, but there is still much to learn. There seems good reason to explore the condensation process experimentally, so far as it is possible to simulate the conditions usually assumed, to compare the results with what we see in the meteorites, and with the calculations. Preliminary results of this experimental investigation are being reported here.

The observations of comets and other primordial objects must be used to deduce the gaseous composition of the solar nebula just before its condensation. Preliminary steps in this direction are reported here.

Recent observational data on the volatile fraction of comets are confronted with a model based on the fractional condensation, in the 80-100 °K range, of a higher-temperature equilibrium obtained from a solar mixture, more or less depleted in oxygen and in hydrogen. It is possible to almost duplicate the observational data, only by assuming that the solar ratio of C/0 is at least as large as 0.66 and that the hydrogen was drastically depleted by an unknown process in the primitive solar nebula. Although none of these two assumptions is at variance with present knowledge, the latter is sufficiently exotic to propose a simpler explanation, namely that comets could be made of interstellar grains relatively unprocessed by heat.

The minerals of chondritic meteorites, particularly C2 and C3 are usually believed to be condensates of the (previously unfractionated) solar nebula. This implies two postulates: (1) that chemical reactions between phases ceased at various temperatures; (2) that Ni and Fe were mobilized by diffusion in the solid state. An alternate explanation is proposed, which obviates the two postulates: chondritic minerals may have condensed, in equilibrium with their own vapors, from a fraction already separated from the solar nebula at low temperature and low pressure.

In a centrally condensed solar nebula, the gas is partially supported by a pressure gradient, and rotates at less than the Keplerian velocity. Solid bodies lack this support, and spiral inward due to drag. The radial velocities developed can be significant, even in low-mass nebular models. Possible effects include fractionation of particles by size or density, rapid accumulation of planetesimals, and production of regions of anomalous (non-solar) composition in the nebula.

On the basis of 180/160 and 170/160 ratios, meteorites and planets can be grouped into at least nine categories, as follows (in order of increasing 1°0): (1) type L and LL ordinary chondrites; (2) type H ordinary chondrites, type HE irons, and CI carbonaceous chondrites; (3) the nakhlites and Shergotty; (4) the earth, moon, and enstatite chondrites and achondrites; (5) basaltic achondrites, hypersthene achondrites, mesosiderites, pallasites and type IRB irons; (6) the ureilites; (7) C2 carbonaceous chondrite matrix, Bencubbin, Weatherford, and Kakangari; (8) C3 carbonaceous chondrites; (9) pallasites Eagle Station and Itzawisis. Objects of one category cannot be derived by fractionation or differentiation from the source materials of any other category, but must represent samples of different regions of an i nhomogeneous solar nebula. The isotopic classification, together with major-element abundances, provides a powerful method for recognition of interrelationships of the various meteorites and their parent bodies.

The chondritic meteorites have “solar” compositions, indicating that they have avoided fractionation in planetary processes such as partial melting or fractional crystallization. Since chondrites contain solar proportions of volatiles having nebular condensation temperatures ≤500 K, it follows that agglomeration of grains and accretion to parent bodies occurred after the nebula had cooled to such low temperatures, and, that models calling for simultaneous condensation and accretion of high-temperature minerals such as Fe-Ni metal or ferromagnesian silicates are implausible.

Two independent intergroup fractionations have been recognized in chondritic materials; refractory element abundances (e.g., the Ca/Si ratio) and degree of oxidation measured by the FeO/(FeO + MgO) ratio decrease through the chondrite sequence: carbonaceous-ordinary-IAB-E. Hiatus between groups result from incomplete sampling by the Earth of the original spectrum. A plausible speculation is that this sequence reflects formation at different nebular pressures and temperatures, the fine-grained, oxidized, volatile-rich carbonaceous chondrites forming at lower temperatures >5 AU from the Sun, and the reduced enstatite chrondrites forming at higher temperatures near the Sun. The high fall frequency of the three groups of ordinary chondrites suggests an origin in the asteroid belt. The degree of oxidation of the IAB chondrites appears to be slightly lower than that of the Earth, suggesting an origin near or slightly less than 1 AU from the Sun. The O-isotope compositions of chondrites are consistent with this picture.

Asteroids tend to be light or dark, with a hiatus in properties between these two classes. This can be understood if the light materials are ordinary chondrites originally formed at 2.2-3.5 AU, whereas the dark materials originated at >5 AU, and were deposited in the asteroid belt during a later period as a result of a major increase in the influx of cometary materials through the inner sc1ar system associated with the generation of the Oort cloud of comets.

The role of plasma and hydromagnetic processes in the primeval solar nebula is evaluated. In the light of the present knowledge of particles and fields in space it is difficult to avoid the conclusion that they must have played a crucial role in the emplacement of the material as well as its subsequent condensation into “planetesimals.” Strong evidence in support of these processes in the primeval nebula is provided by the dynamical fine structure of the asteroidal belt and the Saturnian rings. The importance of current planetary magnetospheric as well as cometary research in clarifying certain physical processes believed to have occurred in the primeval nebula is stressed.

A likely origin of the asteroids (and possibly, of the comets?) is the natural outcome of the following scenario that we propose for the formation of the planets. Protoplanets of similar mass and solar composition will segregate in three different ways: For those far enough from the sun (like Uranus and Neptune), the segregation of icy grains releases enough energy to drive the remaining gases to infinity. For all other planets, the segregation of refractory material only does not release enough energy to disrupt the protoplanet; however, while spiraling inwards in a resisting medium, the terrestrial protoplanets cross their Roche limit and lose their gaseous outer layers. Asteroids (or comets) could therefore originate from the disruption of protoplanets before the settling of their refractory (or icy) grains is completed.

A few of the questions proposed earlier to the contributors are reviewed to identify some of the unsolved problems related to the different interfaces between comets, asteroids, and meteorites and their evolution and origin.