Barnard (1891) appears to have been the first to recognize the significance of systematic observations of comets at large distances from the sun. His successful tracing of two 1889 comets to heliocentric distances over 5 and even 6 a. u. caused him to notice that some of the short-period comets might be within the reach of the Lick Observatory’s 36-inch refractor throughout their entire orbits around the sun. Although it is clear nowadays that the short-period comets would be a good deal fainter at comparable distances than the two nearly parabolic comets referred to by Barnard, his original idea proved basically correct, except for the necessity of using photographic plates. Periodic Comet Encke was probably detected near aphelion during Barnard’s lifetime, in September 1913 (Barnard 1914a; Marsden and Sekanina 1974). Undisputed images of the comet just several days off aphelion were obtained in 1972 (Roemer 1972; McCrosky and Shao 1972).

My approach to the nature and origin of the nucleus is strongly influenced by the quantitative data on amounts of dust and gas being. released near perihelion. Calibrated spectra and photoelectric photometry (Gebel 1970, O’Dell 1971, Stokes 1972) have shown that the morphological division of Oort and Schmidt into dust-rich and dust-poor seems to be correct. More accurately, I can say that we have observed a wide variation in the ratio of scattered light continuum to molecular emission. This may or may not reflect a large variation of the dust to gas ratio in the nucleus. In fact, the apparently dustiest comets may have smallest dust to gas ratio at the nucleus! This is because the process of accelerating the particles out from the nucleus by means of viscous gas flow depends sensitively on the amount of gas leaving. (Finson and Probstein 1968). The comet with weak scattered light is probably one that is unable to lift the particles from the nucleus, leaving a residual surface of particles. Since small particles can be lifted more easily, the remaining surface would selectively become one of large particles, forming an insulating layer of low albedo with internal degas if ication occuring at an even slow rate. This in turn would diminish the particle loss rate, with a rapid convergence to a particle cover and nucleus. This model would explain the variation from continuum strong to continuum poor in the Oort-Schmidt new and old comets, in addition to the intrinsically low luminosity and photometric radius of old comet nuclei.

Almost all information about the physics of the nucleus is based on deductions from observations of the coma and tails. It is well to keep in mind the hierarchy of events on which these deductions are based:

1. The material properties of the constituents of the nucleus and the detailed physical and chemical structure of the nucleus form the basis for the behavior of coma and tails.

2. Interaction of solar radiation with the surface of the nucleus determines the overall temporal development of the coma.

3. The subsequent interaction of solar radiation and solar wind with the coma determine the gross features of the tails.

4. Short term fluctuations primarily in the solar wind (and associated magnetic, field) cause disturbances of comparable duration observable mostly in the tail but also in the coma.

What I would like to discuss here is the chemical nature of the nucleus.

This implies the consideration of the vaporization of the nucleus into the coma because the coma is our only source of information on the nucleus. The nature of the parent molecules vaporizing from the nucleus has been for a long time the missing link for our understanding of the molecular processes. A major development has been brought about by the first identification of three neutral parent molecules HCN and CH3 CN identified in Comet Kohoutek, H20 in Comet Bradfield, confirmed by H2O+ in Kohoutek.

My remarks will present some views of the behavior of the nucleus and problems with their explanation.

One thing we definitely know about comets is that there has to be a permanent structure which revolves around the sun in the comets orbit. Permanent means carrying over from apparition to apparition during the lifetime of the comet. I adopt some form of the “icy nucleus” model as proposed by Whipple. This structure reasonably fits most of the known cometary features although no completely consistent model accounting for all known phenomena in a satisfactory manner has yet been described. The icy nucleus does not appear to have any major flaws. It has a real advantageous feature in that detailed models can be constructed and their behavior more or less accurately predicted, e.g. (Donn 1963; Delsemme and Miller, 1971; Sekanina 1972, 1973; Huebner, 1975).

In discussing the nucleus of comets and the role of comets in the evolution of the solar system I summarize the observation only in Table 1, the observed composition. Note that gas-phase chemistry obscures the nature of the active atoms, molecules and radicals in the nucleus.

I wish to stress the fact, so obvious from this symposium, that the comprehensive observational attack on Comet Kohoutek (1973f) has led to an enormous step forward in deciphering the comet enigma.

The cosmogony of the planetary and satellite systems consists of understanding the physio-chemical processes leading to their formation and also trying to decide at what time and over what period their formation took place. The cosmogony of the comets require answers to not only these two questions but also as to where, in relation to the solar system, the observed and inferred distributions of comets were formed.

The current evidence concerning the nature of comet nuclei suggests that comets may be sizeable aggregations of interstellar grains. This is a progress report on an effort to find circumstances and processes whereby such aggregations might be formed in the solar system at distances far beyond the proto-planets during the early stages of solar-system development. Under investigation are interactions between the early solar “gale” and the surrounding interstellar gas and dust—the so-called “snowplow effect.” Compression of the gas and resultant motions of the dust coupled with the pressure radiation from the Sun and nearby new stars may, under certain idealized circumstances, produce a high enough concentration of dust for gravitational instability to occur in the dust, thereby producing km-sized coherent bodies. The likelihood or probability of actual comet formation by such processes remain to be determined.

It is now a generally accepted concept that comets are a residue of the early history of the solar system from the time when the planets were forming. Because of the approximately 0.1% loss of material from the nucleus during perihelion passage near 1 A.U., lifetimes of short period comets are limited to 104-105 years. This requires an astronomically recent source of the comets seen at the present epoch. From the statistics of the aphelia of parabolic and long period comets, Oort (1951) proposed the existence of a comet cloud between 50,000 and 100,000 A.U. which serves as a reservoir from which presently observed comets have recently been perturbed. Although there are various difficulties with populating the cloud (Opik, 1973) and its subsequent evolution (I.A.U. Symposium 45, 1972; Everhart, 1974) it is the basis for nearly all current studies on the origin and evolution of comets.

At present only four polyatomic molecules or ions have been identified in the spectra of comets and their tails. They are C3, CO2 +, NH2, and H2O+. The first two are linear molecules. The C3 radical gives rise to the well-known 4050 group. It was first obtained in the laboratory in an interrupted discharge through CH4; was definitely identified by Douglas as being due to C3, and was later investigated in considerable detail in absorption in the flash photolysis of CH2N2. The complicated vibrational structure of this spectrum was first understood when it was realized that the bending frequency in the ground state is very low (64 cm-1) and that in the excited state the interaction of the vibrational angular momentum with the electronic angular momentum leads to large splittings (Renner-Teller splittings).

Many years ago K. Wurm (1) suggested that the photodissociation of stable molecules such as H2O, HCN, CH4, NH3, etc., could account for the observed cometary radicals. This postulate can be represented schematically by the following photochemical reaction,

A relatively new technique called Laser Induced Photoluminescence Spectroscopy has been applied to laboratory studies of cometary radicals. This technique can be used to measure properties of radicals, to determine photodissociation processes in parent molecules, and to investigate reactions of radicals in specific vibration-rotation levels. Thus far, the LIPS method has been applied to the CN radical to determine: (1) the radiative lifetime and quenching constants for the B2Σ+ state and (2) the photodissociative formation of CN from several parent molecules.

Most records of the first half billion years of the solar system have been wiped out from the planets and from their satellites by their evolution and their morphological differentiation. However, two sources of information seem still to be available on this early period: some meteorites give us clues on the non-volatile fraction that condensed from the primeval nebula, whereas the clues on a more volatile fraction, possibly condensed at a colder temperature, may come from the comets.

I would just like to make a brief summary of some of the major points in my review two days ago. The most important results from the cometary UV observations are: (1) We know now that the gas production rates of a medium bright comet is on the order of 1030 molecules s-1 based on hydrogen Lα observations and interpretations. (2) The large amounts of OH indicate that water is an abundant molecule. I do not know of any other probable parent molecule of OH. The case for water as a parent molecule is strengthened by the parallel decrease of OH and H in comet Bennett and by arguments given by Delsemme. (3) Disregarding the observed amounts of OH and O, one would not necessarily conclude that the hydrogen is a dissociation product of water. The uncertainty is due to the deduced outflow velocities of about 8 km s-1. I feel this value for the velocity is pretty mysterious and not yet explained. We have yet to connect this velocity with the dissociation — or formation — producing the hydrogen atoms. The possibility that an appreciable amount of hydrogen does not stem from water cannot be excluded. (4) The UV observations of comet Kohoutek seem to indicate that water is not predominate over other molecules by two orders of magnitudes. The production of carbon atoms (and, therefore, of parent molecules containing carbon) seems to be nearly as great as that of water. The uncertainties of the numbers for the production rates are at least factors of two. I, personally, think it is too early to draw definite conclusions on the various abundance ratios. H, O, OH, and C may well be produced in the same order of magnitude. But a C deficiency by an order of magnitude (or even more) is not rules out either.

Well, I just said that I disagree with nearly all that was said in the review paper. Now I have to illustrate this point a little, so I will first write something on the board.

Present theories for the formation of molecular species observed in comets predict the sublimation of parent molecules such as H2O, CH4, CO2, and NH3, from the surface of the nucleus and their subsequent photodissociation and ionization to form the observed species (Delsemme 1973). It can be shown (Oppenheimer 19 75) that gas phase chemical reactions occur between these fragments which have characteristic timescales which are short compared to the timescale for significant variation in the solar flux incident on the comet. Hence, a steady-state approximation may be used for determining the densities of many species. It can also be shown (Oppenheimer 1975) that the rate of formation of many species is faster by gas phase reactions than by photoprocess. For instance, the formation of OH+ from H2O+ by the reaction

H2O+ + O + → OH+ + OH

The spectral analysis of the coma and type I tails of a comet gives a clue to clarify the composition of the volatiles in its nucleus. The ultraviolet observation of strong H, O, and OH emissions in comets 1969 g and i by OAO II (Code and Savage, 1972) and the recent identification of H2O+ in the spectrum of 1973f (Wehinger et al., 1974) have suggested that the main constituent of cometary coma is H.O. It was also suggested from the computation of the ionization processes (Jackson and Donn, 1968) that CO and N. may be the second most abundant gases (some ten% by number) in comets. Other gases detected in the optical (C2, CN, C3…), ultraviolet (NH, CN) and radio (CH, CHg, CN) regions appears to be minor constituents of cometary atmospheres.

The recent observations of atomic oxygen and carbon in the far ultraviolet spectrum of Comet Kohoutek (1973f) (Feldman et al. 1974; Opal et al. 1974) have demonstrated the existence of these atomic species in the cometary coma. However, in order to identify the source of their origin, it is necessary to relate the observed ultraviolet flux to the atomic production rate. Assuming the only excitation mechanisms allowed are those produced by resonance scattering and fluorescence of solar ultraviolet radiation, the problem reduces to finding the emission rate factors (g-factors) as a function of the heliocentric comet velocity. Since the widths of the solar emission lines are smaller than the maximum heliocentric Doppler shift, given by

where q is the perihelion distance in A.U., it is necessary to consider the detailed multiplet structure of the transition, the solar line shape and the relaxation of excited fine structure levels.

This report will be devoted to a discussion of the theoretical interpretation of the spectra of comets and in this discussion we shall consider successively a number of molecules that have been studied recently: CN, CH, C2, C3, OH, CH+. The first two of this list, CN and CH, have been analysed in greatest detail until now, so that we shall be rather brief on the other molecules. We shall try to indicate what kind of information can be derived from these studies concerning the conditions prevailing in cometary atmospheres and to show which fundamental data are needed in the calculation of the theoretical spectra.

Up to now a few hundred of spectra of several tens of comets have been obtained at different observatories over the whole world. On the one hand, there are high dispersion spectra photographed with slit spectrographs attached to large telescopes, and on the other, small dispersion spectra obtained by means of objective prisms. These objective prism spectra, showing the principal emission bands only, are known for a considerable number of comets, and its appearance is usually described verbally.