Weidenschilling (64) examined the formation of comets in a minimum mass nebula by using a one-dimensional model to follow accretion of µm-sized dust particles into kilometer-scale bodies. Comets form on time scales of a few hundred thousand years, even neglecting action of several factors that result in swifter accretion rates. A growing comet accretes material from a large volume in the nebula; the initial coagulation process is aided greatly by gas-drag-induced orbital migration. Migration homogenizes material accreted into comets by giving them feeding zones from 10 to 100 AU in radius that obscure small compositional differences between comets that end up at different nebular radii. Comets form much more rapidly in higher mass nebulae where gas-drag-induced orbital migration and accumulation due to gravitational instabilities would be more important. The differences we might expect to observe in the dust and volatile compositions of individual comets depends on the ratio of comet accumulation time scale to nebular lifetime. If these are comparable, then all comets look similar. If the nebular lifetime were much longer than the time required to accumulate comets, then substantial diversity will exist in this population as the crystalline fraction of the dust and the complex organic content of the volatiles increases with time. These latter predictions certainly appear to be more consistent with observations of both the dust (50, 65) and gas (66, 67) content of recent comets.
Comets formed early in nebular history will consist of amorphous silicates and unaltered interstellar ices; no processed material is yet available. Comets formed late contain more hydrocarbons, ammonia, and annealed dust than those formed earlier. This increase will not necessarily be linear. However, after accretion of fresh material from the surrounding molecular cloud ceases, accumulation of processed gas and dust in the comet formation region should at least be monotonic. The time-dependent nature of dust and gas accreted into comets might easily obscure less significant differences in cometary chemistry such as potential distinctions between comets accreted at higher temperatures in the Jupiter-Saturn region and those accreted in cooler zones near, or even beyond, Uranus-Neptune.
Older comets should be rich in CO, CO2, N2, and amorphous dust, whereas younger comets contain an abundance of crystalline olivine, hydrocarbons, NH3, and prebiotic organics. We predict that the fraction of crystalline dust is correlated to the ratios of hydrocarbons to CO and of NH3/amines/amides to N2. Fig. 4 shows vapor pressures (68, 69) for compounds formed in molecular clouds, present as icy mantles on interstellar grains (N2, CO, and CO2) along with more hydrogenated species synthesized in the nebula (CH4, C2H4, C2H6, C3H6, C3H8, and NH3). Interstellar grains heated above 50 K lose CO and N2 from their mantles, whereas interstellar CO2 could remain trapped. Hence, CO and N2 are both suitable indicators of interstellar volatiles in comets. Alternatively, if NH3 and most hydrocarbons synthesized in the nebula are cooled below 150 K, they will be trapped on grain surfaces. CH4 requires much cooler temperatures to achieve the same degree of condensation and might not condense into comets even if it were present in the nebula. Therefore, CH4 would not make a useful indicator of processed nebular gas in comets. One good proxy for the ratio of the processed-to-primitive nebular gas in comets is the ratio of C2H4 to CO, another is the ratio of C2H6 to CO. Hydrocarbons are synthesized primarily in the nebula and thereafter trapped on icy grains, whereas CO is associated with grain mantles formed within a giant molecular cloud core that had never been sufficiently heated to permit vaporization. Using similar reasoning, the ratio of NH3 to N2 should also be a good measure of the ratio of processed-to-primitive gas in comets. Hence, the ratio of crystalline (to total) cometary dust will be positively correlated to ratios of C2H4/CO, C2H6/CO, and NH3/N2 in cometary comae.