The ensemble properties of comets: Results from narrowband photometry of 85 comets, 1976-1992

We present the results of narrowband photometry of 85 comets observed over a period of 17 years. The data have been reduced homogeneously to molecular production rates and a proxy for the dust production rate. We confirm previous investigations, both our own and those of others, showing that there is no differentiation with depth in the cometary nucleus, that most comets are very similar to each other in chemical composition, and that the dust-to-gas ratio does not vary with the dynamical age of the comet. There is little variation of relative abundances with heliocentric distance, implying that for the species we observe the role of density-dependent processes in the coma is small. There is also little variation from one apparition to the next for most short-period comets. We show, however, that there are significant compositional groupings of comets, apparently related to place of formation. Specifically, the majority of comets originating in the Kuiper belt appear to be significantly depleted in the carbon-chain molecules that lead to the observed C-2 and C-3. Kuiper belt comets also exhibit smaller active areas, steeper slopes, and greater asymmetries about perihelion than do other comets, implying smaller active fractions. We argue that at least the compositional effect is primordial rather than evolutionary. We also show a significant but unexplained correlation of some properties, notably the dust-to-gas ratio, with perihelion distance. We provide strong evidence that most CN, some C-2, and virtually no NH are produced from grains in the coma rather than from nuclear ices. Evidence suggests that the mixing between water and other ices occurs at the level of grains rather than at the molecular level. We also show that there must be a large population of comets having very low activity. The data appear to require mixing of cometesimals with different compositions into individual nuclei, with some nuclei containing nearly the full range of compositions seen in the ensemble while others contain a much smaller range of compositions. (C) 1995 Academic Press, Inc.