A comparative analysis of narrowband photometry observations of 85 comets yields a wealth of information about the nature of comets. The data set consists of 2020 observations obtained over 429 nights between 1976 and 1992 that are fully reduced in a consistent manner to molecular production rates of 5 emission species (OH, NH, CN, C _3, and C_2) as well as a comparative measure of the dust production rate. Ratios of these production rates serve as a means of exploring compositional variation between comets. Vaporization models are used to estimate the active surface area for each comet, which is indicative of intrinsic differences in activity level among comets. Orbital parameters distinguish comets by dynamical age and separate the database into several dynamical classes. The current investigation confirms a number of results from earlier studies. There are no significant or systematic variations of the various abundance ratios with heliocentric distance. Neither the cometary dust -to-gas ratio nor the activity level vary systematically with dynamical age or with heliocentric distance. There is no conclusive evidence suggesting changes in composition based on dynamical evolution of the comet. Also, this investigation finds for many comets the strong degree of homogeneity in abundance ratios suggested by other studies, particularly among the carbon bearing species. Results unique to the present database include a significant correlation between the cometary dust-to -gas ratio and perihelion distance of a comet, suggesting mantling effects on the nucleus. Additional evidence for mantling is provided by the large number of comets with very low levels of activity as measured by the surface active area and the low fractional active areas calculated for those comets with nucleus size estimates available. The most important result from this study, however, is a taxonomic classification of comets separating chemically distinct compositional groupings most likely due to the original formation region. Gradients in temperature or cosmic ray exposure as a function of heliocentric distance in the proto-solar nebula can be invoked to qualitatively explain the observed compositional differences. Future observations measuring other emission species may serve to further delineate chemically distinct groups among the comet population.
- Pub Date:
- January 1995
- COMPOSITIONAL VARIATION;
- Physics: Astronomy and Astrophysics