Chemical abundances in comets

Photoelectric filter photometry and spectrophotometry undertaken to establish homogeneous, quantitative data on chemical abundances in comets is discussed. Absolute column densities of a particular species were derived from a comparison of the absolute flux of an emission feature due to the species with a fluorescence calculation, and converted into species production rates by the use of a Haser model and the species lifetime. Observations of C2 and CN reveal a constant production rate ratio at heliocentric distances up to 1.5-2.0 AU, which implies a compositional homogeneity with depth and the formation of most comets from a homogeneous volume in the solar nebula. Observations of the trace species C3 indicate no apparent variation in relative abundances, while observations of low levels of NH2 and CH imply that NH3 and CH4 are not abundant species. H2O is considered to be the most abundant species based on detections of O, H and OH, while variations in CO or CO2 content may account for the very large differences in cometary gas-to-dust ratios and morphologies.