Comets are believed to be the most primitive remnants of the epoch of solar system formation. As such, they provide important clues to the physical and chemical processes that occurred at that time. The determination of the chemical composition and variation therein of cometary nuclei is essential to our understanding the history of our solar system and thus is one of the principle goals of cometary studies. While in situ studies, like those of Halley and Grigg-Skjellerup by Giotto and other spacecraft, have provided a wealth of data, it has been long-term groundbased observational programs that establish the context for such data and allow the maximum possible amount of information to be extracted.
While the PAL is not currently studying comets, we have been involved in the Lowell Observatory Cometary Database, an ongoing program of narrow-band photometry of comets. This research resulted in the first taxonomic classification of comets based on their chemical compositions. The majority (~70%) of comets having "typical" molecular abundances were separated out from a second distinct class of comets that exhibit significant depletions in the carbon chain species (C3 and C2) with respect to water.