Emerging trends and a comet taxonomy based on the volatile chemistry measured in thirty comets with high-resolution infrared spectroscopy between 1997 and 2013

A systematic analysis of the mixing ratios with respect to H2O for eight species (CH3OH, HCN, NH3, H2CO, C2H2, C2H6, CH4, and CO) measured with high-resolution infrared spectroscopy in thirty comets between 1997 and 2013 is presented. Some trends are beginning to emerge when mixing ratios in individual comets are compared to average mixing ratios obtained for all species within the population. The variation in mixing ratios for all measured species is at least an order of magnitude. Overall, Jupiter-family comets are depleted in volatile species with respect to H2O compared to long-period Oort cloud comets, with the most volatile species showing the greatest relative depletion. There is a high positive correlation between the mixing ratios of HCN, C2H6, and CH4, whereas NH3, H2CO, and C2H2 are moderately correlated with each other but generally uncorrelated or show only weak correlation with other species. CO is generally uncorrelated with the other measured species possibly because it has the highest volatility and is therefore more susceptible to thermal evolutionary effects. Most of these correlations appear to be independent of dynamical class with a few possible exceptions. Molecular mixing ratios for CH3OH, HCN, C2H6, and CH4 show an expected behavior with heliocentric distance suggesting a dominant ice source, whereas there is emerging evidence that the mixing ratios of NH3, H2CO, C2H2, NH2, and CN may increase at small heliocentric distances, suggesting the possibility of additional sources related to the thermal decomposition of organic dust. Although this provides information on the composition of the most volatile grains in comets, it presents an additional difficulty in classifying comet chemistry because most comets within this dataset were only observed over a limited range of heliocentric distance. Although there is remarkable compositional diversity resulting in a unique chemical fingerprint for each comet, a hierarchical tree cluster analysis is used to determine a taxonomic classification system containing four groups and eleven subgroups. Optical and infrared comparisons indicate that mixing ratios of daughter species and potential parents from cometary ices are sometimes but not always consistent with one another. This suggests that in many comets there are significant sources of C2 and/or CN from grains, and that the importance of these sources is variable within the comet population.