Comprehensive Analyses of the Strongly Carbon-chain Depleted Comets in Lowell Observatory’s Narrowband Photometry Database

Abstract We present measurements, analyses, and results for the 17 strongly carbon-chain depleted comets in Lowell Observatory’s narrowband photometry database. The majority of comets exhibit a very similar, i.e., typical, composition in optical wavelengths, though the existence of anomalous comets with lower abundances of carbon-chain molecules relative to CN has long been known. M. F. A’Hearn et al. identified an entire class of these carbon-chain depleted comets, and the most recent full analysis of our database reveals that there are varying degrees of carbon-chain depletion. Here we focus on the most depleted comets, the strongly carbon-chain depleted class, which is the largest nontypical compositional class to emerge from our taxonomic analyses. All comets in this class are strongly depleted in both C 2 and C 3 with respect to OH and CN, with depletions for these ratios being 3–27× below the mean abundance ratios for comets with typical composition. Several comets in this class additionally exhibit depletion in NH, with the largest depletions being 11× below mean typical values. A number of these comets exhibit asymmetry in production rates as a function of time and heliocentric distance, and one exhibits evidence of small secular changes. Almost all of the strongly depleted comets are Jupiter-family comets, indicating a Kuiper Belt origin for this compositional class. Multiple lines of evidence from our full database indicate that this composition is due to primordial conditions when and where these comets were formed and is not due to thermal heating after their arrival in the inner solar system.