Document Type



Doctor of Philosophy



Date of Defense


Graduate Advisor

Dr. Erika Gibb


Dr. Shun Saito


Dr. Alexey Yamilov

Dr. David Horne

Dr. Bruce Wilking

Dr. Erika Gibb

Dr. Shun Saito


The study of interstellar molecules such as CO is crucial because interstellar ices in the core of a pre-solar molecular cloud provide the starting point for volatile evolution in the protoplanetary disk. A record of the initial volatile composition of the protoplanetary disk can be obtained from the study of the chemical composition of cometary nuclei. Because of their long residence in the Oort cloud and infrequent passage through the inner solar system, long-period comets are one of the most primitive bodies in our solar system that can tell us about the composition of the early solar system. High-resolution infrared spectroscopy allows for measuring the chemical abundances of primary volatile species (sublimed native ices directly from the nucleus) in the comae of comets, which are crucial for their chemical taxonomic classification. Characterizing the chemical composition of comets over a range of heliocentric distances (Rh) is crucial to investigate whether the composition of the cometary nucleus varies with heliocentric distance or remains constant. There is a systematic enhancement of some molecules (C2H2, NH3, H2CO) for comets observed close to the Sun compared to those observed beyond ~1 AU. Measurements of the chemical abundance of primary volatile species in the long-period comet C/2020 S3 (Erasmus) observed near Rh ~ 0.5 AU, presented in this study, contribute to the effort of investigating the chemical abundances at small heliocentric distances. Within 2 AU from the Sun, cometary activity is mainly driven by the sublimation of H2O ice. On the other hand, cometary outgassing beyond the sublimation region of H2O (~ 3.0 AU) is driven by hypervolatile species such as CO and CO2. There have only been a few comet measurements made over the transitional heliocentric distance range Rh ~ 2.5-3.0 AU because the majority of comets are either very faint or show low activity at this distance from the Sun. This leaves it uncertain where the transition from hypervolatile to H2O-driven activity occurs. Observations of long-period comet C/2017 K2 (Pan-STARRS) spanning a range of Rh ~2.35 - 3.15 AU were conducted to address this critical topic in cometary science.