Near-IR Spectroscopic Analysis of the Primary Volatile Composition of Long and Short-Period Comets

摘要

Comets are among the most well-preserved objects that formed in the protosolar nebula ～4.5 Gyr ago. Hence, they are important for understanding various aspects of the formation, evolution, and habitability of the solar system. Multiple primary volatiles (molecules directly sublimating into the coma from the nucleus) emit via rovibrational transitions in the near-IR, providing opportunities to calculate their abundances. To date, only ～50 comets have been characterized for their primary volatiles, with the short-period Jupiter-family comets (JFCs) being significantly underrepresented. In contrast, hundreds of comets have been sampled at optical/UV wavelengths, primarily for the composition of daughter species, leading to the emergence of taxonomic schemes. To develop a near-IR comet taxonomy, more comets need to be characterized. This dissertation focuses on understanding the chemical composition of JFC 46P/Wirtanen. The comet was observed during its historic 2018 apparition using the high-resolution, near-IR spectrograph iSHELL. This work presents the first comprehensive analysis of molecular production rates (molecules s?1) and abundances (production rates relative to H2O and C2H6) of a large set of primary volatiles, H2O, HCN, C2H2, NH3, CH3OH, CH4, C2H6, H2CO, CO, and HC3N in 46P/Wirtaten. The multiple pre, near, and postperihelion measurements spanning ～2 months allowed for testing temporal variability in productions and abundances. This work places the results in context by comparing them with those from the comet population observed using ground-based near-IR observations. Using iSHELL's long-slit, spatial profiles of multiple primary volatiles were obtained to understand their outgassing sources, and the results are discussed in detail in this work. In particular, H2O exhibited spatial profiles that are indicative of the presence of extended outgassing sources in the coma, similar to the EPOXI mission target and 46P/Wirtanen's "twin" comet 103P/Hartley 2. This work will add important measurements to the pool of already sampled comets, in particular to underrepresented JFCs, which will in turn improve our understanding of the solar system.