Primary Volatile Composition of Comet 46P/Wirtanen During its Exceptional 2018 Apparition

Comets are thought to have preserved the compositional signature of the regions in the early solar system where they formed ~ 4.5 billion years ago. Knowledge of the chemical composition of comets, therefore, can help in understanding the chemical and physical conditions in the protosolar nebula. Detailed studies of the chemical composition of comets will also help in gaining a better understanding of how much post-formation processing affects cometary composition. To assess this quantitatively, a large sample of comets needs to be studied with particular emphasis on Jupiter-family comets, which remain underrepresented in the near-IR observational database. 46P/Wirtanen is a small, hyperactive, Jupiter-family comet that is a potential and favorable future mission target. The historically-favorable 2018 apparition of 46P/Wirtanen was characterized by a close approach to Earth and excellent observing geometry. It presented a unique opportunity to measure the volatile abundances, outgassing patterns and spatial associations among different molecules in the comet in great detail. We used the long slit, high sensitivity and superior spectral grasp of the iSHELL spectrograph at the NASA Infrared Telescope Facility (IRTF) at Mauna Kea to observe 46P/Wirtanen on several dates between 2018 December 6 and 2019 February 5. This study is the first comprehensive compositional study of 46P/Wirtanen in the near-IR that targets a large suit of molecules over a range of dates, thus addressing the so-called “snapshot bias” of a single observational visit that often limits interpretation of near-IR observations of comets. We report production rates and mixing ratios with respect to H₂O (or sensitive upper limits) for the primary volatiles H₂O, HCN, CH₄, C₂H₆, CH₃OH, H₂CO, NH₃, CO, OCS, C₂H₂ and HC₃N. In addition, rotational temperatures and spatial profiles are reported for several of these molecules. We will discuss the chemical composition of 46P/Wirtanen at different heliocentric distances as well as a sensitive search for temporal variability and asymmetries in mixing ratios about perihelion. Various co-authors gratefully acknowledge support by NSF AARG, by NASA Earth and Space Science Fellowship, EW, SSW, NPP, and SSO Programs.