First Comet Observations with NIRSPEC-2 at Keck: Outgassing Sources of Parent Volatiles and Abundances Based on Alternative Taxonomic Compositional Baselines in 46P/Wirtanen

Abstract A major upgrade to the NIRSPEC instrument at the Keck II telescope was successfully completed in time for near-infrared spectroscopic observations of comet 46P/Wirtanen during its exceptionally close flyby of Earth in 2018 December. These studies determined the abundances of several volatiles, including C 2 H 2 , C 2 H 6 , CH 3 OH, NH 3 , HCN, H 2 CO, and H 2 O. Long-slit spatial distributions of gas rotational temperature and column density are diagnostic for the presence of icy grains in the coma and understanding if different volatiles are associated with common or distinct outgassing sources. These spatial distributions suggest that C 2 H 2 , C 2 H 6 , and HCN have a common outgassing source, whereas H 2 O and CH 3 OH have additional, more extended sources. The synergy of these findings with observations by space missions (Rosetta and EPOXI) motivates continuing studies to address whether or not C 2 H 6 , C 2 H 2 , and HCN have a common source of release (plausibly associated with CO 2 ) in a larger sample of comets and whether systematic differences exist in the release of these species compared to H 2 O and CH 3 OH. Abundances of volatiles are reported relative to H 2 O, as traditionally done, as well as C 2 H 6 . While not unique, the choice of C 2 H 6 demonstrates the value of extending the chemical taxonomy of parent volatiles in comets toward additional compositional “baselines” and, importantly, closer integration between coma abundances and the underlying volatile associations as revealed by spatial distributions. Our findings on composition and sources of outgassing include information relevant to future evaluations of 46P/Wirtanen as a prospective spacecraft target.