Testing Short-term Variability and Sampling of Primary Volatiles in Comet 46P/Wirtanen

The exceptionally favorable close approach of Jupiter-family comet 46P/Wirtanen in 2018 December enabled characterization of its primary volatile composition with exceptionally high spatial resolution and sensitivities using the iSHELL spectrograph at the NASA Infrared Telescope Facility on Maunakea, H I . We sampled emissions from H 2 O, HCN, C 2 H 2 , NH 3 , C 2 H 6 , and CH 3 OH on UT 2018 December 21 using two instrumental settings that spanned the 2.9–3.6 μ m spectral region. We also obtained a sensitive 3 σ upper limit for H 2 CO and for the rarely studied molecule HC 3 N. We report rotational temperatures, production rates, and mixing ratios (relative to H 2 O as well as to C 2 H 6 ). We place our results in context by comparing them with other comets observed at near-IR wavelengths. We also compare our results with those obtained using the NIRSPEC-2 spectrograph on Keck II on UT December 17 and 18 and with results obtained from iSHELL on other dates during the same apparition. Within 1–2 σ uncertainty, production rates obtained for all molecules in this work were consistent with those obtained using NIRSPEC-2 except H 2 O, indicating low-level variability on a timescale of days. Mixing ratios with respect to H 2 O in 46P/Wirtanen were consistent with corresponding values from NIRSPEC-2 within the uncertainty with the exception of CH 3 OH, which yielded a higher ratio on December 21. Our measurements afforded a high temporal resolution that spanned ∼2/3 of the rotational period of 46P/Wirtanen, enabling us to test short-term variability in the production rates of H 2 O and HCN due to rotational effects. Both H 2 O and HCN production rates showed similar temporal variability, resulting in nearly constant HCN/H 2 O.