Cometary Science Newsletter

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

• Khan, Y. ^{1, 10}
• Gibb, E. L. ^{1, 10}
• Bonev, B. P. ^{2, 10}
• Roth, N. X. ^{3, 4, 10}
• Saki, M. ^{1, 10}
• DiSanti, M. A. ^{4, 10}
• Dello Russo, N. ^{5, 10}
• Vervack, R. J., Jr. ^{5, 10}
• McKay, A. J. ^{2, 4, 10}
• Combi, M. R. ⁶
• Shou, Y. ⁶
• Cordiner, M. A. ^{4, 7}
• Kawakita, H. ^{8, 10}
• Fougere, N. ⁶
• Protopapa, S. ⁹

1. Department of Physics and Astronomy, University of Missouri-St. Louis, One University Boulevard, St. Louis, MO, USA
2. Department of Physics American University 4400 Massachusetts Avenue NW Washington, DC 20016, USA
3. Universities Space Research Association Columbia, MD, USA
4. Solar System Exploration Division Code 690, NASA-Goddard Space Flight Center, Greenbelt, MD, USA
5. Johns Hopkins Applied Physics Laboratory, Laurel, MD, USA
6. Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, USA
7. Department of Physics, Catholic University of America, Washington DC, USA
8. Koyama Astronomical Observatory Kyoto Sangyo University Motoyama, Kamigamo, Kita-ku, Kyoto, 603-8555, Japan
9. Southwest Research Institute, Boulder, CO, USA
10. Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawai’i under contract NNH14CK55B with the National Aeronautics and Space Administration

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, HI. We sampled emissions from H2O, HCN, C2H2, NH3, C2H6, and CH3OH on UT 2018 December 21 using two instrumental settings that spanned the 2.9–3.6 μm spectral region. We also obtained a sensitive 3σ upper limit for H2CO and for the rarely studied molecule HC3N. We report rotational temperatures, production rates, and mixing ratios (relative to H2O as well as to C2H6). 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 1–2σ uncertainty, production rates obtained for all molecules in this work were consistent with those obtained using NIRSPEC-2 except H2O, indicating low-level variability on a timescale of days. Mixing ratios with respect to H2O in 46P/Wirtanen were consistent with corresponding values from NIRSPEC-2 within the uncertainty with the exception of CH3OH, 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 H2O and HCN due to rotational effects. Both H2O and HCN production rates showed similar temporal variability, resulting in nearly constant HCN/H2O.

The Planetary Science Journal (Published)

DOI: 10.3847/PSJ/abc95c