An Investigation of the Abundances of Hypervolatiles CO, CH₄, C₂H₆ in Jupiter-family Comet 46P/Wirtanen

Comets are thought to retain volatiles from the time of their formation. Therefore, characterizing their composition should provide insights into the conditions in the early solar system. Roughly 15 Jupiter Family comets (JFCs) have been sampled in the near-IR, albeit at vastly different levels of detail. Detections (or significant upper limits) are even sparser for the hypervolatiles CO and CH₄ due to Doppler shift limitations. Although differences in abundances of parent species have been noted, such a small sample size has hampered development of a classification system. 46P/Wirtanen is a JFC whose favorable and close approach to Earth in Dec 2018 - Feb 2019 provided a great opportunity to help understand the distribution of volatiles in the early solar system and in this potential mission target.

We observed 46P/Wirtanen using the high-resolution (λ/Δλ ∼ 40,000) IR spectrograph iSHELL at the NASA Infrared Telescope Facility (IRTF) on UT Feb 4 and 5, 2019. JFCs are generally faint, and maximum brightness is often near their closest approach to Earth, when geocentric velocities (Δ_dot) are small. Measuring CO and CH₄ requires sufficiently high Δ_dot to shift cometary emissions from their highly opaque telluric counterparts to wavelengths with high atmospheric transmission. At the time of 46P’s closest approach (Dec 16), the geocentric velocity was insufficient for studies of CO and CH₄; however, by early February 2019 the geocentric velocity became large enough (Δ_dot ~ 13 km s^-1) while the comet was still sufficiently bright to place stringent upper limits on CO and CH₄. These measurements are particularly critical because abundances of the highly volatile CO and CH₄ are the most sensitive to differences in natal temperatures, and also to the degree of post-formative thermal evolution. Additionally, our observations included fluorescent emission from other volatile species, most notably CH₃OH, H₂CO, CN, and OH* (prompt emission as a proxy for water).

We present rotational temperatures, production rates (Qs), and mixing ratios (with respect to both H₂O and C₂H₆), discuss the implications of the Q_x/Q_H2O vs. Q_x/Q_C2H6 content of 46P/Wirtanen and for the evolution of JFCs, and place our results in the context of findings from the Rosetta mission and ground-based studies of comets.

These observations were obtained at the NASA-IRTF, operated by the University of Hawai’i under contract NN14CK55B with the National Aeronautics and Space Administration. This work was supported by the NASA Earth and Space Science Fellowship, Solar System Workings, Solar System Observations, and Astrobiology Programs, and NSF’s AAG.