Comprehensive Study of the Chemical Composition and Spatial Outgassing Behavior of Hyperactive Comet 46P/Wirtanen Using Near-IR Spectroscopy during its Historic 2018 Apparition

Abstract

We present a comprehensive analysis of the chemical composition of the Jupiter-family comet and potential spacecraft target 46P/Wirtanen, in the near-IR wavelength range. We used iSHELL at the NASA Infrared Telescope Facility to observe the comet on 11 pre-, near-, and postperihelion dates in 2018 December and 2019 January and February during its historic apparition. We report rotational temperatures, production rates, and mixing ratios with respect to H2O and C2H6 or 3σ upper limits of the primary volatiles H2O, HCN, CH4, C2H6, CH3OH, H2CO, NH3, CO, C2H2, and HC3N. We also discuss the spatial outgassing of the primary volatiles, to understand their sources and the spatial associations between them. The spatial profiles of H2O in 46P/Wirtanen suggest the presence of extended H2O outgassing sources in the coma, similar to the EPOXI target comet 103P/Hartley 2. 46P/Wirtanen is among the few known hyperactive comets, and we note that its composition and outgassing behavior are similar to those of other hyperactive comets in many ways. We note that the analyzed parent volatiles showed different variations (relative mixing ratios) during the apparition. We compared the chemical composition of 46P/Wirtanen with the mean abundances in Jupiter-family comets and the comet population as measured with ground-based near-IR facilities to date. The molecular abundances in 46P/Wirtanen suggest that although they were changing, the variations were small compared to the range in the comet population, with CH3OH showing notably more variation as compared to the other molecules.

Gas Sources from the Coma and Nucleus of Comet 46P/Wirtanen Observed Using ALMA

“…The total H 2 O production rate on each date was obtained from a linear fit to the measurements between December 2 and December 10 by Combi et al (2020) using Lyα observations by the Solar and Heliospheric Observatory satellite, which gave Q(H 2 O) = 6.1 × 10 27 s −1 on December 2 and Q(H 2 O) = 7.2 × 10 27 s −1 on December 7. These values are consistent with a slow increase in Q(H 2 O) as the comet approached perihelion on December 12 and are in line with the average values of 8 × 10 27 s −1 measured by Lis et al (2019) using the Stratospheric Observatory for Infrared Astronomy between December 14 and December 20 and 7 × 10 27 s −1 measured using the Infrared Telescope Facility (IRTF) between December 6 and December 21 (Khan et al 2023).…”

“…These differences could be explained by temporal variability of the CH 3 OH and H 2 O outgassing rates, since significant shorttimescale variations in Q(CH 3 OH) (over a period of several hours to several days) were observed in this comet by Roth et al (2021b). Indeed, our CH 3 OH production rate of (1.9 ± 0.1) × 10 26 s −1 on December 7 is very close to the value of (2.2 ± 0.1) × 10 26 s −1 observed on December 6 by Khan et al (2023) and (2.1 ± 0.1) × 10 26 s −1 on December 11 (Biver et al 2021), demonstrating good consistency between the ALMA, IRTF, and IRAM measurements in this case.…”

Inferring the CO2 Abundance in Comet 45P/Honda-Mrkos-Pajdušáková from [O i] Observations: Implications for the Source of Icy Grains in Cometary Comae

“…HMP seems to fit into this potential subfamily. In addition to these three comets being hyperactive and having high CO 2 abundances as noted above, they also all have low CO abundances, in addition to similar abundances of other trace volatiles (CO, CH 3 OH, and C 2 H 6 ; McKay et al 2021;Khan et al 2023). Could this possible family be reflective of an ancient breakup of a larger object, or is it indicative of composition in a specific section of the protoplanetary disk?…”

Parent Volatile Outgassing Associations in Cometary Nuclei: Synthesizing Rosetta Measurements and Ground-based Observations

“…For instance, Dello Russo et al (2016b) et al (2017) did not find any correlation between outgassing properties and the sublimation temperature of the corresponding pure ices (see Table 1). MIRO observations show that NH 3 , CH 3 OH, and HCN ices with similar sublimation temperatures (78, 99, and 95 K, respectively; i Based on the short-term variation of volatiles observed in many comets (e.g., see Dello Russo et al 2016b;Biver et al 2018Biver et al , 2022Roth et al 2020;Lippi et al 2021;Saki et al 2021;Khan et al 2023).…”

“…Ammonium salts (NH 4 + X − ) can form with the interaction of ammonia with molecules such as HCN, HNCO, and HCOOH at the low temperatures presented in the comet formation regions in the protoplanetary disk midplane. The main nitrogen-bearing species in comets, NH 3 , NH 2 , HCN (and its isomer HNC), CH 3 CN, and HC 3 N, have emission features in the near-IR and radio wavelengths (see Biver & Bockelee-Morvan 2015, Dello Russo et al 2016bBockelée-Morvan & Biver 2017;Lippi et al 2021;Biver et al 2022), and the detection or stringent upper limits of these molecules have been reported in many comets to date (e.g., Dello Russo et al 2016b;Biver et al 2018Biver et al , 2021Saki et al 2021;Biver et al 2022;Dello Russo et al 2022;Khan et al 2023). Ammonium salts are hard to detect in general, as they are unstable in the gas phase and their infrared signature is often hidden.…”