A Tale of “Two” Comets: The Primary Volatile Composition of Comet 2P/Encke Across Apparitions and Implications for Cometary Science

Abstract

The highly favorable 2017 apparition of 2P/Encke allowed the first comprehensive comparison of primary volatile abundances in a given comet across multiple apparitions. This apparition offered opportunities to address pressing questions in cometary science, including investigating evolutionary and/or heliocentric distance (R h) effects on volatile production, sampling the hypervolatiles CO and CH4 in an ecliptic comet, and measuring volatile release at small R h. The faintness and frequently low geocentric velocity of ecliptic comets during most apparitions make our near-infrared observations of these hypervolatiles rare and of high scientific impact. We characterized the volatile composition of 2P/Encke on three post-perihelion dates using the iSHELL spectrograph at the NASA Infrared Telescope Facility on Maunakea, HI. We detected fluorescent emission from nine primary volatiles (H2O, CO, C2H6, CH3OH, CH4, H2CO, NH3, OCS, and HCN) and three fragment species (OH*, NH2, and CN), and obtained a sensitive upper limit for C2H2. We report rotational temperatures, production rates, and mixing ratios (abundances relative to H2O). Compared to mean abundances in comets observed to date in the near-infrared, mixing ratios of trace gases in 2P/Encke were depleted for all species except H2CO and NH3, which were “normal.” The detection of the hypervolatiles CO and CH4 is particularly notable given the paucity of measurements in ecliptic comets. We observed significant differences in primary volatile composition compared to published pre-perihelion results from 2003 at larger R h. We discuss possible mechanisms for these differences and discuss these results in the context of findings from the Rosetta mission and ground-based studies of comets.

Probing the Evolutionary History of Comets: An Investigation of the Hypervolatiles CO, CH4, and C2H6 in the Jupiter-family Comet 21P/Giacobini–Zinner

“…However, given the uncertainty in T rot in October, it is important to note that the range of possible October CO mixing ratios is in formal agreement with those from July (Table 2). These mixing ratios are depleted with respect to the mean for all comets measured to date at near-infrared wavelengths (5.2 ± 1.3%), but are consistent with the few measurements in ecliptic comets (Dello Russo et al 2016, Roth et al 2018, Dello Russo et al 2019).…”

“…G-Z is the only comet observed at near-infrared wavelengths during three different perihelion passages: 1998 (Weaver et al 1999, Mumma et al 2000), 2005(DiSanti et al 2013, and 2018 (this work), and is just the second comet to have a comprehensive comparison of hypervolatile abundances across apparitions (the other being 2P/Encke; see Roth et al 2018, Radeva et al 2010. Figure 5A shows our individual 2018 measurements of hypervolatile abundances in G-Z.…”

“…CH 4 bears the distinction of being the most severely undersampled hypervolatile in ecliptic comets, having been firmly measured in only six to date (Dello Russo et al 2016and refs. therein, DiSanti et al 2017, Roth et al 2018, Dello Russo et al 2019. Utilizing the large spectral grasp of iSHELL, we detected CH 4 in G-Z at the 4σ level on July 28, at >6σ on July 31, and derived a meaningful constraint on its mixing ratio on October 10 (see Fig.…”

“…Fortunately, the increased sensitivity and long on-source integration times afforded by the high-resolution iSHELL spectrograph, which became available for use at the NASA Infrared Telescope Facility (IRTF) in late 2016, together with unusually favorable apparitions for several short-period comets have to date enabled sensitive searches for these molecules in short-period comets 2P/Encke (Roth et al 2018) and JFC 45P/Honda-Mrkos-Pajdušáková (DiSanti et al…”

The Peculiar Volatile Composition of CO-dominated Comet C/2016 R2 (PanSTARRS)

“…Dello Villanueva et al 2011;. New techniques specific to iSHELL are described in detail in (see also Roth et al (2018)). We provide a brief summary of our reduction procedures below.…”