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 CH 4 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 (H 2 O, CO, C 2 H 6 , CH 3 OH, CH 4 , H 2 CO, NH 3 , OCS, and HCN) and three fragment species (OH*, NH 2 , and CN), and obtained a sensitive upper limit for C 2 H 2 . We report rotational temperatures, production rates, and mixing ratios (abundances relative to H 2 O). 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 H 2 CO and NH 3 , which were “normal.” The detection of the hypervolatiles CO and CH 4 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.