We used iSHELL , the powerful cross-dispersed IR spectrograph at the NASA-IRTF, to measure the native ice composition of long-period comet C/2018 Y1 (Iwamoto) within weeks of its discovery . We recorded spectra between ~ 2.8 and 5.2 µm at resolving power R ~ 40,000 on three pre-perihelion dates (2019 January 13, and February 4-5), permitting a search for temporal variability in production rates and relative abundances in the inner coma. We report production rates for H2O and eight trace parent molecules (CO, CH4, C2H6, CH3OH, H2CO, HCN, C2H2, NH3), and abundance ratios relative to H2O and C2H6. We also recorded spatial distributions along the slit for multiple simultaneously measured volatiles and dust continuum. This allowed assessing coma morphology, as well as potential associations among ices in the nucleus of this moderately bright comet.
On all dates, the geocentric velocity of C/2018 Y1 was sufficiently large to shift cometary CO and CH4 emissions well away from their opaque telluric counterpart absorptions and into regions of highly favorable atmospheric transmittance. This, combined with the large spectral grasp and contiguous wavelength coverage of iSHELL permitted measurement of 11 distinct ro-vibrational lines of CH4, the most complete coverage of methane to date for any comet.
Comparing abundances relative to H2O in C/2018 Y1 with previously observed comets from the Oort cloud  revealed depleted CO, C2H2, H2CO, and NH3, near-median CH4 and HCN, slightly enriched C2H6, but highly enriched CH3OH. Relative to C2H6, all species were depleted, excepting H2O (near-median) and CH3OH (enriched).
A viable pathway for the production of CH3OH and C2H6 is H-atom addition reactions to CO and C2H2, respectively, condensed on the surfaces of interstellar grains at very low temperatures (~ 20 K or lower, ref. ). In this context, our results imply efficient H-atom addition on grains that were assimilated into the nucleus of C/2018 Y1. Furthermore, among parent volatiles systematically measured in comets, CH4 is second in volatility (after CO), and therefore its near-median abundance suggests that thermal processing (e.g., in the proto-solar nebula) may not have been dominant in establishing its volatile composition.
These observations were obtained at the NASA Infrared Telescope Facility, operated by the University of Hawai’i under contract NN14CK55B with the National Aeronautics and Space Administration.
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