Comet C/2016 R2 (PanSTARRS) has a peculiar volatile composition, with CO being the dominant volatile, as opposed to H2O, and one of the largest N2/CO ratios ever observed in a comet. Using observations obtained with the Spitzer Space Telescope, NASA’s IRTF, the 3.5-meter Astrophysical Research Consortium telescope at Apache Point Observatory, the 4.3-meter Discovery Channel Telescope at Lowell Observatory, the Arizona Radio Observatory 10-meter Submillimeter Telescope, and the IRAM 30-meter telescope, we quantified the abundances of 12 different species in the coma of R2 PanSTARRS. We confirm the high abundances of CO and N2 and heavy depletions of H2O, HCN, CH3OH, and H2CO compared to CO reported by previous studies. We provide the first measurements (or most sensitive measurements/constraints) on H2O, CO2, CH4, C2H6, OCS, C2H2, and NH3, all of which are depleted relative to CO by at least 1-2 orders of magnitude compared to values commonly observed in comets. The observed species also show strong enhancements relative to H2O, and, even when compared to other species like CH4 or CH3OH, most species show deviations from typical comets by at least a factor of 2-3. The only mixing ratios found to be close to typical are CH3OH/CO2 and CH3OH/CH4. While R2 PanSTARRS was located at a heliocentric distance of 2.8 au at the time of our observations in 2018 January/February, we argue, using sublimation models and comparison to other comets observed at similar heliocentric distance, that this alone cannot account for the peculiar observed composition of this comet and therefore must reflect its intrinsic composition. We discuss possible implications for this clear outlier in compositional studies of comets obtained to date and encourage future dynamical and chemical modeling in order to better understand what the composition of R2 PanSTARRS tells us about the early solar system. We also present preliminary analysis of post-perihelion observations obtained with Spitzer, Keck HIRES, and NASA's IRTF that show the peculiar coma composition of R2 PanSTARRS is not limited to the pre-perihelion portion of the orbit sampled by previous observations, but is an intrinsic property of the comet. We also discuss possible implications for compositional heterogeneity in R2 PanSTARRS.

Peculiar volatile composition of comet C/2016 R2 (PanSTARRS)

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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