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 Lowell Discovery Telescope (formerly DCT), 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 with respect to CO reported by previous studies. We report the first detections or sensitive constraints on the production rate of H2O, CO2, CH4, C2H6, OCS, C2H2, and NH3, all of which are depleted relative to CO by at least 1-2 orders of magnitude in R2 PanSTARRS compared to values commonly observed in comets. The observed species also show strong enhancements relative to H2O. 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 values observed in other comets 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 the composition of ices in the nucleus of R2 PanSTARRS must be an extreme outlier compared to other comets observed to date. We discuss possible implications for the chemical composition of R2 PanSTARRS in the context of 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 persists beyond the pre-perihelion portion of the orbit sampled by previous observations. We also discuss possible implications for compositional heterogeneity in R2 PanSTARRS.

Pre- and post-perihelion observations of the CO-dominated comet C/2016 R2 (PanSTARRS)

ACS Spring Meeting 2021

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