The extraordinary composition of the blue comet C/2016 R2 (PanSTARRS)

Abstract

We present a multi-wavelength study of comet C/2016 R2 (PanSTARRS). with small amateur telescopes. We report on measurements of CO, CH 3 OH, H 2 CO and HCN production rates, and on the determination of the N 2 /CO abundance ratio. Several other species, especially OH, were searched for but not detected. The inferred relative abundances, including upper limits for sulfur species, are compared to those measured in other comets at about the same heliocentric distance of ∼2.8 AU. The coma composition of comet C/2016 R2 is very different from all other comets observed so far, being rich in N 2 and CO and dust poor. This suggests that this comet might belong to a very rare group of comets formed beyond the N 2 ice line. Alternatively, comet C/2016 R2 (PanSTARRS) could be the fragment of a large and differentiated transneptunian object, with properties characteristic of volatile-enriched layers.

Characterization of the Nucleus, Morphology, and Activity of Interstellar Comet 2I/Borisov by Optical and Near-infrared GROWTH, Apache Point, IRTF, ZTF, and Keck Observations

“…The presence of moderately abundant CO and H 2 O on 2I (Ye et al 2020 and this work) suggests that while 2I has not been heated so thoroughly by its home Sun (as solar system Jupiter family comets and likely 1I have), it could have been ejected from its home system or placed into its star's equivalent of the solar system's Oort cloud more than a few Myr of its formation after its home system's protoplanetary disk midplane had cleared enough to heat its surface above 30 K (Lisse et al 2019). This assumes that in comparison with the solar system comet C/2016 R2, has never been heated above 20K before encountering the Sun, where it is in the process of losing its hypervolatiles but not its H 2 O ice due to hypervolatile supercooling (Biver et al 2018;Lisse et al 2019). Additionally, the host star of 2I may have a higher stellar iron abundance that has been shown to have an effect on the waterice fraction solid building blocks in the protoplanetary disk favoring a higher concentration of CO/CO 2 relative to water ice (Bitsch & Battistini 2020).…”

Probing the Gas Content of Late-stage Protoplanetary Disks with N2H+

“…However, the amount of processing is more limited for primitive meteorites and comets, placing a larger emphasis on accretion of materials and processing occurring within the protoplanetary disk. Observations of comets have found N 2 /CO ratios below the solar nebula value of 0.15, ranging from a comparable value of N 2 /CO∼0.08 for comet C/2016 R2 to the highly depleted value of <6×10 −5 in Hale-Bopp (Cochran et al 2000;Biver et al 2018). An in situ measurement by ROSINA, the mass spectrometer on board the Rosetta spacecraft, determined a N 2 /CO ratio of 5.7×10 −3 for comet 67P/Churyumov-Gerasimenko (Rubin et al 2015).…”

“…The Rosetta mission has revealed a lack of nitrogen in both the volatile ice and refractory phases of comet 67P/Churyumov-Gerasimenko (Rubin et al 2015;Fray et al 2017). In fact, comets with high N 2 /CO abundance ratios, similar to the estimated value for the solar nebula, are rarely observed (Biver et al 2018). This may indicate that CO, and perhaps carbon in general, is better captured and/or retained in solid bodies than volatile nitrogen.…”

On the Origin and Evolution of the Material in 67P/Churyumov-Gerasimenko

“…Finally, ground-based observation of large numbers of comets still reveals surprising cases with very unusual composition (e.g. the N 2rich comet C/2016 R2; Biver et al 2018;McKay et al 2019;Opitom et al 2019), or variation in relative abundance of different species with heliocentric distance (e.g., C/2009 P1;Feaga et al 2013;Gicquel et al 2015).…”