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Recent observations of the long period comet C/2016 R2 (PanSTARRS) indicatean unusually high N2/CO abundance ratio, typically larger than 0.05, and atleast 2-3 times higher than the one measured in 67P/Churyumov-Gerasimenko.Another striking compositional feature of this comet is its heavy depletion inH2O, compared to other comets. Here, we investigate the formation circumstancesof a generic comet whose composition reproduces these two key features. Wefirst envisage the possibility that this comet agglomerated from clathrates,but we find that such a scenario does not explain the observed low waterabundance. We then alternatively investigate the possibility that the buildingblocks of the comet C/2016 R2 (PanSTARRS) agglomerated from grains and pebblesmade of pure condensates via the use of a disk model describing the radialtransport of volatiles. We show that N2/CO ratios reproducing the valueestimated in this comet can be found in grains condensed in the vicinity of theCO and N2 icelines. Moreover, high CO/H2O ratios (>100 times the initial gasphase value) can be found in grains condensed in the vicinity of the COiceline. If the building blocks of a comet assembled from such grains, theyshould present N2/CO and CO/H2O ratios consistent with the measurements made incomet C/2016 R2 (PanSTARRS)'s coma. Our scenario indicates that comet C/2016 R2(PanSTARRS) formed in a colder environment than the other comets that sharemore usual compositions. Our model also explains the unusual composition of theinterstellar comet 2l/Borisov.

Cold traps of hypervolatiles in the protosolar nebula at the origin of comet C/2016 R2 (PanSTARRS)'s peculiar composition