Recent observations of the long-period comet C/2016 R2 (PanSTARRS; hereafter R2) indicate an unusually high N₂/CO abundance ratio, typically larger than ∼0.05, and at least 2–3 times higher than the one measured in 67P/Churyumov–Gerasimenko. Another striking compositional feature of this comet is its heavy depletion in H₂O (H₂O/CO ∼ 0.32%), compared to other comets. Here we investigate the formation circumstances of a generic comet whose composition reproduces these two key features. We first envisage the possibility that this comet agglomerated from clathrates, but we find that such a scenario does not explain the observed low water abundance. We then alternatively investigate the possibility that the building blocks of R2 agglomerated from grains and pebbles made of pure condensates via the use of a disk model describing the radial transport of volatiles. We show that N₂/CO ratios reproducing the value estimated in this comet can be found in grains condensed in the vicinity of the CO and N₂ ice lines. Moreover, high CO/H₂O ratios (>100 times the initial gas-phase value) can be found in grains condensed in the vicinity of the CO ice line. If the building blocks of a comet assembled from such grains, they should present N₂/CO and CO/H₂O ratios consistent with the measurements made in R2’s coma. Our scenario indicates that R2 formed in a colder environment than the other comets that share more usual compositions. Our model also explains the unusual composition of the interstellar comet 2l/Borisov.

最近对长周期彗星 C/2016 R2（PanSTARRS；以下简称 R2）的观测表明，N ₂ /CO 丰度比异常高，通常大于 0.05，至少比 67P/丘留莫夫-格拉西缅科。这颗彗星的另一个显着组成特征是它在 H ₂ O (H ₂O/CO ∼ 0.32%)，与其他彗星相比。在这里，我们研究了一颗通用彗星的形成环境，其成分再现了这两个关键特征。我们首先设想这颗彗星从包合物聚集的可能性，但我们发现这种情况并不能解释观察到的低水丰度。然后，我们还通过使用描述挥发物径向传输的圆盘模型来研究 R2 的构建块从由纯冷凝物制成的谷物和鹅卵石聚集的可能性。我们表明，在 CO 和 N ₂冰线附近凝结的颗粒中可以找到再现这颗彗星中估计值的N ₂ /CO 比率。此外，高 CO/H ₂在 CO 冰线附近凝结的晶粒中可以发现 O 比（> 100 倍初始气相值）。如果彗星的构建块由这些颗粒组装而成，它们的 N ₂ /CO 和 CO/H ₂ O 比率应与 R2 彗发中的测量结果一致。我们的场景表明，R2 形成于比其他彗星更冷的环境中，这些彗星的成分更常见。我们的模型还解释了星际彗星 2l/Borisov 的不寻常组成。