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

O. Mousis, Artyom Aguichine+5 more|arXiv (Cornell University)|Mar 2, 2021

Astro and Planetary Science2 references4 citations

TL;DR

This paper proposes that comet C/2016 R2 (PanSTARRS) formed from pure condensates near the CO and N₂ icelines in the protosolar nebula, explaining its unusually high N₂/CO ratio and extreme H₂O depletion. The model shows that grains condensed in the 10–15 AU region with low viscosity (α = 10⁻⁴) can reproduce the comet’s observed volatile ratios, suggesting it formed in a colder, more distant environment than typical comets.

ABSTRACT

Recent observations of the long period comet C/2016 R2 (PanSTARRS) 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, 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 the comet C/2016 R2 (PanSTARRS) 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 icelines. Moreover, high CO/H2O ratios (>100 times the initial gas phase value) can be found in grains condensed in the vicinity of the CO iceline. 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 comet 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 share more usual compositions. Our model also explains the unusual composition of the interstellar comet 2l/Borisov.

Motivation & Objective

• Explain the origin of comet C/2016 R2 (PanSTARRS)’s unusually high N₂/CO ratio (~0.06–0.08) and extreme H₂O depletion (H₂O/CO ~0.32%).
• Investigate whether clathrate formation can reproduce the comet’s peculiar composition, given its low water abundance.
• Assess the formation conditions in the protosolar nebula that could yield such volatile ratios through pure condensation processes.
• Extend the model to explain the composition of the interstellar comet 2I/Borisov, which also shows CO-rich, H₂O-poor outgassing.
• Quantify the relative likelihood of forming R2-like comets versus H₂O-rich comets based on disk parameters and reservoir mass ratios.

Proposed method

• Used a protosolar nebula disk model to simulate radial transport of volatiles in both gaseous and solid (pure condensate) phases.
• Tracked the formation of pure condensates (e.g., CO, N₂, H₂O ices) at different distances from the Sun, focusing on regions near the CO and N₂ icelines.
• Calculated N₂/CO and CO/H₂O ratios in solid grains formed at varying heliocentric distances (10–15 AU) for different viscosity parameters (α = 10⁻³ and 10⁻⁴).
• Compared the mass of H₂O and CO ices in two annular reservoirs: one where CO > H₂O (R2-like comets), and another with the protosolar CO/H₂O ratio (average comets).
• Simulated time evolution of the reservoir mass ratio over 0.1–1 Myr to assess the likelihood of forming R2-like comets.
• Evaluated the role of N₂ shielding and photodissociation chemistry to assess nitrogen chemistry stability in cold disk regions.

Experimental results

Research questions

• RQ1 Can the high N₂/CO ratio in comet C/2016 R2 (PanSTARRS) be explained by clathrate formation, given its low H₂O abundance?
• RQ2 What physical conditions in the protosolar nebula are required to produce solids with the observed N₂/CO and CO/H₂O ratios in R2?
• RQ3 How does the viscosity parameter (α) of the protosolar disk influence the formation and mass ratio of R2-like versus H₂O-rich cometary reservoirs?
• RQ4 Can the same formation mechanism explain the CO-rich, H₂O-poor composition of the interstellar comet 2I/Borisov?
• RQ5 What is the relative likelihood of forming R2-like comets compared to typical H₂O-rich comets, based on disk mass reservoirs and formation timescales?

Key findings

• Clathrate formation cannot explain the low H₂O/CO ratio in R2, as it would require H₂O/CO ≈ 6, inconsistent with the observed ~0.32%.
• Pure condensates formed near the CO and N₂ icelines (10–15 AU) can reproduce the observed N₂/CO ratio of ~0.06–0.08 in R2.
• CO/H₂O ratios in grains near the CO iceline can exceed 200 times the protosolar gas-phase ratio, consistent with R2’s low H₂O abundance.
• For α = 10⁻⁴, the mass ratio of R2-like comets to average comets can range from <1% to 100% depending on formation epoch (0.1–1 Myr), indicating high sensitivity to disk parameters.
• For α = 10⁻³, the mass of R2-like comets is only about 1% that of average comets, suggesting they are rare under high-viscosity conditions.
• The model also explains the CO-rich, H₂O-poor coma of interstellar comet 2I/Borisov, supporting a common formation mechanism in cold, volatile-rich disk regions.