CO and N2 Produced from H2O, CO2, and NH3 Cometary Ice Analogs

Hypervolatile species such as carbon monoxide (CO) and molecular nitrogen (N2) have been detected in comets, and could be used to constrain comet formation temperature conditions if their presence is due to freeze-out and/or entrapment.

Here we instead explore another plausible origin of cometary hypervolatiles: photodissociation of less volatile species. We characterize CO and N₂ formation following ultraviolet (UV) irradiation and electron bombardment of carbon dioxide (CO₂), ammonia (NH₃), H₂O:CO₂, H₂O:NH₃, and H₂O:CO₂:NH₃ cometary ice analogs.

We find that CO and N₂ form in all photoprocessed ices at temperatures between 10 K and 100 K, resulting in 0.4-0.9 % CO and 0.03-0.7 % N₂ relative to water, and CO/CO₂ and N₂/NH₃ mixing ratios of 2.5-62 % and 0.7-9 %, respectively, across the experiments.

Because our initial ices are reasonably well-matched to interstellar ices and we use UV exposure similar to a dark cloud, we can compare the resulting ratios directly to cometary abundances.

Such a comparison shows that while only a few of CO observations in comets are readily explained by photodissociation, almost all observed cometary N₂ can be accounted for by photodissociation of NH₃ embedded in water ice. The latter result is also consistent with observed similarly elevated isotopic ratios of N₂ and NH₃ in 67P.

Taken together, our results suggest that N₂/H₂O ratios less than 1 % should be used cautiously when inferring a comet’s formation location, while the more substantial CO abundances seen in many comets do likely imply entrapment at low ice temperatures.

Alexandra McKinnon, Alexia Simon, Michelle R. Brann, Elettra L. Piacentino, Karin I. Oberg, Mahesh Rajappan

Comments: Accepted for ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM); Solar and Stellar Astrophysics (astro-ph.SR); Chemical Physics (physics.chem-ph)
Cite as: arXiv:2604.03207 [astro-ph.EP] (or arXiv:2604.03207v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2604.03207
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Related DOI:
https://doi.org/10.3847/1538-4357/ae53e1
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Submission history
From: Alexandra McKinnon
[v1] Fri, 3 Apr 2026 17:32:12 UTC (4,635 KB)
https://arxiv.org/abs/2604.03207
Astrobiology, Astrochemistry,