CO and N 2 Produced from H 2 O, CO 2 , and NH 3 Cometary Ice Analogs

Abstract Hypervolatile species such as carbon monoxide (CO) and molecular nitrogen (N 2 ) 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 N 2 formation following ultraviolet (UV) irradiation and electron bombardment of carbon dioxide (CO 2 ), ammonia (NH 3 ), H 2 O:CO 2 , H 2 O:NH 3 , and H 2 O:CO 2 :NH 3 cometary ice analogs. We find that CO and N 2 form in all photoprocessed ices at temperatures between 10 and 100 K, resulting in 0.4%–0.9% CO and 0.03%–0.7% N 2 relative to water, and CO/CO 2 and N 2 /NH 3 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 a 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 N 2 can be accounted for by photodissociation of NH 3 embedded in water ice. The latter result is also consistent with observed similarly elevated isotopic ratios of N 2 and NH 3 in 67P. Taken together, our results suggest that N 2 /H 2 O ratios