Quantify photolysis/radiolysis contributions to cometary CO and N2 and delineate when entrapment is required

Determine the quantitative fraction of cometary CO and N2 that can be produced through ultraviolet photolysis and electron radiolysis of CO2- and NH3-bearing ices in interstellar and protoplanetary environments, and establish the specific ice compositions, temperatures, and irradiation conditions under which icy-grain chemistry can account for observed cometary CO and N2 versus when the abundances must be attributed to entrapment of gas-phase CO and N2 or direct hypervolatile freeze-out.

Background

Multiple laboratory studies have shown that CO and N2 can form when CO2- and NH3-containing ices are irradiated with UV photons or bombarded with energetic particles, but the reported results have typically been qualitative or lacked yield constraints. This leaves uncertain how much of the hypervolatile content in comets could plausibly originate from in-ice processing versus other mechanisms.

The interpretation of cometary hypervolatile abundances (CO, N2) often assumes entrapment of gas-phase species at low temperatures, which constrains comet formation conditions. If a significant portion of these species can instead be produced photochemically or radiolytically within ices, those constraints may need revision.

The paper frames the need to quantify contributions from in-ice chemistry and to identify the conditions under which entrapment or freeze-out is required to explain observed cometary inventories.

References

As a collection, these experiments show that some CO and N$_2$ are indeed produced when interstellar ice analogs are exposed to UV or electron irradiation, but it is currently unclear how much of a comet's hypervolatile reservoir could be formed through such ice photolysis. At present, we hence do not know under which conditions a cometary CO and N$_2$ reservoir can be explained by icy grain chemistry, and when it must be attributed to entrapment of gas-phase molecules or hypervolatile freeze-out, and therefore can be used to say something about the comet formation temperature.

— CO and N2 Produced from H2O, CO2, and NH3 Cometary Ice Analogs  (2604.03207 - McKinnon et al., 3 Apr 2026) in Section 1, Introduction (paragraph discussing prior irradiation experiments)