As an off-shoot of our interstellar chemical kinetics modeling, we have adapted our in-house chemical code MAGICKAL to allow us to simulate solid-phase chemistry in comets - the first and currently only model of its kind (see Garrod 2019).
The comet model contains all of the same grain-surface and bulk-ice chemical mechanisms as the interstellar version of MAGICKAL, as well as radiolysis caused by interstellar cosmic rays and solar protons. Interstellar and solar UV-vis photons can also influence the chemical evolution of the comet. Because comets are made up of a combination of molecular ices and dust grains (with their origins in the interstellar medium), it is necessary also to simulate the evolving concentration of dust grains in the comet, as well as their effects on the propagation of photons that can dissociate molecules.
The figure below (Fig. 1 from Willis, Christianson & Garrod 2024) shows a schematic of the processes simulated in the model, which includes a total of 25 layers of ice/dust material of increasing thickness, allowing the simulation of comet material to a depth of ~135m.
Garrod (2019) produced the first simulation of solid-phase comet chemical kinetics during the cold storage phase of a comet, prior to its entry into the inner solar system. This work determined that a period of billions of years in the Oort cloud, subjected to Galactic cosmic ray-induced radiolysis, would allow production of some complex organic molecules (COMs) to depths of up to ~10m, which would be most observable in dynamically new comets (i.e. those making their first solar approach).
Willis, Christianson & Garrod (2024) applied the model to the orbital behavior of comet Hale Bopp, producing the first simulations of solid-phase comet chemistry during the active phase. The resultant solar heating and irradiation influenced the comet composition both via active chemistry and by the loss of material from the upper ice layers. The model provided a good match to detected water-loss rates for this comet. However, in general, neither COMs produced during the Oort cloud stage nor the production of COMs during solar approach were sufficient to explain the high abundances of COMs detected toward this and other comets (especially 67P), implying those molecules must be an inheritance from earlier, likely interstellar evolution of cold dust grains.
A follow-up publication using orbital dynamics for 67P is in preparation, which includes an explicit treatment of dust loss from the comet via evaporative erosion of volatile species.