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The first computational model of solid-phase chemistry in cometary nuclearices is presented. An astrochemical kinetics model, MAGICKAL, is adapted totrace the chemical evolution in multiple layers of cometary ice, over arepresentative period of 5 Gyr. Physical conditions are chosen appropriate for"cold storage" of the cometary nucleus in the outer Solar System, prior to anyactive phase. The chemistry is simulated at a selection of static temperaturesin the range 5 - 60 K, while the ice is exposed to the interstellar radiationfield, inducing a photochemistry in the outer ice layers that producessignificant formation of complex organic molecules. A treatment for thechemistry resulting from cosmic-ray bombardment of the ices is also introducedinto the model, along with a new formulation for low-temperaturephotochemistry. Production of simple and complex molecules to depth on theorder of 10~m or more is achieved, with local fractional abundances comparableto observed values in many cases. The production of substantial amounts ofO$_2$ (and H$_2$O$_2$) is found, suggesting that long-term processing byhigh-energy cosmic rays of cometary ices in situ, over a period on the order of1 Gyr, may be sufficient to explain the large observed abundances of O$_2$, ifthe overall loss of material from the comet is limited to a depth on the orderof 10 m. Entry into the inner solar system could produce a further enhancementin the molecular content of the nuclear ices that may be quantifiable usingthis modeling approach.

Simulations of ice chemistry in cometary nuclei