Simulations of ice chemistry in cometary nuclei
  The first computational model of solid-phase chemistry in cometary nuclear
ices is presented. An astrochemical kinetics model, MAGICKAL, is adapted to
trace the chemical evolution in multiple layers of cometary ice, over a
representative period of 5 Gyr. Physical conditions are chosen appropriate for
"cold storage" of the cometary nucleus in the outer Solar System, prior to any
active phase. The chemistry is simulated at a selection of static temperatures
in the range 5 - 60 K, while the ice is exposed to the interstellar radiation
field, inducing a photochemistry in the outer ice layers that produces
significant formation of complex organic molecules. A treatment for the
chemistry resulting from cosmic-ray bombardment of the ices is also introduced
into the model, along with a new formulation for low-temperature
photochemistry. Production of simple and complex molecules to depth on the
order of 10~m or more is achieved, with local fractional abundances comparable
to observed values in many cases. The production of substantial amounts of
O$_2$ (and H$_2$O$_2$) is found, suggesting that long-term processing by
high-energy cosmic rays of cometary ices in situ, over a period on the order of
1 Gyr, may be sufficient to explain the large observed abundances of O$_2$, if
the overall loss of material from the comet is limited to a depth on the order
of 10 m. Entry into the inner solar system could produce a further enhancement
in the molecular content of the nuclear ices that may be quantifiable using
this modeling approach.
