  Comets provide a unique insight into the molecular composition and complexity
of the material in the primordial solar nebula. Recent results from the Rosetta
mission, currently monitoring comet 67P/Churyumov-Gerasimenko in situ, and ALMA
(the Atacama Large Millimeter/submillimeter Array), have demonstrated a
tantalising link between the chemical complexity now confirmed in disks (via
the detection of gas-phase CH3CN; Oberg et al. 2015) and that confirmed on the
surface of 67P (Goesmann et al. 2015), raising questions concerning the
chemical origin of such species (cloud or inheritance versus disk synthesis).
Results from an astrochemical model of a protoplanetary disk are presented in
which complex chemistry is included and in which it is assumed that simple ices
only are inherited from the parent molecular cloud. The model results show good
agreement with the abundances of several COMs observed on the surface of 67P
with Philae/COSAC. Cosmic-ray and X-ray-induced photoprocessing of
predominantly simple ices inherited by the protoplanetary disk is sufficient to
generate a chemical complexity similar to that observed in comets. This
indicates that the icy COMs detected on the surface of 67P may have a disk
origin. The results also show that gas-phase CH3CN is abundant in the inner
warm disk atmosphere where hot gas-phase chemistry dominates and potentially
erases the ice chemical signature. Hence, CH3CN may not be an unambiguous
tracer of the complex organic ice reservoir. However, a better understanding of
the hot gas-phase chemistry of CH3CN is needed to confirm this preliminary
conclusion.
