  Within the framework of The Heterogeneous dust Evolution Model at the IaS
(THEMIS) this work investigates in detail the surface processes and chemistry
relating to core/mantle interstellar and cometary grain structures and its
influence on the nature of these fascinating particles. It appears that a
realistic consideration of the nature and chemical reactivity of interstellar
grain surfaces could self-consistently and within a coherent framework explain:
the anomalous oxygen depletion, the nature of the CO dark gas, the formation of
'polar ice' mantles, the red wing on the 3 um water ice band, the basis for the
O-rich chemistry observed in hot cores, the origin of organic nano-globules and
the \sim 3.2 um 'carbonyl' absorption band observed in comet reflectance
spectra. It is proposed that the reaction of gas phase species with
carbonaceous a- C(:H) grain surfaces in the interstellar medium, in particular
the incorporation of atomic oxygen into grain surfaces in epoxide functional
groups, is the key to explaining these observations. Thus, the chemistry of
cosmic dust is much more intimately related with that of the interstellar gas
than has previously been considered. The current models for interstellar gas
and dust chemistry will therefore most likely need to be fundamentally modified
to include these new grain surface processes.
