  The chemical composition of comets is frequently assumed to be directly
provided by the observations of the abundances of volatile molecules in the
coma. The present work aims to determine the relationship between the chemical
composition of the coma, the outgassing profile of volatile molecules and the
internal chemical composition, and water ice structure of the nucleus, and
physical assumptions on comets. To do this, we have developed a quasi 3D model
of a cometary nucleus which takes into account all phase changes and water ice
structures (amorphous, crystalline, clathrate, and a mixture of them); we have
applied this model to the comet 67P/Churyumov-Gerasimenko, the target of the
Rosetta mission. We find that the outgassing profile of volatile molecules is a
strong indicator of the physical and thermal properties (water ice structure,
thermal inertia, abundances, distribution, physical differentiation) of the
solid nucleus. Day/night variations of the rate of production of species helps
to distinguish the clathrate structure from other water ice structures in
nuclei, implying different thermodynamic conditions of cometary ice formation
in the protoplanetary disc. The relative abundance (to H2O) of volatile
molecules released from the nucleus interior varies by some orders of magnitude
as a function of the distance to the sun, the volatility of species, their
abundance and distribution between the trapped and condensed states, the
structure of water ice, and the thermal inertia and other physical assumptions
(dust mantle, ...) on the nucleus.
