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The chemical composition of comets is frequently assumed to be directlyprovided by the observations of the abundances of volatile molecules in thecoma. The present work aims to determine the relationship between the chemicalcomposition of the coma, the outgassing profile of volatile molecules and theinternal chemical composition, and water ice structure of the nucleus, andphysical assumptions on comets. To do this, we have developed a quasi 3D modelof a cometary nucleus which takes into account all phase changes and water icestructures (amorphous, crystalline, clathrate, and a mixture of them); we haveapplied this model to the comet 67P/Churyumov-Gerasimenko, the target of theRosetta mission. We find that the outgassing profile of volatile molecules is astrong indicator of the physical and thermal properties (water ice structure,thermal inertia, abundances, distribution, physical differentiation) of thesolid nucleus. Day/night variations of the rate of production of species helpsto distinguish the clathrate structure from other water ice structures innuclei, implying different thermodynamic conditions of cometary ice formationin the protoplanetary disc. The relative abundance (to H2O) of volatilemolecules released from the nucleus interior varies by some orders of magnitudeas a function of the distance to the sun, the volatility of species, theirabundance and distribution between the trapped and condensed states, thestructure of water ice, and the thermal inertia and other physical assumptions(dust mantle, ...) on the nucleus.

How to link the relative abundances of gas species in coma of comets to their initial chemical composition ?