Comets are expected to be the most primitive objects in the solar system .
The chemical composition of these objects 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 H _ { 2 } O ) 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 .
For the less volatile molecules such as CO _ { 2 } and H _ { 2 } S , the relative ( to H _ { 2 } O ) abundance of species in coma remain similar to the primitive composition of the nucleus ( relative deviation less than 25 % ) only around the perihelion passage ( in the range -3-2 to +2-3 AU ) , whatever is the water ice structure and chemical composition , and under the conditions that the nucleus is not fully covered by a dust mantle .
The relative ( to H _ { 2 } O ) abundance of highly volatile molecules such as CO and CH _ { 4 } in the coma remain approximately equal to the primitive nucleus composition only for nuclei made of clathrates .
The nucleus releases systematically lower relative abundances of highly volatile species ( up to one order of magnitude ) around perihelion ( in the range -3-2 to +2-3 AU ) in the cases of the crystalline and amorphous water ice structures in the nuclei .
The rate of production , the outgassing profile and the relative abundances ( to H _ { 2 } O ) of volatile molecules are the key parameters allowing one to retrieve the chemical composition and thermodynamic conditions of cometary ice formation in the early solar system .
The coming observations of the coma and nucleus by the Rosetta mission instruments ( VIRTIS , MIRO , … ) should provide the necessary constraints to the model to allow it to infer the primordial ice structure and composition of the comet .