Aeronomical processes in cometary atmospheres: The carbon compounds' puzzle

The deduction of coma abundances, ultimately leading to the abundance of parent molecules escaping the nucleus of a comet, requires to study a very large number of physical phenomena. Those include excitation processes of the emission of the light, production and loss processes of coma species, interaction with the solar wind… etc. In this paper, as an example, we shall review our knowledge on carbon bearing species and attempt to estimate the C/OH abundance ratio in various comets. We shall reach the interesting conclusion that this parameter greatly varies from comet to comet. It will then be appropriate to discuss if this is the effect of an “aging” process or if this is due to a “primordial” difference among the objects under study.