| Abstract: | The main production process of species occurring in the coma of comets is the photodestruction of molecules initially present
in the nucleus ices and non-refractory grains or trapped inside the nucleus "material". Grains can also be a source of molecules
in the coma. Chemical reactions may occur between coma species. Consequently, although chances that an abundant coma species
has not been detected are now small, the coma composition is certainly quite different from that of the nucleus. Except for
the molecules released directly at the nucleus surface, all coma species are produced in an "extended region" or come from
"a distributed source". Since the recent literature is rich in reports on observations of molecules and species possibly not
initially present in the comet ices or not released at the nucleus, a general discussion of how coma species are stored, processed
or produced is presented, based mostly on observational results. What is at stake is the proper modeling of the coma structure,
hence an accurate derivation of the nucleus composition from coma observations.
This revised version was published online in June 2006 with corrections to the Cover Date. |
