COMETS .2. MODELS, EVOLUTION, ORIGIN AND OUTLOOK

In Part II of this paper we comment on the modelling of the complex in teractions which take place in cometary comae and tails between parent molecules, radicals, ions, dust grains and the solar electromagnetic and corpuscular radiation (Sect. 4), and we summarize some of the curr ent thoughts about the nature of the elusive cometary nucleus (Sect. 5 ). Comets are ephemeral phenomena whose lifetimes are short on the cos mic scale; their evolution, statistically and as individual objects, i s a main theme in contemporary research (Sect. 6). Although their orig ins are still not well known, comets undoubtedly carry important clues to the early history and evolution of the solar system (Sect. 7). Fin ally, we mention the main questions now being asked by cometary studie s and illustrate some of the future observational possibilities which may provide crucial data for the next steps forward (Sect. 8).