CHAPTER 11: The Buried Chemical Wealth of Ices in Protoplanetary Discs

Interstellar gas phase chemistry is effective in producing a range of simple molecules. Greater chemical complexity can be achieved through the processing of mixed ices on the surfaces of dust grains in dense, cold regions of interstellar space. The formation of complex species can be tested in appropriate laboratory experiments and is found to be efficient. The number of molecular species detected in interstellar and circumstellar regions of the Milky Way galaxy is currently more than 200, most of them being organic. The detection of complex organic species would have an added value if it occurs in the gas surrounding a young solar-type star, since it would imply that their synthesis is coeval with the formation of planets in a protoplanetary system. However, the cold gas in protoplanetary discs seems to be devoid in complex species. This is to some extent unexpected as chemical and physical conditions in discs do not appear so drastically different from other interstellar regions. This finding is also in striking contrast with cometary and meteoritic findings in the Solar System. We discuss possible reasons for such deficiency.