Organic molecules in the interstellar medium, comets, and meteorites: A voyage from dark clouds to the early earth

Our understanding of the evolution of organic molecules, and their voyage f rom molecular clouds to the early solar system and Earth, has changed drama tically. Incorporating recent observational results from the ground and spa ce, as well as laboratory simulation experiments and new methods for theore tical modeling, this review recapitulates the inventory and distribution of organic molecules in different environments. The evolution, survival, tran sport, and transformation of organics is monitored, from molecular clouds a nd the diffuse interstellar medium to their incorporation into solar system material such as comets and meteorites. We constrain gas phase and grain s urface formation pathways to organic molecules in dense interstellar clouds , using recent observations with the Infrared Space Observatory (ISO) and g round-based radiotelescopes. The main spectroscopic evidence for carbonaceo us compounds in the diffuse interstellar medium is discussed (UV bump at 22 00 Angstrom, diffuse interstellar bands, extended red emission, and infrare d absorption and emission bands). We critically review the signatures and u nsolved problems related to the main organic components suggested to be pre sent in the diffuse gas, such as polycyclic aromatic hydrocarbons (PAHs), f ullerenes, diamonds, and carbonaceous solids. We also briefly discuss the c ircumstellar formation of organics around late-type stars. In the solar system, space missions to comet Halley and observations of the bright comets Hyakutake and Hale-Bopp have recently allowed a reexaminatio n of the organic chemistry of dust and volatiles in long-period comets. We review the advances in this area and also discuss progress being made in el ucidating the complex organic inventory of carbonaceous meteorites. The kno wledge of organic chemistry in molecular clouds, comets, and meteorites and their common link provides constraints for the processes that lead to the origin, evolution, and distribution of life in the Galaxy.