Interstellar Ices as a Source of CN-Bearing Molecules in Protoplanetary Disks

A reliable model for the composition and evolution of interstellar ices inregions of active star formation is fundamental to our quest to determinethe organic inventory of planetesimals in the early Solar System. This hasbecome a realistic goal since the launch of the Infrared Space Observatory,which provides a facility for infrared spectroscopy unhindered by telluricabsorption over the entire spectral range of vibrational modes in solids ofexobiological interest. Interstellar molecules detected in the solid phaseto date include H_2O, NH3, CO, CO2,CH3OH, CH4, H2CO, OCS andHCOOH, together with a C≡N-bonded absorber generically termed`XCN'. In this article, we focus on cosmic synthesis of CN-bearing species,as this important class of prebiotic molecules may not have formedendogenously in significant quantities on early Earth if conditions were nothighly reducing. Experiments in which interstellar ice analogs are subjectto UV photolysis or energetic ion bombardment yield CN-rich residues with aspectral signature that matches a corresponding feature observed in youngprotostars enshrouded in dust and gas. CN-bearing species are also presentin cometary ices, with a combined abundance comparable to the lower end of therange observed in protostars. Energetic processing of interstellar ices isthus a viable and potentially significant source of CN compounds inprotoplanetary disks.