Interstellar ices as a source of CN-bearing molecules in protoplanetary disks

A reliable model for the composition and evolution of interstellar ices in regions of active star formation is fundamental to our quest to determine t he organic inventory of planetesimals in the early Solar System. This has b ecome a realistic goal since the launch of the Infrared Space Observatory, which provides a facility for infrared spectroscopy unhindered by telluric absorption over the entire spectral range of vibrational modes in solids of exobiological interest. Interstellar molecules detected in the solid phase to date include H2O, NH3, CO, CO2, CH3OH, CH4, H2CO, OCS and HCOOH, togeth er 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 o f prebiotic molecules may not have formed endogenously in significant quant ities on early Earth if conditions were not highly reducing. Experiments in which interstellar ice analogs are subject to UV photolysis or energetic i on bombardment yield CN-rich residues with a spectral signature that matche s a corresponding feature observed in young protostars enshrouded in dust a nd gas. CN-bearing species are also present in cometary ices, with a combin ed abundance comparable to the lower end of the range observed in protostar s. Energetic processing of interstellar ices is thus a viable and potential ly significant source of CN compounds in protoplanetary disks.