Low Temperature Surface Formation of NH3 and HNCO: hydrogenation of
  nitrogen atoms in CO-rich interstellar ice analogues
  Solid state astrochemical reaction pathways have the potential to link the
formation of small nitrogen-bearing species, like NH3 and HNCO, and prebiotic
molecules, specifically amino acids. To date, the chemical origin of such small
nitrogen containing species is still not well understood, despite the fact that
ammonia is an abundant constituent of interstellar ices toward young stellar
objects and quiescent molecular clouds. This is mainly because of the lack of
dedicated laboratory studies. The aim of the present work is to experimentally
investigate the formation routes of NH3 and HNCO through non-energetic surface
reactions in interstellar ice analogues under fully controlled laboratory
conditions and at astrochemically relevant temperatures. This study focuses on
the formation of NH3 and HNCO in CO-rich (non-polar) interstellar ices that
simulate the CO freeze-out stage in dark interstellar cloud regions, well
before thermal and energetic processing start to become relevant. We
demonstrate and discuss the surface formation of solid HNCO through the
interaction of CO molecules with NH radicals - one of the intermediates in the
formation of solid NH3 upon sequential hydrogenation of N atoms. The importance
of HNCO for astrobiology is discussed.
