Ultraviolet photolysis and proton irradiation of astrophysical ice analogs containing hydrogen cyanide

Hydrogen cyanide (HCN) has been identified in the gas phase of the interstellar medium as well as in the comae of several comets. Terrestrially, HCN is a key component in the synthesis of biologically important molecules such as amino acids. In this paper, we report the results of low-temperature (18 K) ice energetic processing experiments involving pure HCN and mixtures of HCN with H2O and NH3. Ice films, 0.1 to several microns in thickness, were exposed to either ultraviolet photons (110–250 nm) or 0.8-MeV protons to simulate the effects of space environments. Observed products include HCNO (isocyanic acid), NH4+ (ammonium ion), CN− (cyanide ion), OCN− (cyanate ion), HCONH2 (formamide), and species spectrally similar to HCN polymers. Product formation rates and HCN destruction rates were determined where possible. Results are discussed in terms of astrophysical situations in the ISM and the Solar System where HCN would likely play an important role in prebiotic chemistry. These results imply that if HCN is present in icy mixtures representative of the ISM or in comets, it will be quickly converted into other species in energetic environments; pure HCN seems to be polymerized by incident radiation.