Chemistry induced by energetic ions in water ice mixed with molecular nitrogen and oxygen

Context. Several molecular species have been observed as frozen gases in cold environments such as grains in the interstellar/circumstellar medium or icy objects in the outer solar system. Because N2 and O2 are homonuclear, symmetric molecules are not easily observed. It is therefore relevant to find indirect methods to prove their presence from astronomical observations. Aims. Here we investigate one of the possible indirect methods, namely the formation of specific molecules by cosmic ion bombardment of ices in astrophysical environments that contain O2 and N2. The observation of these molecules in astronomical environments could act as a trojan horse to detect the presence of frozen molecular oxygen and/or nitrogen. Methods. We have conducted ion bombardment experiments of frozen O2 ,H 2O and their mixtures with N2 at the laboratories of CIMAP-GANIL at Caen (France) and LASp at Catania (Italy). Different ions ( 13 C 2+ ,A r 2+ and H + ) and energies (30–200 keV) have been used. Results. We have found that 13 CO2 is formed when carbon ions are implanted in ices containing H2 Oa nd/or O2. Ozone and nitrogen oxides (NO, N2O, NO2) are formed in the studied ices containing O2 and N2 with different relative abundances. Conclusions. We suggest that ozone and nitrogen oxides are present and have to be searched for in some specific environments such as dense clouds in the interstellar medium and the surfaces of Pluto, Charon and Triton. Their observation could demonstrate the presence of molecular oxygen and/or nitrogen. A possible interest for the observations of atmospheres in exo-planetary objects is also discussed.