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 N-2 and O-2 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 O-2 and N-2. 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 O-2, H2O and their mixtures with N-2 at the laboratories of CIMAP-GANIL at Caen (France) and LASp at Catania (Italy). Different ions (C-13(2+), Ar2+ and H+) and energies (30-200 keV) have been used. Results. We have found that (CO2)-C-13 is formed when carbon ions are implanted in ices containing H2O and/or O-2. Ozone and nitrogen oxides (NO, N2O, NO2) are formed in the studied ices containing O-2 and N-2 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.