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Context. Ices present in different astrophysical environments are exposed to ion irradiation from cosmic rays (H to heavier than Fe) in the keV to GeV energy range.

Laboratory data of CH4ice radiolysis promoted by fast heavy ions are obtained by infrared spectroscopy (FTIR). CH4molecules are condensed on a CsI substrate at 15 K, and the ice layer is bombarded by 220 MeV 16O7+ion beam. The ice thickness is thin enough to be traversed by projectiles at constant velocity close to the equilibrium charge state. The induced CH4dissociation gives rise to the formation of molecular species CH3, C2H2, C2H4, C2H6, and C3H8. Their formation and dissociation cross sections are determined. C2H6represent the most abundant daughter molecules. The carbon budget analysis of CH4and its radiolysis products shows that the column density of carbon atoms contained in the methane destroyed during ion irradiation is 30 − 50% greater than the sum for the column densities of the newly formed species. As an astrophysical implication, the current results allow estimation of chemical reaction rates in ices covering interstellar grains.

An experimental study of the interaction of heavy, highly charged, and energetic ions (52 MeV 58Ni13+) with pure H2O, pure CO2and mixed H2O:CO2astrophysical ice analogs is presented. This analysis aims to simulate the chemical and the physicochemical interactions induced by heavy cosmic rays inside dense and cold astrophysical environments, such as molecular clouds or protostellar clouds. The measurements were performed at the heavy ion accelerator GANIL (Grand Accélérateur National d’Ions Lourds in Caen, France). The gas samples were deposited onto a CsI substrate at 13 K. In-situ analysis was performed by a Fourier transform infrared (FTIR) spectrometer at different fluences. Radiolysis yields of the produced species were quantified. The dissociation cross sections of pure H2O and CO2ices are 1.1 and 1.9  × 10-13cm2, respectively. For mixed H2O:CO2(10:1), the dissociation cross sections of both species are about 1  ×  10-13cm2. The measured sputtering yield of pure CO2ice is 2.2  ×  104molec ion-1. After a fluence of 2−3  ×  1012ions cm-2, the CO2/CO ratio becomes roughly constant (~0.1), independent of the initial CO2/H2O ratio. A similar behavior is observed for the H2O2/H2O ratio, which stabilizes at 0.01, independent of the initial H2O column density or relative abundance.