Cosmic ray-ice interaction studied by radiolysis of 15 K methane ice with MeV O, Fe and Zn ions

Methane (CH₄)ice is found in the interstellar medium and in several bodies of the Solar system, where it is commonly exposed to cosmic rays and stellar winds. The chemical, physical and structural effects induced by fast heavy ions in thin layers of pure CH₄ ices at 15 K arc analysed by mid-infrared spectroscopy (Fourier transform infrared). Different pure CH ₄ ice samples were irradiated with 6 MeV ¹⁶O²⁺. 220 MeV ¹⁶O⁷⁺, 267 MeV ⁵⁶Fe²²⁺ and 606 MeV ⁷⁰Zn²⁶⁺ ions at Grand Accélérateur National d'Ions Lourds/France. Results show that C_nH_m molecules, where n - 2-4 and m = 2(n - 1) to 2(n + 1) and radical species CH₃, C₂H₃ and C₂H₅ are formed. The destruction cross-sections of CH₄ ice and the formation cross-sections of new molecules C_nH_m are reported. The extrapolation of current results allow us to estimate the half lives of CH ₄ ices in the interstellar medium and the Solar system (Earth orbit) as about 600 × 10⁶ and 600 yr. respectively. This huge ratio strongly suggests that the vast majority of chemical or even biochemical processes induced by ionizing radiation occur close lo stars.