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

Abstract

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 are analysed by mid-infrared spectroscopy (Fourier transform infrared). Different pure CH4 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 CnHm 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 CnHm are reported. The extrapolation of current results allow us to estimate the half lives of CH4 ices in the interstellar medium and the Solar system (Earth orbit) as about 600 x 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 to stars.