Radiolysis of Ices by Cosmic-Rays: CH₄ and H₂O Ices Mixtures Irradiated by 40 MeV ⁵⁸Ni¹¹⁺ Ions

Physico-chemical modifications induced by swift heavy ions on methane-water (CH₄:H₂O) ices at 15 K are analyzed. Ice films, at concentrations of (1:3) and (1:15), were irradiated by 40 MeV ⁵⁸Ni¹¹⁺ ions. Fourier transform transmission spectroscopy in the mid-range was used to monitor the evolution ices at 15 K as a function of projectile fluence. New IR bands appearing for the irradiated (CH₄:H₂O) (1:3) ice are attributed to the synthesized molecules: C₃H₈, HCO, H₂CO, CO, CO₂, H₂O₂, HCOOH, CH₃OH, C₂H₅OH, and CH₃CHO. For the irradiated (CH₄:H₂O) (1:15) ice, the abundances of the compounds containing two carbons atoms are lower than those for the (1:3) ice; in contrast, CH₃OH and H₂O₂ abundances increase when compared to the values obtained with the (1:3) ice. After irradiation, the ices were warmed up until 110 K, when the IR spectra reveal features of complex organic molecules. The destruction and formation cross sections and the sputtering yields of the ice mixtures are estimated. These findings provide possible pathways for the occurrence of compounds rich in C, O, and H, which are indeed observed in the cold regions of the universe such as ices in grain mantles of the interstellar medium and circumstellar envelopes.