Radiolysis of H2O:CO2 ices by heavy energetic cosmic ray analogs

An experimental study of the interaction of heavy, highly charged, and energetic ions (52 MeV ⁵⁸Ni¹³⁺) with pure H₂O, pure CO₂ and mixed H₂O:CO₂ astrophysical 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 CO₂ ices are 1.1 and 1.9 × 10^-13 cm², respectively. For mixed H₂O:CO₂ (10:1), the dissociation cross sections of both species are about 1 × 10^-13 cm². The measured sputtering yield of pure CO₂ ice is 2.2 × 10⁴ molec ion^-1. After a fluence of 2-3 × 10¹² ions 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.