Complex Ice Chemistry: A comparative study of electron irradiated planetary ice analogues containing methane

Various Solar System objects are covered in layers of ice that are dominated by H2O, CH4, and N-2 and in which complex chemical processes take place. In this work, the influence of composition and irradiation duration on the volatile irradiation products of mixed CH4:N-2, CH4:H2O, and CH4:H2O:N-2 ices after electron irradiation are studied. The ices were irradiated for 2 or 4 h with 5 keV electrons, followed by a temperature programmed desorption, where the desorption of the volatile irradiation products was observed. The formation of C2Hx and C3Hx is observed in all ices and for both irradiation times. For the ices containing H2O, molecules as large as tentatively identified C4Hx and C5Hx are observed to co-desorb with water, whereas for CH4:N-2 a continuous desorption signal is observed instead of a sharp desorption peak. A decrease in signal intensity from the 2 to the 4 h irradiation is observed for most m/z signals in CH4:H2O and CH4:H2O:N-2 ices, whereas the opposite is recorded for CH4:N-2, where in general larger signal for longer irradiation duration is seen. The addition of nitrogen to the CH4:H2O ice did not lead to clear identification of different molecules, but instead to a decrease of the observed signal for complex molecules, suggesting that the addition of nitrogen to the CH4:H2O mixture primarily leads to a more effective incorporation of material in an organic residue. The analysis of the residue will be subject of future work to complement the findings in this study.