Water-group ion irradiation studies of Enceladus ice analogues: Can radiolysis account for material in and around the south polar plume?

Saturn's magnetosphere contains trapped plasma and energetic charged particles which constantly irradiate the surface of Enceladus. In this study, we exposed Enceladean surface ice analogues containing H2O, CO2, CH4, and NH3 to water-group ions (e.g., O+, O3+, OH+, and H2O+) having energies between 10 and 45 keV with the aim of exploring the chemical evolution of these ices and characterising the extent to which the surface material on Enceladus is weathered by Saturn's radiation environment. Each irradiation process was monitored in situ using Fourier-transform mid-infrared transmission absorption spectroscopy, and post-irradiative warming of the ices was performed to better characterise complex organic molecules formed as a result of the mobilisation of radiolytically generated radicals. Irradiation resulted in the formation of CO, OCN−, and NH4+ in all experiments, and the radiolytic formation of formamide, acetylene, acetaldehyde, and hydroxymethyl radicals was also tentatively suggested in most experiments. Post-irradiative warming of the ices resulted in the formation of carbamic acid, ammonium carbamate, and an alcohol species. Although many of these products have not been previously observed on Enceladus' surface, some have been detected in Enceladus' plumes. Since our results demonstrate that the radiolytic formation of these molecules can occur over timescales similar to the exposure times of plume and surface material to magnetospheric radiation, questions must be raised as to whether such material originates directly from the subsurface ocean or is instead formed within the radiation-rich space environment.