Evolution of icy surfaces: an experimental approach

We present new laboratory results on some effects induced by ion irradiation (30 keV He+ and 60 keV Ar++) of frozen NH3, CH4, H2O, and their mixtures. These species have been chosen in view of their possible presence on the surface of Saturnian satellites and rings. In fact these surfaces are exposed to intense irradiation by magnetospheric and/or solar energetic particles. We find that the abundance ratio ammonia:water in irradiated mixtures H2O:NH3 (similar or equal to 2.1) decreases as irradiation dose increases. The decrease is greater when the temperature of the bombarded target is higher. New species have been synthesized after irradiation of mixtures H2O:NH3:CH4 (similar or equal to 2.5:1:2) and some firmly identified : C2H6, CO, and CO2. Refractories containing OCN groups and, possibly, amino acids, are also observed. The results are discussed in view of their potential interest in the Saturnian environment. Tn particular they confirm that ion irradiation produces a decrease of the ammonia! water molecular number ratio. However IR signatures of ammonia could be found on the surfaces of the "bright" Saturnian moons (Mimas, Enceladus, Tethys, Dione, and Rhea), especially those in the spectral region of the fundamental bands, covered by the instruments on board Cassini. We also suggested that the presence of ammonia could be indirectly sustained from the observation of XCN compounds on the surfaces of the "dark" satellites (Hyperion, Iapetus, Phoebe). Prebiotic chemistry could also be activated by particle irradiation on Saturnian moons. (C) 1998 Elsevier Science Ltd. All rights reserved.