Ozone Synthesis on the Icy Satellites

Condensed O₂ and ozone on the surfaces of some icy satellites are thought to originate from the radiolytic decomposition of surface water ice by the impact of energetic magnetospheric ions, but decades of laboratory studies have produced no evidence for ozone from the radiolysis of pure water ice. Here we report for the first time the production of ozone in ice by 100 keV ions. Using a method that departs drastically from those used in all previous experiments, we have simulated more closely conditions on the icy satellites by performing ion irradiation while depositing water concurrently, which takes into account the effects of gravity and surface porosity. This codeposition causes the burial of a high concentration of radiolytic O₂ from which ozone is formed. Our results demonstrate that the enhanced trapping of oxygen in surface ices depends on temperature and should vary locally, depending on the rates of irradiation and recondensation. The burial of radiolytic products by redeposition will likely occur in many varied astronomical environments, besides icy satellites.