Oxidant generation in the ice under electron irradiation: Simulation and application to Europa

Electron irradiation of ice is an important process in the Solar System, especially for icy satellites (e.g., Europa) whose tenuous atmospheres originate from the thermal and radiation-induced outgassing of their icy surfaces. Laboratory experiments have been performed on the electron irradiation of water ice to study the volatile gases (e.g., H₂ and O₂) that leave the surface and the chemical compounds (e.g., H₂O₂) that are produced inside the ice. Semi-empirical models have also been developed to estimate the production of the chemical species in the ice during irradiation as functions of electron energy and ice temperature. In this study, we build a chemistry-transport model to simulate chemical processes occurring in the ice during irradiation by 10 keV electrons and to describe how the chemical species of interest are formed, transported, and distributed in the ice. The simulated H₂O₂ mixing ratio agrees well with experiments performed by Hand and Carlson (2011). Our model can be applied to the surfaces of icy satellites (e.g., Europa) to estimate the oxidant generation in the ice and to evaluate the potential habitability of those satellites. This study calls for further experimental studies on ice under electron irradiation to constrain critical parameters such as rate coefficients and diffusion coefficients of volatile species produced by radiation in the ice.