Europa maintains a thin oxygen exosphere produced when energetic particles from Jupiter’s magnetosphere bombard its icy surface — a process called sputtering. While ion bombardment had long been considered the dominant driver, the role of electrons was poorly constrained.
For my M.Sc. thesis at Northern Arizona University, working with Mark Loeffler, I measured the sputtering yield of water ice by 0.5 keV electrons across a range of temperatures relevant to Europa’s surface. The results were surprising: the global O2 production rate from electron-induced sputtering exceeds the combined contribution from all ionic species previously considered. This means electrons are a critical — and previously underappreciated — source of Europa’s oxygen exosphere.
See the full paper: Davis et al. (2021), The Astrophysical Journal Letters, 908, L53.