Energetic electrons and ions, with the flux of the latter being more isotropic, bombard surface ice, sputtering atoms and molecules off and causing radiolysis of water and other chemical compounds. The energetic particles break water into oxygen and hydrogen, maintaining the thin oxygen atmospheres of the icy moons (since the hydrogen escapes more rapidly).
High-energy electrons and ions, with the latter's flux being more uniform, collide with surface ice, ejecting atoms and molecules and causing radiolysis of water and other chemical substances. The energetic particles split water into oxygen and hydrogen, preserving the thin oxygen atmospheres of icy moons (since hydrogen escapes more quickly).