Reactive scattering of water group ions on ice surfaces with relevance to Saturn's icy moons

Collisions of hyperthermal water group ions with ices are known to occur in astrophysical environments, for example, solar wind-comet interactions and moon-magnetosphere interactions (at Saturn and Jupiter), yet their effect on surface and gaseous composition is little understood. Here, we report molecular dynamics simulations of hyperthermal water-group molecules impacting water-ice surfaces using the ReaxFF reactive force fields to describe bond breaking and forming processes dynamically during the reactive collision events. Simulation conditions are chosen for relevance to Saturn's moons Dione and Rhea. At impact velocities of 10-35 kms(-1) (8 to 102 eV for O), we find that water group projectiles undergo Eley-Rideal reactions with the ice surface to form O-2, HO2, and H2O2. At a 45o angle of incidence the O-O bonded products tend to embed within a few molecular layers of the surface. Smaller angles of incidence lead to formation of some molecular oxygen that promptly rebounds from the surface with-3 eV kinetic energy. Impact velocity, angle of incidence, and hydrogenation of the impactor strongly influence O-2 formation, with the yield peaking at 19% for atomic oxygen impactors at 20 kms(-1). Notably, this reaction is relatively temperature independent and proceeds in hydrogen rich pristine ice, where radiolysis is hindered. This mechanism for O-2 production from low energy impacts may contribute to O-2 exospheres at Saturn and Jupiter's icy moons, where hyperthermal ion fluxes are large.