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 km s⁻¹ (8 to 102 eV for O), we find that water group projectiles undergo Eley-Rideal reactions with the ice surface to form O₂, HO₂, and H₂O₂. At a 45° angle of incidence the Osingle bondO 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 O₂ formation, with the yield peaking at 19% for atomic oxygen impactors at 20 km s⁻¹. Notably, this reaction is relatively temperature independent and proceeds in hydrogen rich pristine ice, where radiolysis is hindered. This mechanism for O₂ production from low energy impacts may contribute to O₂ exospheres at Saturn and Jupiter's icy moons, where hyperthermal ion fluxes are large.
Additional Information© 2022 Elsevier. Received 23 December 2021, Revised 17 February 2022, Accepted 22 February 2022, Available online 26 February 2022, Version of Record 7 March 2022. WAG acknowledges support from JPL (Fund IA-1642096- Hypervelocity Sampling Across the Solar System). KPG acknowledges support from JPL (Fund IA-1653621- PDRDF: Plasma-surface interactions as a way to probe the composition of icy worlds). KPG and RWG would like to thank Tom Nordheim of the Jet Propulsion Laboratory for useful discussions concerning moon-magnetosphere interactions. Data availability. The relevant data are available from the corresponding author, WAG, upon reasonable request. Declaration of Competing Interest. There are no conflicts to declare.
Supplemental Material - 1-s2.0-S0019103522000872-mmc1.zip
Supplemental Material - 1-s2.0-S0019103522000872-mmc2.docx