Saturn's neutral torus versus Jupiter's plasma torus

[1] With the recent discovery of an atmospheric plume of H2O it is thought that Enceladus could deliver as much as 300 kg/s of neutral gas to Saturn's inner magnetosphere. Io is the source of roughly 1 ton/s of sulfur and oxygen gas at Jupiter. Despite the apparent similarity, the neutral/ion ratio at Saturn is 3 orders of magnitude higher than at Jupiter. We explore the flow of mass and energy at Saturn and Jupiter using a simplified homogeneous physical chemistry model to understand why these two system are so different. Our results suggest that ionization at Saturn is fundamentally limited by the slower corotational flow velocity at Enceladus, resulting in a factor of 4 lower ion pickup temperature. The net result of cooler ions at Enceladus is a cooler thermal electron population (similar to 2 eV) that is insufficient to generate significant ionization.