Cassini detection of water-group pick-up ions in the Enceladus torus

This study reports direct detection by the Cassini plasma spectrometer of freshly-produced water-group pick-up ions within the proposed Enceladus torus, a radially narrow toroidal region surrounding Saturn that contains a high density of water-group neutrals. This torus is produced by the icy plumes observed near the south pole of Enceladus. The ions are created by charge exchange collisions between water-group neutrals in the Enceladus torus and thermal ions corotating with Saturn. They are identified in the Cassini data via their characteristic ring-like signatures in ion velocity distributions. In the radial distance range of 4.0 to 4.5 R-S, the density of these non-thermalized ions is estimated to be at least 5.2 cm(-3), about 8% of the total ion density. The estimated density together with ionization, charge exchange, and loss times, yield an ion thermalization time of at least 3150 s, in reasonable agreement with hybrid particle simulations.