Composition and dynamics of plasma in Saturn's magnetosphere

During Cassini's initial orbit, we observed a dynamic magnetosphere composed primarily of a complex mixture of water-derived atomic and molecular ions. We have identified four distinct regions characterized by differences in both bulk plasma properties and ion composition. Protons are the dominant species outside about 9 R_S (where R_S is the radial distance from the center of Saturn), whereas inside, the plasma consists primarily of a corotating comet-like mix of water-derived ions with ∼3% N⁺. Over the A and B rings, we found an ionosphere in which O₂⁺ and O⁺ are dominant, which suggests the possible existence of a layer of O₂ gas similar to the atmospheres of Europa and Ganymede.