Encelad us: A potential source of ammonia products and molecular nitrogen for Saturn's magnetosphere

The detection of nitrogen species in Saturn's magnetosphere could, in principle, provide clues to the origin and evolution of its satellites and tenuous rings. Smith et al. (2005) first identified low-energy N⁺ using the Cassini Plasma Spectrometer (CAPS). N⁺ was predominantly seen in the Saturn's inner magnetosphere (<∼14 Rs), indicating an Enceladus nitrogen source rather than the expected Titan source. However, the parent molecular species was not confirmed. Subsequent modeling showed that a small N₂ source at Enceladus consistent with ion neutral mass spectrometer observations could produce the observed spatial distribution of N⁺. Considering the significance of understanding the Enceladus plumes, identifying the molecular parent for the observed N⁺ (N ₂ or NH_x) can provide clues to the subsurface composition of Enceladus and the processes generating this plume activity. In this paper, we expand on the work of Smith et al. (2007) to identify the source molecules for nitrogen ions detected in Saturn's inner magnetosphere. We conduct an extensive study of all available CAPS data to determine if N₂⁺ or ammonia is the parent molecule for these nitrogen ions. We present evidence for the detection of product ions (NH_x⁺), likely from ammonia, and provide upper limits on the amount of N₂⁺ that may be present in the plasma in the inner magnetosphere.