Cassini INMS Observations of Neutral Molecules in the E-Ring

In 2008 and 2009, the Cassini Ion Neutral Mass Spectrometer (INMS) investigation made in situ measurements of neutral species near the equatorial plane in a torus within 0.5 Saturn radii (Rs) of the orbit of Enceladus. The observations are made in the Closed Source Neutral (CSN) mode, which is the most sensitive mode for measuring neutrals. These observations show an azimuthal asymmetry in the neutral densities and provide measurements used to calibrate simulations of the neutrals emitted from Enceladus. Far from Enceladus, the neutral densities, at less than 103 particles/cm3, are at the detection limit of INMS. Near Enceladus, but outside of the plumes and north of the equatorial plane, particles are detected within 5,000 km of Enceladus, with the density increasing to approximately 105 particles/cm3 at the equatorial plane. In the radial direction, neutral H2O density appears to be sharply peaked at 4 Rs, in agreement with simulations of the neutral torus. H2O and CO2 are the only species identified, to date. Other predicted species, atomic nitrogen and OH, react with the walls of the instrument and are not measured in the CSN mode. Relatively-high counts at 2 Daltons are infrequently observed near the equatorial plane. These measurements are interpreted as H2 released during ice-grain impacts and occur with a frequency consistent with the number densities measured by the Cassini Cosmic Dust Analyzer.