The Composition of ~96 keV W + in Saturn's Magnetosphere

Enceladus produces plumes of water vapor and ice particles whose gaseous H 2 O molecules dissociate into OH and O. The gas ejecta form a large toroidal planetary cloud colocated with and partially sourced by a similarly large torus of ice grain ejecta. Gas torus neutrals are ionized by charge exchange, solar ultraviolet, and electron impacts, producing the thermal water group ions, W + (O + , OH + , H 2 O + , and H 3 O + ), which become energized in Saturn's magnetosphere. We study the components of energetic (~96 keV) W + using Cassini Charge‐Energy‐Mass Spectrometer (CHEMS) data from 78 near‐equatorial main ring current passes (dipole L  = 7–16, ±10° in latitude) in 2004–2010. W + fractional abundances are ~53% O + , ~22% OH + , ~22% H 2 O + , and ~3% H 3 O + , when averaged over L  = 7–16, resulting in a mean water group mass of 16.7 amu. This large energetic O + fractional abundance, over twice that observed for thermal O + at L  < 10, qualitatively agrees with the broad atomic O cloud observed by Cassini and predicted by some models. Fractional abundance ratios for O + /W + , OH + /W + , and H 2 O + /W + vary little over L  = 7–21. Our observation of energetic H 2 O + /W +  ≈ 22% out to L  ~ 21 suggests that neutral H 2 O spreads throughout the magnetosphere rather than being confined to a narrow torus centered on Enceladus' orbit. Although nearly constant at <2% for L  > 13, H 3 O + /W + tends to increase significantly to ~4–5% at L  ~ 7–8.