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

Abstract

AbstractEnceladus produces plumes of water vapor and ice particles whose gaseous H2O 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+, H2O+, and H3O+), 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% H2O+, and ~3% H3O+, 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 H2O+/W+ vary little over L = 7–21. Our observation of energetic H2O+/W+ ≈ 22% out to L ~ 21 suggests that neutral H2O spreads throughout the magnetosphere rather than being confined to a narrow torus centered on Enceladus' orbit. Although nearly constant at <2% for L > 13, H3O+/W+ tends to increase significantly to ~4–5% at L ~ 7–8.