The composition and structure of the Enceladus plume

Abstract

The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed an occultation of the Sun by the water vapor plume at the south polar region of Saturn's moon Enceladus. The Extreme Ultraviolet (EUV) spectrum is dominated by the spectral signature of H2O gas, with a nominal line-of-sight column density of 0.90 0.23 × 1016 cm-2 (upper limit of 1.0 × 1016 cm-2). The upper limit for N2 is 5 × 1013 cm-2, or <0.5% in the plume; the lack of N2 has significant implications for models of the geochemistry in Enceladus' interior. The inferred rate of water vapor injection into Saturn's magnetosphere is ∼200 kg/s. The calculated values of H2O flux from three occultations observed by UVIS have a standard deviation of 30 kg/s (15%), providing no evidence for substantial short-term variability. Collimated gas jets are detected in the plume with Mach numbers of 5-8, implying vertical gas velocities that exceed 1000 m/sec. Observations at higher altitudes with the Cassini Ion Neutral Mass Spectrometer indicate correlated structure in the plume. Our results support the subsurface liquid model, with gas escaping and being accelerated through nozzle-like channels to the surface, and are consistent with recent particle composition results from the Cassini Cosmic Dust Analyzer.