The Water Vapor and Dust Plumes of Enceladus
Hill, Thomas W.
Doctor of Philosophy
Enceladus is the most active moon of Saturn. Its south polar plume, composed mostly of water vapor and ice grains, is one of the groundbreaking discoveries made by the Cassini spacecraft. During Cassini’s E2, E3, E5 and E7 encounters with Enceladus, the Ion and Neutral Mass Spectrometer (INMS) measured high neutral water vapor densities up to ~10^9 cm-3 (Waite et al., 2006; Teolis et al., 2010; Dong et al., 2011). We have constructed a physical model for the expected water density in the plumes, based on supersonic radial outflow from one or more of the surface vents. We apply this model to possible surface sources of water vapor associated with the multiple jets observed in the visible dust plumes (Spitale and Porco, 2007). Our model predictions fit well with the INMS measurements of neutral water vapor density along the E3, E5, and E7 trajectories. The fit is optimized by values of outflow velocity in the range ~550–750 m/s and values of total source rate in the range ~1.5 − 3.5 10^28 H2O molecules/s ~ 450 – 1050 kg/s. The model can be extended to incorporate the jet features within the plume observed during the E7 encounter. The dust (ice grain) plumes of Enceladus have been observed by multiple Cassini instruments. We propose a composite ice grain size distribution covering a continuous size range from nanometer to micrometers, by combining the CAPS (Cassini Plasma Spectrometer) nanograin size distributions (Hill et al., 2012) and the CDA (Cosmic Dust Analyzer) and RPWS (Radio and Plasma Wave Science) dust power-law size distribution (Kempf et al., 2008; Ye et al., 2012, 2013). We also study the grain charging process using the RPWS-LP (Langmuir Probe) data (Morooka et al., 2011). Based on the size distribution and charge per grain, the densities, source rate, motion, and currents of the ice grains can be calculated. We found that the grains ~2-20 nm dominate in both charge density and number density. The total grain mass density is likely to be ~ 1- 10 times that of the water vapor, and the grain mass loading rate is ~100 kg/s. The motion of the charged grains in Saturn’s magnetosphere implies a transition from ion-like motion to neutral-like motion as the grain size increases from ~nm to ~μm. The grains carry a total current of ~10^5 A at Enceladus, which leads to a different current system from that of the ion pick-up and associated Birkeland currents. The grain current system may be dominant or at least comparable to the ion pick-up current system in accounting for the magnetic perturbations observed near Enceladus.