The water vapor plumes of Enceladus

摘要

The Cassini E3, E5, and E7 encounters with Enceladus probed the south polar plumes, where the Ion and Neutral Mass Spectrometer (INMS) measured neutral H_2O molecular densities up to ～10~9 cm~(-3). 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. Our model predictions fit well with the INMS measurements of neutral H_2O 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) H_2O molecules/s. The model can be extended to incorporate the jet features within the plume observed during the E7 encounter.