Linking Enceladus’ plume characteristics to the crevasse properties

Abstract

Supersonic plumes consistent of water vapor and ice particles have been observed by the Cassini spacecraft when flying over the tiger stripes of Enceladus at the south polar terrain of the satellite. Measurements of the salinity of the plumes (0.5-2%) and the size of the emitted particles, indicate a subsurface liquid ocean beneath the crust of ice. The vent temperature of the plumes were measured to be relatively high (170-210 K) compared to the cold surface temperature of Enceladus. The mass flow, velocity and particle size show a wide range of possible values, whereas the solid fraction is estimated between 7 - 20 %. Here we demonstrate a fluid dynamics model that accounts for nucleation, particle growth, wall accretion and sublimation and the viscous interaction with the walls. The channels behave as converging-diverging nozzles, which form supersonic plumes due to a pressure difference between the reservoir and the exosphere. The geometry of the channel, the reservoir conditions and wall interactions are studied to reproduce the characteristics of the plumes observed by Cassini. From a parameter study performed on the crevasse properties we find that the velocity and the solid fraction increase with the expansion ratio, whereas the exit temperature is inversely related to the expansion ratio. The interactions with the walls reduce the mass flow in the channel and start condensation earlier in the channel, this increases the solid fraction, particle size and velocity and reduces the exit temperature. The particle size is dominantly dependent on the length of the channel, large particles (75 *10(-6)m) must originate from up to a kilometre below the surface, while smaller particles (3 *10(-6) m) can originate from 150 m below the surface. The channel requires an expansion ratio close to Dthroat/Dexit = 4, to generate the observed exit temperature <210 K with velocities up to 950 m/s and a solid fraction ranging from 0.07-0.2. The minimum width of the channel needs to be in the order of meters to withhold the channel from freezing up within days and a liquid reservoir at a temperature near triple point is most probable to agree with the observed exit temperature and velocity.