The active southpole region on Enceladus is probably the most important discovery in planetology of the last ten years because most models associate this phenomenon with a subsurface reservoir of liquid water (Porco et al., 2006; Matson et al., 2007; Nimmo and Pappalardo, 2006; Nimmo et al., 2007) – the prerequisite for life. On the other hand demonstrated this discovery how limited our knowledge of the geophysics of Enceladus actually is. Most of the Cassini findings raised new questions rather than providing answers. In particular the mechanism providing the energy to maintaining the “hot spot” at the southpole is not known.

Formation of the Dust Jets inside the Surface Fractures

Obviously, knowledge of the physical conditions inside the vents would allow us to verify the existence of subsurface liquid water reservoirs as well as would provide precious constraints for the energy generation in the interior of the moon. Schmidt et al. (2008) noted first that the dynamical properties of the ejected ice particles must be due the dynamics of the dust growth inside the vents. Thus, a model capable to reproduce both remote sensing and in-situ data of the dust plumes provides a mean to look inside the vents.

Schmidt et al. (2008) proposed that the speed distribution of the ejected dust grains required to reproduce the observed plume brightness and the dust number density is maintained by frequent dissipative collisions of the growing ice grains with the walls of the vents. In our model the the cracks in the moon’s ice shell are approximated as random channels of variable cross section through which water vapour and entrained ice particles escape from the hot interior. The gas production is driven by a subsurface heat reservoir, located below the south pole of the moon. The ice grains are assumed to condensate below the surface in the vapour. Because the cracks in the ice crust are curved, the streamlines of gas and dust will be dif- ferent. In particular, dust particles will undergo many dissipative collisions with the vent walls before they escape from the interior of the moon. After a collision they are re-accelerated by drag forces due to friction with the ambient gas. The speed distribution of the escaping grains is established mostly near the nozzle of the vents where the grain size distribution is only weakly affected by mantle growth.

Vertical E ring Structure

The plot compares model calculations of the Enceladus jets emerging into the E ring with CDA measurements during a steep crossing of Saturn’s ring plane at a radial distance to Saturn of 3.93RS on 2005-177 UTC. The detector was sensitive to grains larger than 0.9µm and the vertical resolution was 190 km (Kempf et al., 2008). The spikes in the vertical ring profile correspond with some of the strongest Enceladus dust jets identified by Spitale & Porco (2008).
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