Constraining the Enceladus Plume and Understanding Its Physics via Numerical Simulation from Underground Source to Infinity

Abstract:

The Enceladus ice/vapor plume not only accounts for the various features observed in the Saturnian system, such as the E-ring, the narrow neutral H₂O torus, and Enceladus’ own bright albedo, but also raises exciting new possibilities, including the existence of liquid water on Enceladus. Therefore, understanding the plume and its physics is important. Here we assume that the plume arises from flow expansion within multiple narrow subsurface cracks connected to reservoirs of liquid water underground, and simulate this expanding flow from the underground reservoir out to several Enceladus radii where Cassini data are available for comparison. The direct simulation Monte Carlo (DSMC) method is used to simulate the subsurface and near-field collisional regions and a free-molecular model is used to propagate the plume out into the far-field. We include the following physical processes in our simulations: the flow interaction with the crack walls, grain condensation from the vapor phase, non-equilibrium effects (e.g. freezing of molecular internal energy modes), the interaction between the vapor and the ice grains, the gravitational fields of Enceladus and Saturn, and Coriolis and centrifugal forces (due to motion in non-inertial reference frame). The end result is a plume model that includes the relevant physics of the flow from the underground source out to where Cassini measurements are taken. We have made certain assumptions about the channel geometry and reservoir conditions. The model is constrained using various available Cassini data (particularly those of INMS, CDA and UVIS) to understand the plume physics as well as estimate the vapor and grain production rates and its temporal variability.