Model Insights into the Variability of the Vapor Plume of Enceladus

Variability in the water vapor emanating from the south polar region of Enceladus stems from factors that affect the source rate or the release parameters of the vapor. Understanding of the variability and the factors that influence it provides insight into the mechanism producing the plume. Variability was first reported by Saur et al. (GRL 35, L20105, 2008) and was based on magnetometer data gathered during the first three Enceladus encounters. Later, Smith et al. (JGR 115, A10252, 2010) found variability in the molecular emissions as measured by Cassini INMS (Ion and Neutral Mass Spectrometer). Recently, Hedman et al. 2013 used VIMS observations of the dust to link plume variability to the mean anomaly of Enceladus, indicating that tidal forces may modulate the emissions. The Cassini INMS profiles of volatiles in the plumes of Enceladus show variations from flyby to flyby. Owing to the different geometries of the flybys, temporal and spatial variations are necessarily convolved in the data. We use a model of the plume to disentangle spatial and temporal differences in water densities for the seven Enceladus encounters that provided good viewing geometry for INMS. The model implements a Monte Carlo approach to study the distribution of water molecules in the vapor plume. The code solves the equation of motion for the water molecules following their release from Enceladus given initial position and velocity. The primary input to the model is the velocity distribution, both in angle and magnitude. In addition, initial position can be implemented as a point source, set of point sources, or linear feature. Using a range of initial conditions, we compare INMS tracks through model runs to INMS data to trace structures observed at altitude to the properties of the plume close to the surface of Enceladus. Three encounters in 2011 and 2012, E14, E17, and E18, were nearly collinear, providing an opportunity for our modeling to separate spatial and temporal variability. Moreover, all of these flybys passed through the plume at low altitudes and measured the emissions with the highest resolution to date.