The Enceladus Dust Plume from the Cassini Cosmic Dust Analyzer

We analyse data recorded by the Cosmic Dust Analyzer (CDA) on board the Cassini spacecraft that were obtained during traversals of the Enceladus dust plume. The focus of our work are profiles of relative abundances of grains of different compositional types derived from mass spectra recorded with the Dust Analyzer (DA) subsystem during the Cassini flybys E5 and E17. The profile from E5, corresponding to a steep and fast (17.7 km/s) traversal of the plume, was analyzed in previous work (Postberg et al, 2011). Here we present and include into the analysis a second compositional profile obtained at a very different geometry during flyby E17, with a nearly horizontal traversal of the South Polar Terrain (SPT) at a significantly lower relative velocity (7.5 km/s). Additionally, we employ in our analysis rates of dust detections registered in the plume by the High Rate Detector (HRD) subsystem of CDA at two different Enceladus flybys (E7 and E21). We derive the ranges of grain sizes that were sampled by the two CDA subsystems at these flybys and use the data sets to constrain the parameters of a new dust plume model. That model we construct from a recently developed mathematical description of dust ejection (Ershova and Schmidt, 2021) using the software package DUDI, publicly available at https://github.com/Veyza/dudi. Further constraints we use for our model are published velocities of gas ejection and the positions of gas and dust jets on the SPT. From our model we derive production rates of dust mass for the different compositional types of grains detected by CDA, amounting to a total rate equal to or larger than about 28 kg/s. The contribution of salt-rich dust to the plume was previously believed to be dominant in mass, based on the analysis of E5 flyby data alone (Postberg et al, 2011). However, including both compositional profiles (E5 and E17) in our analysis, we find that the salt-rich dust contribution is only about one percent by mass or less. This finding follows in part from an improved understanding of the masses of grains of various compositional types that implies a generally smaller size for salt rich grains than previously thought. Furthermore, the E17 compositional profile exhibits a dominance of organic enriched grains over the SPT, a region of the plume that was poorly constrained, if at all, by the E5 data. Our new dust plume model can be used to predict numbers and masses for grains of various compositional types that a detector on a future mission will collect during a plume traversal.