Compositional profiles of the Enceladus dust plume from CDA measurements at flybys E5 and E17

Plumes of gas and dust are ejected from the south polar terrain of Saturns moon Enceladus [1-4]. Part of the material escapes Enceladus gravity and the dust forms Saturns E ring. The Cosmic Dust Analyzer (CDA) onboard the Cassini spacecraft recorded information on the spatial distribution and composition of dust in the E ring and in the plume. Initially, three compositional types of dust were identified type I dust grains composed of pure water ice, type II containing simple organic compounds, and type III with presence of sodium salts on the percent level. Later type I and the type II particles were found to have the same composition but to differ in size [5,6]. Type II, as well as type III grains, are on average larger than the type I particles. The compositional profiles obtained with CDA during flybys E5 and the E17 at Enceladus both show a rise in fraction of the type II and the type III spectra near the plume sources, but quantitatively the profiles are different. We reproduce the observed profiles utilizing a new model for dust ejection and configuration in the plume [7]. We find that the variations in the observed composition of dust along the spacecraft trajectory in the plume can be explained by a dynamical filtering of the differently sized dust particles. We consider diffuse ejection and collimated jets as two possible modes of ejection. Only salt-rich dust is ejected in the diffuse mode, while the jets contain all the three compositional types. This separation is connected in our model to different plausible processes of dust formation. To constrain the size distributions of the three types of dust particles, we use the data on the size distribution of dust grains of different spectra observed in the E ring in the close vicinity of Enceladus and dust number density profiles recorded during the E7 flyby at Enceladus. Taking into account the E5, the E7, and the E17 profiles altogether, we also derive the total dust production rate of the plume. [1] Hansen et al, 2006, Science, 311[2] Porco et al, Science, 311[3] Spahn et al, Science, 311[4] Spencer et al, Science, 311[5] Khawaja et al, 2019, MNRAS, 489[6] Postberg et al, 2018b, Univ. of Arizona Press[7] Ershova & Schmidt, 2021, A&A, 650.