Aggregate Particles in the Plumes of Enceladus

TL;DR

This study revisits the particulate mass estimates of Enceladus's plumes by modeling particles as spheres and aggregates, revealing possible lower mass plumes and implications for particle formation mechanisms.

Contribution

It introduces a model combining spherical particles and aggregates to better estimate plume mass and composition, expanding understanding of particle formation in Enceladus's plumes.

Findings

High mass plumes have approximately 166,000 kg of particles.

Low mass plumes have approximately 25,000 kg of particles.

Aggregates could form via Brownian coagulation before ejection.

Abstract

Estimates of the total particulate mass of the plumes of Enceladus are important to constrain theories of particle formation and transport at the surface and interior of the satellite. We revisit the calculations of Ingersoll & Ewald (2011), who estimated the particulate mass of the Enceladus plumes from strongly forward scattered light in Cassini ISS images. We model the plume as a combination of spherical particles and irregular aggregates resulting from the coagulation of spherical monomers, the latter of which allows for plumes of lower particulate mass. Though a continuum of solutions are permitted by the model, the best fits to the ISS data consist either of low mass plumes composed entirely of small aggregates or high mass plumes composed of mostly spheres. The high particulate mass plumes have total particulate masses of (166 $\pm$ 42) $\times$ 10$^3$ kg, consistent with the results of Ingersoll & Ewald (2011). The low particulate mass plumes have masses of (25 $\pm$ 4) $\times$ 10$^3$ kg, leading to a solid to vapor mass ratio of 0.07 $\pm$ 0.01 for the plume. If indeed the plumes are made of such aggregates, then a vapor-based origin for the plume particles cannot be ruled out. Finally, we show that the residence time of the monomers inside the plume vents is sufficiently long for Brownian coagulation to form the aggregates before they are ejected to space.