Friday, October 30, 2015

Enceladus' Plumes are 1/6th the Mass Previously Thought

Aggregate particles in the plumes of Enceladus


Gao et al


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 and Ewald (Ingersoll, A.P., Ewald, S.P. [2011]. Icarus 216(2), 492–506), 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 ± 42) × 103 kg, consistent with the results of Ingersoll and Ewald (Ingersoll, A.P., Ewald, S.P. [2011]. Icarus 216(2), 492–506). The low particulate mass plumes have masses of (25 ± 4) × 103 kg, leading to a solid to vapor mass ratio of 0.07 ± 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.

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