Search

Negatively and positively charged nano-sized ice grains were detected in the Enceladus plume by the Cassini Plasma Spectrometer (CAPS). However, no data for uncharged grains, and thus for the total amount of dust, are available. In this paper we estimate this population of uncharged grains based on a model of stochastic charging in thermodynamic equilibrium and on the assumption of quasi-neutrality in the plasma-dust system. This estimation is improved upon by combining simulations of the dust component of the plume and simulations for the plasma environment into one self-consistent model. Calibration of this model with CAPS data provides a total dust production rate of about 12 kg s−1, including larger dust grains up to a few microns in size. We find that the fraction of charged grains dominates over that of the uncharged grains. Moreover, our model reproduces densities of both negatively and positively charged nanograins measured by Cassini CAPS. In Enceladus׳ plume ion densities up to ~ 10 4 cm − 3 are required by the self-consistent model, resulting in an electron depletion of about 50% in the plasma, because electrons are attached to the negatively charged nanograins. These ion densities correspond to effective ionization rates of about 10 − 7 s − 1 , which are about two orders of magnitude higher than expected.

The structure of Enceladus׳ south polar plume of charged dust is studied by simulations of the dust grain dynamics. The model considers the Lorentz force and charging of the grains by the plasma environment within the plume. Simulated dust plumes are investigated by applying 10 selected sets of dust parameters that include variations of the grain production rate, the slope of the grain size distribution and the start conditions (velocity, direction) of the grains. The modeled dust plume profiles are in good agreement with nanograin data of Cassini Plasma Spectrometer (CAPS). Major results are (1) due to the local plasma environment the nanograins are accelerated by the Lorentz force and form a structured tail; (2) due to the finite charging time the peak dust charge density is located about 0.3 – 0.6 r E below Enceladus׳ south pole; (3) nanograins smaller than 10 nm are more than 99% of the produced dust; (4) CAPS data are best matched if the nanograins are launched with high, collimated start velocities; (5) the grain charging time is crucially affected by inhomogeneities in the local plasma environment.