In 2005, the Cassini spacecraft a large plume of water vapor emitted from the south pole region of Saturn’s moon, Enceladus. This plume adds approximately 100 kg/s of water molecules and ice particles to the Saturn system and produces an interacts with Saturn’s magnetosphere similar to the Io-Jupiter interaction.
A key difference between Enceladus and Io is the role of charged dust in the region around the satellite. Observations by the Cassini plasma wave, Langmuir probe and plasma spectrometer show that the electron density is strongly depleted in the plume itself. In contrast, the ion density is slightly enhanced. The balance of the electrons, required by charge neutrality, are on the surface of the ice particles embedded in the plume.
Interpretation of the Langmuir probe data suggests that over 99.9% of the electrons are on these dust particles. At the same time, the Cassini Plasma Spectrometer (CAPS) directly observed charged dust particles of order one nanometer in size (~100 water molecules). These nanograins are both positively and negatively charged. The charge state of these particles provides an indicator of the free electron abundance. In contrast to the Langmuir probe analysis, this shows that the free electron abundance is most likely 5-10% rather than under 0.1%.