  The Cassini spacecraft's Grand Finale flybys through Saturn's ionosphere
provided unprecedented insight into the composition and dynamics of the gas
giant's upper atmosphere and a novel and complex spacecraft-plasma interaction.
In this article, we further study Cassini's interaction with Saturn's
ionosphere using three dimensional Particle-in-Cell simulations. We focus on
understanding how electrons and ions, emitted from spacecraft surfaces due to
the high-velocity impact of atmospheric water molecules, could have affected
the spacecraft potential and low-energy plasma measurements. The simulations
show emitted electrons extend upstream along the magnetic field and, for
sufficiently high emission rates, charge the spacecraft to positive potentials.
The lack of accurate emission rates and characteristics, however, makes
differentiation between the prominence of secondary electron emission and
ionospheric charged dust populations, which induce similar charging effects,
difficult for Cassini. These results provide further context for Cassini's
final measurements and highlight the need for future laboratory studies to
support high-velocity flyby missions through planetary and cometary
ionospheres.
