The icy moon Enceladus is among the most dynamic objects in the Saturn system. Observations by NASA’s Cassini spacecraft revealed a plume of nearly pure water erupting from fissures in the moon’s south polar region. The resulting ice particles form one of Saturn’s largest rings, the E ring. Ionization of particles in the plume also provides most of the plasma in Saturn’s magnetosphere.
Cassini measurements show that the activity and brightness of the plume are variable, peaking when Enceladus is farthest from Saturn (apoapsis) and reaching a minimum when the moon is nearest its host planet (periapsis). This occurs because tidal stresses on the south pole cause the fissures through which the icy plume particles emanate to expand near apoapsis and to compress near periapsis.
Persoon et al. searched for a similar pattern in the electron density near Enceladus. They analyzed data collected by Cassini’s Radio and Plasma Wave Science instrument during 13 equatorial flybys of the moon when the spacecraft passed through the plume as well as from one flyby when the moon was at periapsis. The researchers determined the magnitude of Enceladus’s position-based electron density enhancement and found a consistent peak in the density after the moon passes apoapsis. This postapoapsis enhancement differs notably from the near-apoapsis peak in plume activity.
The authors suggest that the lag in the electron density enhancement stems from the time needed for the enhanced plume particles to become ionized. However, none of the ionization processes previously observed at Enceladus account for the magnitude and timing of the electron density peak. Previous theory and computer modeling have indicated that interactions between the water plume and Saturn’s existing plasma environment may lead to increased ionization, but these processes may also be too slow to account for the observed density enhancement.
These discrepancies suggest that one or more ionization processes not previously considered may drive the variation. A future mission to Enceladus, advocated by some planetary scientists, could provide an opportunity to resolve this question. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1029/2019JA027768, 2020)
—Morgan Rehnberg, Science Writer