Time variability of the Enceladus plumes: Orbital periods, decadal periods, and aperiodic change

The Enceladus plumes vary on a number of timescales. Tidal stresses as Enceladus revolves in its eccentric orbit lead to a periodic diurnal variation in the mass and velocity of solid particles in the plume. Tidal stresses associated with an orbital resonance with Dione lead to a periodic decadal variation. Aperiodic variations occur on time scales of months, and may be due to ice buildup and flow of the walls of the fissures that connect the ocean to the surface. We document these variations using all the relevant data taken by the ISS instrument from 2005 to 2017. Key questions now include how a 5% peak-to-peak variation in orbital eccentricity, which itself is only 0.0045, could lead to a 2-fold decadal variation in plume properties. Another question is how the plumes stay open if ice builds up every month and clogs the vents. Other questions include why the solid particles exit the vents several times slower than the gas, and why the speeds vary inversely with the mass of the plumes. The Cassini data are in, but the modeling has just begun.