Enceladus' plume consists mainly of a mixture of water vapor and solid ice
particles that may originate from a subsurface ocean. The physical processes
underlying Enceladus' plume particle dynamics are still being debated, and
quantifying the particles' size distribution and launch velocities can help
constrain these processes. Cassini's Visual and Infrared Mapping Spectrometer
(VIMS) observed the Enceladus plume over a wavelength range of 0.9 micron to
5.0 microns for a significant fraction of Enceladus' orbital period on three
dates in the summer of 2017. We find that the relative brightness of the plume
on these different dates varies with wavelength, implying that the particle
size distribution in the plume changes over time. These observations also
enable us to study how the particles' launch velocities vary with time and
observed wavelength. We find that the typical launch velocity of particles
remains between 140 m/s and 148 m/s at wavelengths between 1.2 microns and 3.7
microns. This may not be consistent with prior models where particles are only
accelerated by interactions with the vent walls and gas, and could imply that
mutual particle collisions close to the vent are more important than previously
recognized.Comment: 13 pages, 8 figures, accepted for publication in PS