  Water vapor produces a series of diagnostic emission lines in the near
infrared between 2.60 and 2.75 microns. The Visual and Infrared Mapping
Spectrometer (VIMS) onboard the Cassini spacecraft detected this emission
signal from Enceladus' plume, and so VIMS observations provide information
about the variability of the plume's water vapor content. Using a data set of
249 spectral cubes with relatively high signal-to-noise ratios, we confirmed
the strength of this water-vapor emission feature corresponds to a
line-of-sight column density of order 10^20 molecules/m^2, which is consistent
with previous measurements from Cassini's Ultraviolet Imaging Spectrograph
(UVIS). Comparing observations made at different times indicates that the
water-vapor flux is unlikely to vary systematically with Enceladus' orbital
phase, unlike the particle flux, which does vary with orbital phase. However,
variations in the column density on longer and shorter timescales cannot be
ruled out and merit further investigation.
