Presentation #206.01 in the session The Mist: Outgassing on Enceladus.
The plume of ice particles and water vapor ejected from the south polar region of Saturn’s moon, Enceladus, contains information about the moon’s geologic activity and its variability over time. Variations in Enceladus’ particle plume activity have been observed by Cassini in visible and near-infrared wavelengths (e.g. Hedman et al. 2013 Nature, Nimmo et al. 2014 ApJ, Ingersoll et al. 2020 Icarus). However, the plume’s water vapor variations have been difficult to quantify because the most sensitive measurements of the vapor flux come from relatively infrequent observations like occultations and close flybys (e.g. Hansen et al. 2020 Icarus, Teolis et al. 2017 Astrobiology). By modeling the 2.85-micron water-ice absorption signature, we have found that plume spectra obtained by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) instrument show evidence of the water-vapor emission feature peaked around 2.6-2.7 microns. Analysis of the strength of this possible water-vapor emission feature yields estimates of the column density of order 1020 m-2 which is consistent with measurements from Ultraviolet Imaging Spectrograph (UVIS) studies (Hansen et al. 2011 GRL). We are currently quantifying this potential signal by mining VIMS observations taken over a wide range of orbital phases. This analysis should help us constrain the temporal variations in Enceladus’ water vapor plume.