Potential Spectral Signatures in the Visible Spectra of the Enceladus Plume

A plume of both vapor and small particles constantly erupts from the South Polar Terrain on Saturn’s moon Enceladus. The particle component of this plume consists primarily of particles composed very pure water ice, but Cassini’s in situ investigations of the Enceladus’ plume with the Cosmic Dust Analyzer (CDA) revealed that subsets of these particles are rich in either salts or organics. These direct samplings of the plume provide detailed compositional information at discreet points in time along different sampling trajectories. However, plume activity clearly varies along individual fractures, between the tiger stripes, and location in Enceladus’ orbit. Remote sensing observations of plume composition could therefore complement the in-situ measurements by providing a broader picture of how the plume-particle’s composition varies with time and space. In this context, we describe a spectral signature at 0.5 µm in the visible spectrum observed by both Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) and its Imaging Science Subsystem (ISS). This feature is consistent with the expected spectral signature of tholin-like organics within the ice particles, and therefore should provide a new tool for assessing variations in the plume particle’s composition with time and space.