SPECTRAL OBSERVATIONS OF THE ENCELADUS PLUME WITH CASSINI-VIMS

On 2005 November 27 (day 331), the Visual and Infrared Mapping Spectrometer instrument onboard the Cassini spacecraft obtained high signal-to-noise, spatially resolved measurements of Enceladus' particle plume. These data are processed to obtain spectra of the plume at a range of altitudes between 50 and 300 km from the surface. These spectra show that the particulate component of the plume consists primarily of fine-grained water ice. The spectral data are used to derive profiles of particle densities versus height, which are in turn converted into measurements of the velocity distribution of particles launched from the surface between 80 and 160 m s-1 (that is, between one-third and two-thirds of the escape speed). These calculations indicate that particles with radii of 1 μm are approximately equally likely to have launch speeds anywhere between 80 and 160 m s-1, while particles with radii of 2 and 3 μm have progressively steeper velocity distributions. These findings should constrain models of particle production and acceleration within Enceladus.