The E-ring in the vicinity of Enceladus: II. Probing the moonʹs interior—The composition of E-ring particles

The population of Saturnʹs outermost tenuous E-ring is dominated by tiny water ice particles. Active volcanism on the moon Enceladus, embedded in the E-ring, has since late 2005 been known to be a major source of particles replenishing the ring. Therefore particles in the vicinity of Enceladus may provide crucial information about the dynamical and chemical processes occurring below the moonʹs icy surface. Here we present a statistical evaluation of more than 2000 impact ionisation mass spectra of Saturnʹs E-ring particles, with sizes predominantly below 1 μm, detected by the Cosmic Dust Analyser onboard the Cassini spacecraft. We focus on the identification of non-water features in spectra otherwise dominated by water ice signatures. Here we specify the categorisation of two different spectrum types, which probably represent two particle populations. Type I spectra imply pure water ice particles, whereas in Type II spectra organic compounds and/or silicate minerals are identified as impurities within the icy particles. This finding supports the hypothesis of a dynamic interaction of Enceladusʹ rocky core with liquid water.