Revealing the Neutral Composition of Enceladus’ Plume

Saturn’s moon Enceladus ejects a substantial plume of icy material from surface fractures near its south pole. The Cassini spacecraft’s Ion and Neutral Mass Spectrometer (INMS) has been instrumental in determining the neutral composition of Enceladus’ plume, which is dominated by H2O, CO2, CH4, and includes a host of complex organic species. INMS observations in 2008 provided the positive identification of NH3 and the likely presence of 40Ar, hinting at the presence of liquid water reservoirs below the moon’s ice shell. Encounters with Enceladus in the fall of 2009 have brought the Cassini spacecraft substantially closer to the plume sources than previous flybys, with trajectories optimized for measurements of neutral composition. Results from the latest Cassini-INMS data sets are presented here to highlight and further understand the complex neutral composition of Enceladus’ plume.