New constraints on the composition and physical properties of the icy surface on Enceladus’ South Polar Terrain

The observation of active jets of ice grain and vapour emanating from four warm faults (called tiger stripes) at Enceladus’s South Pole was one of the major discoveries of the Cassini-Huygens mission (Porco et al. 2006, Spencer et al. 2006, Waite et al. 2006, Spahn et al. 2006).  Infrared mapping carried out by the Visual and Infrared Imaging Spectrometer (VIMS) on board the Cassini spacecraft provided information on surface composition, but also on the physical state (grain size and degree of crystallinity) near active faults (Brown et al. 2006, Jaumann et al., 2008; Taffin et al., 2012; Filacchione et al., 2016, Combes et al. 2018, Robidel et al. 2020). However, many spectral characteristics were not fully exploited by previous studies. Here, by acquiring laboratory infrared spectra of ice powder analogues, we identified several salt compounds, and potentially CO2 clathrate, at the surface, with a higher concentration along active faults. Our analysis shows that the spectral signatures in the inter-stripe regions are consistent with fresh, cold, fine-grained ice deposits, while ice near the tiger stripes has been thermally processed. The higher concentration of salts observed along the tiger stripes, as well as the main spectral features of water ice, imply significant sublimation and sintering processes in the vicinity of active jet sources. These new results provide essential constraints for identifying the best landing site for a future mission to Enceladus and for anticipating the mechanical properties of the icy regolith.