Cassini CDA observes compositional segregation of Enceladus' ice grains from slow freezing and fragmentation of...

Salt-rich ice particles, termed Type 3, are a major compositional group within Enceladus' plume and Saturn's E ring and are thought to represent frozen micrometer-sized aerosolized droplets of Enceladus' salty subsurface ocean. Here, we present an analysis on the basis of ≈1000 mass spectra of individual Type 3 E ring ice grains recorded by Cassini's Cosmic Dust Analyzer (CDA), revealing a large compositional diversity among these grains. We find at least five basic compositional subtypes dominated by either sodium chloride (NaCl), sodium bicarbonate (NaHCO₃)/sodium carbonate (Na₂CO₃), disodium phosphate (Na₂HPO₄)/trisodium phosphate (Na₃PO₄), sodium hydroxide (NaOH), potassium chloride (KCl), or potassium hydroxide (KOH). By conducting experiments and thermodynamic calculations, we show that salt separation in agreement with CDA observations occurs as a consequence of salt mineral precipitation only within droplet sizes >10 micrometers and freezing rates below 20 kelvins per minute. We suggest that oceanic spray forms with an average size of 100s of micrometers entrained in a slow gas flow, causing slow freezing and separation of salts. Subsequently, they are accelerated to speeds >100 meters per second in narrow ice vents where frequent wall collisions lead to much smaller fragments mostly containing a single type of salt, matching CDA observations. The gradual cooling of ocean droplets in combination with wall collisions determines the final size distribution and provides an efficient chemical separation of both inorganic and organic oceanic constituents into differently composed ice grains ejected into Enceladus' plume.