Scientists have solved a long-standing mystery regarding the chemical diversity of ice particles erupting from Saturn’s moon Enceladus. An international team, including researchers from the Earth-Life Science Institute (ELSI) at the Institute of Science Tokyo, demonstrated that the variation in salt compositions within these grains is likely caused by slow freezing processes deep within the moon’s crust, rather than implying multiple distinct ocean sources.
Between 2004 and 2017, NASA’s Cassini spacecraft collected data on Saturn’s E-ring, which is continuously fed by material spewing from Enceladus’s south pole. Prof Frank Postberg of Freie Universität Berlin led an analysis of 961 mass spectra from salt-rich “Type 3” particles. The study revealed that while the grains originated from the same global subsurface ocean, their chemical makeups varied drastically. Some were rich in sodium chloride, while others contained higher concentrations of carbonates, phosphates, or potassium chloride. Notably, chloride and carbonate rarely appeared together in the same sodium-rich particle.
To understand this discrepancy, Professor Yasuhito Sekine and his colleagues recreated Enceladus’s ocean conditions in the laboratory. They froze droplets containing major ocean salts under varying cooling rates and analyzed how elements distributed themselves as the water solidified. The results indicated that freezing speed is a critical factor. In droplets approximately 200 micrometers across, slow freezing—defined as roughly 10 Kelvin per minute or slower—caused salts to separate into distinct regions. Conversely, rapid freezing kept chemical ingredients evenly mixed.
“What surprised us was that the diversity seen by Cassini could emerge from droplets originating from essentially the same ocean water,” Sekine said. “Our experiments show that when relatively large ocean droplets freeze slowly, different salts can separate within them. If those frozen droplets are later broken apart, they can produce much smaller ice grains, each with very different chemical compositions.”
These findings suggest a more complex trajectory for ocean spray than previously assumed. Earlier models posited that seawater spray froze quickly and escaped rapidly into space. The new data points to a scenario where droplets travel slowly through a labyrinthine network of fractures in the icy crust. During this prolonged journey, they gradually freeze, allowing salts to segregate. Closer to the surface, faster-moving gas causes these frozen droplets to collide with channel walls at high speeds, shattering them into tiny fragments. Each fragment retains the unique chemical signature of its specific region within the original droplet.
Postberg noted that this process provides a physical explanation for the Cassini observations: the spacecraft was likely sampling shards of larger frozen droplets, each preserving different components separated during their ascent. This natural separation mechanism may also concentrate organic substances, making key compounds easier to detect for future missions. Furthermore, the trapping of liquid brine pockets during slow freezing could create environments conducive to prebiotic chemistry, offering new insights into the potential habitability of Enceladus’s hidden ocean.
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