Abstract DGP2026-24

Cassini CDA observes compositional diversity in Enceladus' cryo-volcanic ice grains from slow freezing and fragmentation of large oceanic droplets

Frank Postberg (1), Zenghui Zou (1,2), Yasuhito Sekine (3), Minori Koga (3), Jürgen Schmidt (1), Mark Fox-Powell (4), Fabian Klenner (5,6), Jon K. Hillier (1), Nozair Khawaja (1), Toshihiko Kadono (7), Sascha Kempf (8), Ralf Srama (9)

(1) Institut für Geologische Wissenschaften, Freie Universität Berlin, Berlin, Germany. (2) School of Mathematics and Physics, Qinghai University, Xining, China. (3) Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, Tokyo, Japan. (4) School of Environment, Earth & Ecosystem Sciences, The Open University, Milton-Keynes, UK. (5) Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA. (6) Department of Earth and Planetary Sciences, University of California, Riverside, CA 92521, USA. (7) Department of Basic Sciences, University of Occupational and Environmental Health, Kitakyushu, Japan. (8) Laboratory for Atmospheric and Space Physics, University of Colorado in Boulder, Boulder (CO), USA. (9) Institut für Raumfahrtsysteme, Universität Stuttgart, Stuttgart, Germany

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 (sub-)micron-sized, frozen aerosols from Enceladus’ salty subsurface ocean (Postberg et al. 2009, 2011). Here we present new insights into the plume formation process based on an updated analysis of approximately 1,000 CDA mass spectra of individual salt-rich ice grains from Enceladus, typically ranging from approximately 0.5 to 5 µm in diameter. Analyzing the compositional diversity within the Type 3 group, substantial experimental and theoretical efforts aid in the inference of processes forming Enceladus’ ice plume from the subsurface oceanic water table.

We find at least five basic compositional subtypes dominated by either NaCl, NaHCO3/Na2CO3, Na2HPO4/Na3PO4, NaOH or KCl/KOH. With the exception of hydroxides, these individual salts are rarely found together within a single ice grain. From our freezing experiments, we arrive at consistent boundary conditions and find that salt separation, in agreement with CDA observations, occurs as a consequence of salt mineral precipitation only within droplet sizes above ≈ 10 µm and freezing rates below ≈ 20 K/min.

Supported by thermodynamic calculations and plume modelling, we suggest that oceanic spray droplets originally form with typical size of several 100 µm, containing a solution of the observed salt species. These are entrained in a slow gas flow causing slow freezing and separation of salts. Subsequently they are accelerated to speeds >100 m/s in narrow ice vents where frequent wall collisions lead to much smaller fragments mostly containing a single type of salt, matching CDA observations. The size distribution of ice grains observed in the plume by several Cassini instruments (e.g. Hedman et al. 2009) indeed matches results from high-velocity fragmentation experiments (Hauck et al. 2015, Burke et al. 2023).

Such a complex plume formation process bears several implications for constraining conditions in the ocean by plume observation and sampling. Previous studies have already established that the plume composition is spatially heterogeneous (e.g., Ershova et al. 2024) and that organic material across the ice grain population also varies significantly in composition (Postberg et al. 2018; Khawaja et al. 2019, 2025). Our results further underscore the high level of compositional diversity of ice grains emitted by Enceladus, which future sampling missions must account for when inferring the ocean’s composition and its habitability.