1,1,5,6- 1Freie Universität Berlin, Institute of Geological Sciences, Planetary Science and Remote Sensing, Berlin, Germany (frank.postberg@fu-berlin.de)
- 2School of Mathematics and Physics, Qinghai University, Xining, China
- 3Tokyo Institute of Technology, Earth Life Science Institute (ELSI), Japan
- 4The Open University, Milton Keynes, UK
- 5University of Washington, Department of Earth and Space Sciences, Seattle, USA
- 6University of Stuttgart, Institute of Space Systems, Stuttgart, Germany
Salt-rich ice particles constitute a major compositional group observed by Cassini’s Cosmic Dust Analyzer (CDA) in Enceladus' plume and Saturn’s E-ring. Previously, these particles were believed to be frozen aerosolized droplets of relatively homogeneous composition, formed through bubble bursting at the water table of Enceladus' salty subsurface ocean (Postberg et al. 2009, 2011). Here, we present an updated analysis based on approximately 1000 CDA mass spectra of individual salt-rich ice grains from Enceladus, typically ranging from 0.2 to 2 µm in size, focusing on the compositional diversity within this group.
We find at least five basic compositional subtypes, each usually dominated by one of the following salts: NaCl, NaHCO₃/Na₂CO₃, Na₂HPO₄/Na₃PO₄, NaOH, KCl and KOH. With the exception of the hydroxides, these individual salts are rarely found together within a single ice grain. We carried out freezing experiments on water droplets containing mixed salts accompanied thermodynamical modelling of the freezing process. We arrive at consistent boundary conditions and find that salt separation in agreement with CDA observations, occurs only under very specific conditions. From that, we infer that the freezing process of Enceladus ice grains is much more complex than the initially reported flash freezing of water droplets (e.g., Postberg et al. 2009). By combining CDA observations with experimental data, we now gain unprecedented insights into the physical and chemical conditions above the subsurface ocean within Enceladus’ ice vents, where ice grains freeze from a liquid ocean spray.
Previous studies have established that the plume composition is spatially heterogeneous (Ershova et al. 2024) and that organic material in different ice grains greatly varies in composition (Postberg et al. 2018; Khawaja et al. 2019). Our results further underscore the high level of compositional diversity of Enceladus' emitted ice grains. This reflects a range of processes shaping the plume, highlighting the need for future space missions to account for the plumes compositional diversity when analyzing plume material to assess the ocean’s composition and its habitability.
References:
- Postberg, S. Kempf, J. Schmidt et al., Nature 459, 2009
- Postberg, J. Schmidt, J.K. Hillier et al., Nature 474, 2011
- Ershova, J. Schmidt, F. Postberg et al., A & A 689, 2024
- Postberg. N. Khawaja, B. Abel et al., Nature 558, 2018
- Khawaja, F. Postberg, J.K. Hillier et al., MNRAS 489, 2019
How to cite: Postberg, F., Zou, Z., Sekine, Y., Koga, M., Schmidt, J., Fox-Powell, M., Klenner, F., Hillier, J. K., Khawaja, N., and Srama, R.: Salt diversity observed in Enceladus' ice grains suggest a complex plume formation process from the subsurface ocean, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–12 Sep 2025, EPSC-DPS2025-618, https://doi.org/10.5194/epsc-dps2025-618, 2025.
