Size-Dependent Composition of Ice Grains Relevant to Salt-Rich Particles in the Plumes of Enceladus

Background

The plumes of Saturn’s moon Enceladus emit water vapour and ice grains from cracks in the ice shell, fed by vents that transport subsurface ocean material upwards to the surface [1]. These ice grains have varied compositions, but the salt-rich population (“Type III”) are interpreted to originate as dispersed ocean spray droplets representative of the subsurface ocean composition [2]. It is assumed that Type III grains can therefore be used as a tool to interpret the chemistry and habitability of the otherwise inaccessible subsurface.  However, from the subsurface ocean to space, ocean fluid ascends through an extreme temperature and pressure gradient, and it is not yet understood how this influences the composition of ejected ice grains. There have also been observations of size-dependent, compositional stratification in the plumes, wherein larger grains fall back to the surface and smaller grains are ejected further from the plume source [3]. This implies that different droplet sizes experience a different cooling rate when exposed to the same plume temperatures. Since cooling rate influences composition [4], there is a likely compositional difference between grain sizes that deposit on the surface and those that escape. Therefore, high-altitude plume sampling by spacecraft and observations of surface fallout may provide different compositional information about the subsurface ocean material.

The focus of this work is to understand the relationship between droplet size and composition, and specifically how salts such as sodium chloride, carbonates, and phosphates, which are believed to be present in the subsurface ocean [2, 5], behave during freezing. By quantifying the solid phase composition of frozen droplets of simulated Enceladus ocean composition, we can establish whether compositional differences exist across a range of grain sizes.

 

Methods

We designed a fluid simulant representative of Enceladus’ ocean derived from salt constituents confirmed by Cassini measurements of the plume [5, 6], and with a pH of 10 (a midpoint encompassing estimates [7, 8]). Experimental simulations of ice grain formation allowed us to assess how grain size affected the composition and spatial distribution of salts within a droplet. Through quenching aliquots of the fluid across a wide range of droplet volumes (≤ 1× 10 ^─ 4 µL, 0.5 µL, 5 µL)  in liquid nitrogen (Figure 1), fluids undergo flash-freezing (>10 K s^-1) designed to simulate freezing rates relevant to the plume-forming regions on Enceladus. Utilising scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD), we studied the elemental composition, mineralogy and physical partitioning of solid phases within ice grains and how this varies across grains of different volumes. Introducing a range of droplet sizes into the same thermal environment provides variation in bulk cooling rate [10], applicable to grains of different size fractions in the Enceladus plumes.

Figure 1 – Optical microscopy image of flash-frozen ice grains mounted onto quartz slide (10-100 µm size).

 

Results

Preliminary SEM-EDS analysis of the 0.5 and 5 µL grains detected the formation of sodium chloride, sodium carbonate and sodium phosphate salts. The micro-scale arrangement of these salts differed - sodium carbonate/bicarbonate manifested as relatively flat sheets of rounded, globular nodules of sub-micron size, whereas sodium chloride salt fibres are isolated in striated, parallel ridges across both droplet sizes (Figure 2). At the 100 µm scale, all salts appear embedded together in a matrix across the droplet sizes, but heterogeneities in their distribution become visible under higher magnification. Textural differences between droplet sizes were expressed, with vesicle-like pore spaces and compositional heterogeneity between salts visible at the 10 µm scale in the 5 µL grains. By contrast, the 0.5 µL grains consist of a finer microstructure, with morphological differences from the partitioning of various salts displayed at the 5 µm scale. Compositional heterogeneities are expressed at different scales dependent on grain size, and whether such differences are visible at mineralogical level will be studied in follow-up analyses.

Figure 2 - SEM image capturing the microstructure of NaCl features within a flash-frozen 0.5 µL ice grain.

 

Next steps

The focus of ongoing analysis will be the solid phase composition of the smallest grain size (≤ 1× 10 ^─ 4 µL) and quantifying the phase abundance in all ice grains using cryo-Raman and XRD. Additionally, future work will assess how composition and grain microstructure is affected by the presence and absence of organics that preferentially form with specific solid phases and may be present in plume material [11]. These findings enable us to predict potential compositional differences between grain sizes, and to understand how organics are incorporated, between the largest grains that fall back to the surface and the smaller grains that achieve escape velocity.

 

References

[1] Porco, C. C. et al. (2006). Science. 311(5766), 1393-1401; [2] Postberg, F. et al. (2009). Nature 459, 1-4; [3] Postberg, F. et al. (2011). Nature 474, 620-622; [4] Fox‐Powell, M. G. (2021). J. Geophys. Res. Planets 126; [5] Postberg, F. et al. (2023). Nature, 618(7965), 489-493; [6] Postberg, F. et al. (2009). Nature, 459(7250), 1098-1101; [7] Zolotov, M. Y. (2012). Icarus 220, 713–729; [8] Glein, C. R. et al. (2018). The Geochemistry of Enceladus: Composition and Controls. Enceladus and the Icy Moons of Saturn, 39; [9] Postberg, F. et al. (2018). Nature 558, 564; [10] Adda-Bedia, M. et al. (2016). Langmuir 32.17, 4179-4188; [11] Khawaja, N. et al. (2019). MNRAS 489.4, 5231-5243.