Supercooling and Glass Formation upon Freezing of Enceladus-relevant Salt Solutions

Analysis of ice grains emitted from Saturn’s moon Enceladus revealed that the moon’s subsurface ocean represents a potentially habitable place in the Solar System [1-4].

The emitted ice grains could be crystalline, glassy, or a mixture of both [5,6]. These phase states of the grains are ultimately linked to their formation, i.e. liquid-solid phase transitions. Recent work indicates that emitted plume material does not directly reflect ocean composition [7]. However, even a small fraction of glass within the grains may be favorable for the preservation of organics or even cells [8,9], potentially present in Enceladus’s ocean.

Supercooling, vitrification (glass formation) and heat capacities of aqueous solutions can be measured with or derived from Differential Scanning Calorimetry (DSC). This technique was recently used to study Mars-relevant brines [10]. For Enceladus-relevant salt systems (described below), liquid-solid phase transitions remain an open area of research with limited thermodynamic data.

Here, we present results from DSC experiments with aliquots of aqueous solutions of NaCl, KCl, Na₂CO₃, NaHCO₃, NH₄OH, Na₂HPO₄, K₂HPO₄, as well as mixtures thereof. Measured salt concentrations covered the range of estimated concentrations of these compounds in Enceladus’s ocean [3,7,11]. We analyzed samples (volumes from 4 to 40 μL) over a wide range of cooling rates, from as low as 10 K/min up to ~1000 K/min via drop-quenching into liquid nitrogen (flash freezing). We then modeled the freezing process of these solutions and associated mineral formation using the aqueous chemistry package PHREEQC and compared the modeling results with our DSC experiments.

Our preliminary results show that at least 60 K supercooling is possible to occur during freezing of salty ice grains from Enceladus. Between 0.5 – 15 percent of the grain’s total volume form a glassy state, with salt-rich grains containing more glass than salt-poor grains. Flash freezing leads to a significantly higher degree of vitrification and lower glass transition temperatures (T_g) than other cooling rates.

Our work is an important step toward understanding the formation and structure of ice grains from Enceladus as well as their capability for cryopreservation of organics and cells. Thermodynamic and kinetic data derived from our experimental results, such as heat capacities and T_g, help inform future models. Our results are also relevant to Jupiter’s moon Europa where a potential plume might also be sourced from the moon’s underlying water ocean.

 

References

[1] Postberg et al. (2018) Nature 558, 564–568.

[2] Khawaja et al. (2019) Mon. Not. R. Astron. Soc. 489, 5231–5243.

[3] Postberg et al. (2023) Nature 618, 489–493.

[4] Hsu et al. (2015) Nature 519, 1098–1101.

[5] Newman et al. (2008) Icarus 193, 397–406.

[6] Fox-Powell & Cousins (2021) J. Geophys. Res.: Planets 126, e2020JE006628.

[7] Fifer et al. (2022) Planet Sci. J. 3, 191.

[8] Fahy & Wowk (2015) in Cryopreservation and freeze-drying protocols, pp.21–82.

[9] Berejnov et al. (2006) J. Appl. Cryst. 39, 7848–7939.

[10] Bravenec & Catling (2023) ACS Earth Space Chem. 7, 1433–1445.

[11] Postberg et al. (2009) Nature 459, 1098–1101.