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

Analysis of ice grains emitted from Saturns moon Enceladus revealed that the moons 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 Enceladuss 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 Enceladuss ocean 3,7,11. We analyzed samples (volumes from 4 to 40 L) 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 grains 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 (Tg) 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 Tg, help inform future models. Our results are also relevant to Jupiters moon Europa where a potential plume might also be sourced from the moons underlying water ocean. References 1 Postberg et al. (2018) Nature 558, 564568. 2 Khawaja et al. (2019) Mon. Not. R. Astron. Soc. 489, 52315243. 3 Postberg et al. (2023) Nature 618, 489493. 4 Hsu et al. (2015) Nature 519, 10981101. 5 Newman et al. (2008) Icarus 193, 397406. 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.2182. 9 Berejnov et al. (2006) J. Appl. Cryst. 39, 78487939. 10 Bravenec Catling (2023) ACS Earth Space Chem. 7, 14331445. 11 Postberg et al. (2009) Nature 459, 10981101.