Supercooling, Glass Formation, and Mineral Assemblages upon Freezing of Salty Ice Grains from Enceladus's Ocean

The analysis of micrometer-sized ice grains emitted into space by Saturn's moon Enceladus suggests that the moon's subsurface ocean may be habitable. However, the formation conditions of these ice grains are largely unknown. Upon cooling, ocean droplets may supercool and then form a crystalline or glassy state, or a mixture of both. To investigate the processes of supercooling and glass formation in Enceladus's ice grains, we performed differential scanning calorimetry experiments with Enceladus-relevant salt mixtures at cooling rates ranging from 5 K minute$^{-1}$ to ~1227 K minute$^{-1}$ and extrapolated our results to faster cooling rates. We modeled the freezing of these solutions and associated mineral assemblages using the thermodynamic chemistry packages PHREEQC and Reaktoro. Our results indicate supercooling of ~25-30 K upon freezing from Enceladus's saline ocean. Freshly formed ice grains should be predominantly crystalline but contain up to 5% glass. Fast cooling rates and high salt concentrations favor the formation of glasses, potentially enabling the preservation of organics and cells, if present. Salts in the grains crystallize in the following sequence: first phosphate, followed by carbonates, and then chlorides. We find that the recently detected phosphates in Enceladus's ice grains are likely Na₂HPO₄:12H₂O. The pH values appear to vary among individual ice grains, depending on the stage of the freezing process, and these values may slightly differ from the pH of the moon's bulk ocean. Our experiments and models are relevant to other icy worlds with salty water reservoirs in their subsurfaces, such as Jupiter's moon Europa or the dwarf planet Ceres.