Identification of a novel metastable NaCl polyhydrate formed by rapid freezing of brines relevant to cryovolcanism at icy worlds

Introduction: The transport of material from the internal oceans to the surfaces of icy moons such as Europa and Enceladus can provide information on ocean chemistry and thus potential habitability. Areas that have experienced brine-and-ice volcanism (cryovolcanism) could contain a record of recently exposed ocean material, and identifying such regions will be a major priority for upcoming missions. NaCl on the surface of icy moons is considered endogenic 1, however, the composition of NaCl ices formed under conditions relevant to cryovolcanic eruptions is not well understood, limiting our ability to identify materials emplaced by cryovolcanism. We investigated how the composition of salt-rich ices in the Na-Cl-H2O system varies as a function of salt concentration and freezing rate. Our work reveals the presence of a previously unrecognised metastable NaCl hydrate formed upon freezing of Na+/Cl- solutions at cooling rates relevant to cryovolcanic activity on icy moons. This new NaCl hydrate expands on the recently identified NaCl hydrates formed in high-pressure experiments 2, and together with these reveals a rich phase behavior in the low temperature Na-Cl-H2O system that had been overlooked for over 150 years.Methods: We used neutron and x-ray diffraction (XRD) and differential scanning calorimetry (DSC) to study the compositional diversity of solid phases in Na+/Cl--bearing ice created with varying concentrations and freezing rates.To understand the effects of rapid freezing that might be experienced by fluids in cryovolcanic eruptions, we compared two different freezing rates: flash frozen (FF) samples, frozen within 10s of seconds by exposing fluids directly to liquid nitrogen temperature, and slowly frozen (SF) samples, cooled first to 193 K over multiple hours. The salt concentrations investigated ranged from dilute solutions relevant to probable ocean salt concentrations (0-0.5 M NaCl) 3, to the eutectic concentration (5.2 M), representative of intra ice-shell brines close to complete solidification. Samples were heated from 100 K to room temperature to assess the evolution of their structure and composition with temperature.Results and Discussion: We found that the phase composition of Na+/Cl- bearing ice is dependent on the freezing rate, indicating that compositional properties of salt-rich ices can act as a record of thermal history.Neutron diffraction showed the presence of an unknown crystalline phase in the FF samples only, hereafter referred to as NaCl.xH2O (Figure 1a). Because this phase was only formed by rapid freezing, its presence on icy moons could indicate cryovolcanic regions where brines have frozen rapidly. Upon heating, this unknown phase decomposed irreversibly between approximately 180 and 200 K, producing ice I and hydrohalite (NaCl.2H2O), the known NaCl hydrate formed through freezing at ambient pressure, indicating that it is metastable (Figure 1b). The latent heat released during the metastable transition was quantified by DSC. If this metastable transition occurs at icy moon surface conditions, the presence of NaCl.xH2O would indicate recent emplacement. Furthermore, the release of latent heat could have geological implications. From a detailed analysis of the neutron data, we found that the new NaCl.xH2O is likely to have ~5 water molecules associated with each NaCl molecule, based on the increase in the peak intensity of the water ice Bragg peaks after all the NaCl.xH2O has fully transitioned into hydrohalite and ice I.Figure 1: A) Neutron diffraction patterns of 2 M NaCl ice at 100 K showing the presence of an unknown crystalline phase in the FF sample only. B) Heating neutron diffraction patterns of 2 M NaCl ice from 100 K to 240 K showing the transition from NaCl.xH2O to hydrohalite.Targeted flash-freezing and heating experiments were carried out to generate a sample containing only NaCl.xH2O with Ice I. A high-resolution XRD pattern of this sample was obtained at 100K. Structural refinement of the NaCl.xH2O phase is current ongoing.Figure 2: A high resolution XRD pattern of Ice I with NaCl.xH2O (black) and hydrohalite (red) at 100 K Conclusion: Rapid freezing of Na+/Cl- fluid under icy moon relevant cryovolcanic freezing rates has produced a newly identified NaCl hydrate with a hydration state of approximately 5H2O. This phase upon heating decomposes irreversibly into ice I and hydrohalite between 180-200 K. The identification of this phase on icy moons could indicate regions of recent cryovolcanic activity where ocean material was rapidly frozen. Seeking evidence for this hydrate in surface materials should therefore be a priority for the Europa Clipper and JUICE missions.References: 1 Trumbo, S. K. et al. (2019) Sci. Adv. 5, 71 2 Journaux et al., (2023), PNAS, 120, 9 3 Postberg, F et al. (2009) Nature. 459