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Brine systems in Europa's ice shell have been hypothesized as potential habitats that could be more accessible than the sub‐ice ocean. We model the distribution of sub‐millimeter‐scale brine pockets in Europa's ice shell. Through examination of three habitability metrics (water activity, ionic strength, and salinity), we determine that brine pockets are likely not geochemically prohibitive to life as we know it for the chloride and sulfate‐dominated ocean compositions considered here. Brine volume fraction is introduced as a novel habitability metric to serve as a proxy for nutrient transport and recycling—because of its role in governing permeability—and used to define regions where nutrient‐open, nutrient‐closed, and relict habitats are stable. Whereas nutrient‐closed habitats could exist wherever brine is stable, nutrient‐open habitats are confined to meter‐scale regions near the ice‐ocean interface where freezing is occurring. This classification scheme can help guide future life‐detection missions to ocean worlds. Plain Language Summary: Pockets of salty liquid water (brines) could exist in the ice shell of Jupiter's moon Europa. Because brines would be stable over long timescales within these pockets, they represent places that could be inhabited by microorganisms. We model where sub‐millimeter‐scale brine pockets might exist in Europa's ice shell and then study the properties of the brine using a geochemical model. Our results demonstrate that the conditions of the brine do not fall beyond the limits of where life can exist on Earth, indicating that brine pockets may be suitable habitats in Europa's ice shell. We also model the amount of brine in the ice shell to see if organisms inhabiting these brine pockets could have access to ocean‐sourced nutrients via their transport along brine networks in the ice. By considering these factors, we classify potential brine habitats in Europa's ice shell. Key Points: Brine pockets in Europa's ice shell may not be geochemically prohibitive to life as we know it, and as such could be potential habitatsBrine volume fraction, as a proxy for nutrient transport and recycling, may be a critical factor for the habitability of Europa's ice shellIce shell habitats sustained by drainage of brine and recharge by ocean water (brine convection) can exist where the ice shell is freezing [ABSTRACT FROM AUTHOR]

The composition of impurities in ice controls the stability of liquid water and thus the distribution of potential aqueous habitats. We present a framework for modeling the brine volume fraction in impure water ice as a function of temperature and bulk ice salinity, inspired by models originally developed for sea ice. We applied this framework to examine the distribution of brine within the thermally conductive layer of Europa's ice shell, considering binary (NaCl and MgSO4) and multi‐ion "analog" (Cl‐dominated and SO4‐dominated) endmember impurity compositions. We found the vertical extent of brine in a conductive ice layer, expressed as a fraction of the total layer thickness, to be <12% for NaCl, <2% for MgSO4, and <18% for both the analog endmember impurity compositions, suggesting that the depth where brine is stable in an ice shell is more sensitive to composition when only two ionic species are present. For the same temperature and bulk ice salinity, the brine volume fraction is higher in a Cl‐dominated ice shell than a SO4‐dominated ice shell. Pressure, governed by the ice thickness, was found to have only a minor effect on the vertical extent of brine within an ice shell, relative to temperature and bulk ice salinity. The minimum stable bulk ice shell salinity formed through freezing of an ocean was found to be insensitive to composition and ultimately governed by the magnitude of the assumed percolation threshold. Plain Language Summary: When ice forms from salt water, salt is excluded from the ice causing the remaining liquid water to become more saline, forming brine. We developed a method to build models that estimate how much brine exists in ice at a certain temperature for a certain amount of salt. Because water is a necessary ingredient for life, places where brine exists in ice represent potential habitats. We applied our method to the ice shell of Jupiter's moon Europa, assuming the ice shell formed from an ocean. Two possible oceans made up of different types of dissolved salts were considered. We found that the depth where brine is stable in an ice shell is more sensitive to the type of salt when two ions are present than when four ions are present. When the salt is made mostly of chloride, a larger volume of brine exists than when the salt is made up mostly of sulfate. The thickness of the ice shell did not affect the percentage of the ice layer where brine existed as much as temperature or the amount of salt did. The type of salt in the ocean did not affect how much salt was retained in the ice. Key Points: The brine volume fraction of impure water ice can be represented as a function of temperature and bulk ice salinityFor the same temperature and bulk salinity, the brine volume fraction in a Cl‐dominated ice shell is higher than a SO4‐dominated ice shellThe magnitude of the percolation threshold governs the minimum fraction of salt entrained in ice formed through freezing of an ocean [ABSTRACT FROM AUTHOR]

Despite evidence for plumes on Jupiter's moon Europa, no surface features have been definitively identified as the source of the plumes to date. Furthermore, it remains unknown whether the activity originates from near‐surface water reservoirs within the ice shell or if it is sourced from the underlying global ocean. Here we investigate brine pocket migration, studied previously in the context of sea ice on Earth, as a process for transporting brine along thermal gradients. We show that the fracture system located in the center of Europa's Manannán crater is consistent with the formation of a subsurface brine reservoir. After the initial impact, residual aqueous melt concentrated via brine pocket migration as the target material cooled. Freezing and overpressurization then resulted in a cryovolcanic eruption. The volume of the emptied reservoir and the critical composition at the end of migration provide further constraints on the average salinity of Europa's ice shell. Plain Language Summary: Jupiter's satellite Europa has a subsurface ocean covered by an icy shell. We show how small pockets of brine can migrate within the ice from colder areas to warmer areas. This can happen even at very low temperatures, below the point where pure water would freeze, because the water becomes saltier and saltier as it migrates. By looking at an impact crater on Europa, which was initially warm in the center and cooled inward from its colder surroundings, we can study how the water migrated toward the center and formed a central water reservoir. As the final water pocket at the center of the crater started to freeze, the increasing pressure lead to a cryovolcanic eruption that emplaced brine onto the surface to form a prominent "spider" feature before the ice collapsed into the cavity below. Using a digital terrain model of the crater and collapse feature, we estimate how much water erupted and how salty Europa's ice shell is. Key Points: We introduce brine pocket migration as a mechanism for melt mobilization and a driver for cryovolcanism on icy worldsWe apply brine pocket migration to Europa's Manannán crater and its central "spider" (araneiform) fracture system and collapse structureWe show that understanding the concepts of brine pocket migration can also constrain the salinity of Europa's ice shell and ocean [ABSTRACT FROM AUTHOR]