Binary Systems for Ocean Worlds: the Influence of Soluble Impurities on Ice Crystal Growth and Morphology

The eutectic microstructures of four two-phase binary systems [FeSO4–H2O, Na2CO3–H2O, KCl–H2O, and (NH4)2SO4–H2O] have been characterized by using light microscope imaging. These salts have received comparatively little previous attention in terms of their potential presence on icy moons of the outer solar system (specifically, Europa, Enceladus, and Titan), making these systems important candidates for microstructural study. In this work, a solute was introduced via liquid solution during the flooding phase of the standard ice method, such that eutectic growth occurred within the triple junctions between pure ice seed grains. The resulting morphologies are generally consistent with a previously used classification scheme for eutectic microstructures, with some modification. We additionally provide refractive index (RI) measurements for liquid solutions of nine binary systems and one multiphase system as a function of composition. The resulting curves are meant to aid experimentalists in determining the bulk compositions of fabricated saline ice samples and are used in this work to determine the bulk compositions of samples for which microstructures were characterized.