Salt Chemistry Regulates Habitable Microenvironments in Brinicles: Implications for Icy Ocean Worlds.

Brinicles are dynamically growing ice structures that have been proposed as chemically active systems on Ocean Worlds, potentially serving as habitable niches for life. However, experimental studies of their internal microstructures and implications for habitability remain limited. This study examined how brine composition influences the formation of liquid habitats within brinicles and how these differences impact the transport and distribution of cellular proxies. Flow cytometry and cryo-Raman spectroscopy were used to quantify the spatial distribution of cell surrogates, liquid brine, and hydrated salts across the radial and vertical axes of brinicles grown under two salt compositions (NaCl and CaCl 2 ). Our findings demonstrate fundamental differences in brinicle microstructure between the two salt compositions. CaCl 2 grown brinicles exhibited confined localization of liquid brine towards the inner channel, whereas NaCl brinicles showed a more homogeneous distribution of the liquid fraction. Ultimately, these differences provide a physical mechanism for cellular surrogate localization in the ice. CaCl 2 brinicles resulted in the entrapment of microspheres near the inner hole while the NaCl brinicles produced uniform distribution throughout the ice. These results demonstrate that brine chemistry controls habitat structure and biological transport within brinicles, highlighting the importance of ocean composition when assessing habitability on Ocean Worlds.