Abstract
Brinicles are self-assembling tubular ice membrane structures,
centimeters to meters in length, formed by the downward migration of
supercooled brine rejected from ice sheets, and found beneath sea ice in
the polar regions of Earth. They provide a plausible setting for
geochemical gradients amenable to life at the ice-ocean interface, in
some ways analogous to hydrothermal vents at the seafloor-ocean
interface. Their occurrence in icy ocean worlds like Europa and
Enceladus remains hypothetical. The context of brinicles on Earth
includes influences from oceanic flow, which will differ in other
worlds, and surficial inputs from the atmosphere that do not exist in
oceans with kilometers-thick global coverings of ice formed from the
underlying ocean. Thus, it is difficult to project the likely occurrence
and role of brinicles based on field observations of their earthly
analogues. We discuss brinicles as they are currently understood,
including their electrochemical properties in connection with potential
habitats at the ice-ocean interface on Europa and Enceladus. We employ a
fluid mechanical model (Cardoso and Cartwright, 2017) to assess the
properties of brinicles on other worlds and consider their longevity
relative to potential brine outflows from the overlying ice. We
demonstrate how brinicles may grow by thermal diffusion, and provide
simple scaling for their growth and outflow rates. The specifics of the
composition and dynamics of both the ice and the ocean in these worlds
remain poorly constrained. We demonstrate through calculations using
FREZCHEM that sulfate likely fractionates out of accreting ice in Europa
and Enceladus, and thus that an exogenous origin of sulfate observed on
Europa’s surface need not preclude additional endogenous sulfate in
Europa’s ocean. We suggest that, like hydrothermal vents on Earth,
brinicles in icy ocean worlds constitute ideal places where ecosystems
of organisms might be found.