Stable Brine Layers beneath Europa’s Chaos

Abstract

The formation mechanism of Europa’s large chaos terrain (>∼100 km diameter) and associated lenticulae (<∼10 km diameter) has been debated since their observations by the Galileo spacecraft. Their geomorphology and distribution suggest there may be reservoirs of saline liquid water 1–3 km beneath the surface—the “shallow water” model—generated by injection of ocean water or melting of the ice shell. Recent investigations on the evolution of small shallow-water bodies (≤103 km3) suggests that salts with a small effect on melting point (MgSO4) can extend the lifetime of saline bodies by ∼5% compared to freshwater reservoirs. However, sodium chloride, identified as a potential oceanic salt, has a significantly stronger impact on the freezing point, suggesting a further extension of liquid lifetimes. Moreover, the substantial volumes of liquid water (∼104 km3) beneath large chaos could be melted in situ rather than injected through a fracture, implying a distinct chemistry and formation environment. Here, we use numerical models to explore how the chemistry and disparate origins of shallow water control its evolution and lifetime. For small, injected sills, we find that NaCl can extend their liquid lifetime to ∼140 kyr—up to a ∼60% increase over freshwater sills. Saline melt lenses will last at least 175 kyr but, in contrast to sills, may persist as a stable layer of brine beneath the surface for over 500 kyr. Our results provide further support for the presence of liquid water at shallow depths within Europa’s ice shell today.