Thermal and chemical evolution of small, shallow water bodies on Europa

Abstract:

The young surface of Europa’s ice shell is dotted with typically low-albedo elliptically shaped features ~10 km in diameter, collectively called lenticulae. Although their formation mechanism is not well constrained, recent work suggests lenticulae morphology is most consistent with bodies of liquid water, or sills, emplaced at shallow depths (~1-3 km) below the surface; the so-called “shallow water” model. The low albedo reflects the presence of salts, possibly magnesium sulfates, and suggests that salts may be important to the formation mechanism. Liquid water near the surface is enticing for future remote and in situ exploration, as it may provide a habitable niche and a way to assess the potential habitability of Europa’s subsurface ocean.

To quantify the longevity of small bodies of water in Europa’s ice shell and understand their chemical evolution to constrain how shallow water impacts the ice shell composition, and regional surface geology over time, we developed a multiphase, two-dimensional, finite difference model that describes the thermal and chemical evolution of small bodies of liquid water after they are emplaced in the ice shell of Europa. Using sea ice as an analog at the roof of the sill and terrestrial magma chambers at the floor, a conservative parameterization of salt entrainment and rejection is implemented using constitutive equations relating bulk salinity in the ice to its thermochemical formation environment.

We find that lenticulae are more geologically transient than previously described. Our results imply that lenticulae may be actively forming, suggesting liquid water may still be present in Europa’s shallow ice shell. If salts are present in the liquid, meters-thick layers of salt may precipitate out of solution during freezing creating distinct mechanical, thermal, and dielectric layers in the shell. Finally, we suggest how both active and refrozen lenticulae may be detectable with the upcoming Europa Clipper mission providing a window into the ice shell’s history and habitability.