Porosity and Salt Content Determine if Subduction Can Occur in Europa's Ice Shell

Motivated by recent evidence for subduction in Europa's ice shell, we explore the geophysical feasibility of this process. Here we construct a simple model to track the evolution of porosity and temperature within a slab that is forced to subduct. We also vary the initial salt content in Europa's ice shell and determine the buoyancy of our simulated subducting slab. We find that porosity and salt content play a dominant role in determining whether the slab is nonbuoyant and subduction in Europa's ice shell is actually possible. Generally, we find that initially low porosities and high salt contents within the conductive lid are more conducive to subduction. If salt contents are laterally homogenous, and Europa has a reasonable surface porosity of (0)=0.1, the conductive portion of Europa's shell must have salt contents exceeding 22% for subduction to occur. However, if salt contents are laterally heterogeneous, with salt contents varying by a few percent, subduction may occur for a surface porosity of and overall salt contents of 5%. Thus, we argue that under plausible conditions, subduction in Europa's ice shell is possible. Moreover, assuming that subduction is actively occurring or has occurred in Europa's recent past provides important constraints on the structure and composition of the ice shell. Plain Language Summary Europa, one of Jupiter's icy moons, has features that look similar to mid-ocean spreading ridges, and recent observations suggest that Europa may also have something similar to subduction zones. This implies that Europa may be the only planetary body other than Earth to have plate tectonics. We construct a computer simulation to track the temperature and compaction of ice as it sinks into warmer underlying ice. When ice warms up, it becomes less dense and easier to compact. The salt content of Europa's plates is also important because salt is denser than ice. The temperature, salt content, and how compacted the ice is determine the density and whether the ice can sink. We find that if the salt content of Europa's ice shell varies by few percent from place to place, then the plates can sink. This supports the idea that something like plate tectonics is occurring on Europa and may tell us about the composition of Europa's ice shell. Our work also implies that the plates will sink all the way to Europa's subsurface ocean. This is important because material from the surface of Europa could act as food for life that may exist in Europa's ocean.