The Growth of Europa's Icy Shell: Convection and Crystallization

The thickness of Europa's icy shell, which controls its thermal and rheological structure, cannot currently be directly measured. However, it can be estimated through indirect methods, including geodynamic modeling. We simulate the crystallization of the ice shell from a liquid water ocean and its subsequent thermal equilibration in the presence of tidal heat. We evaluate the effects of solid-state stagnant lid convection on the shell's thermal equilibrium thickness and shell formation timing. The thermal equilibrium thickness of the ice shell ranges from 5 to 30 km, depending on the imposed tidal flexure strain rate. The ice lithospheric thickness remains less than 8 km, independently of imposed tidal flexure strain rate, overall shell thickness, and surface temperature, suggesting the Europan ice lithospheric strength is globally uniform. Thermal convection significantly reduces the thermal equilibration time scale, with the Europan ice shell able to fully crystallize out from a liquid water ocean in less than 2 Myr. Heterogeneous thermal equilibrium thickness values across the satellite suggest that widespread crystallization and melting may occur at the base of the shell. Rapid melting and crystallization timing suggest that the young surface age of the Europan ice shell may be the result of rapid catastrophic whole-shell melting and recrystallization entirely recycling the surface. Plain Language Summary. The icy shell of Jupiter's moon Europa rests on top of a water ocean that may harbor life. As space exploration probes to date were unequipped to measure ice shell thickness, which has important geological and astrobiological implications, we must rely on theoretical models to estimate that thickness. Due to the size of Jupiter and Europa's unique orbital path around it, the Europan ice is heated by especially strong tidal forces, which will eventually balance with ice cooling to halt freezing. Simulating this process allows us to estimate a stable shell thickness, as well as the time it takes for the Europan ice shell to fully form. Our ice shell model additionally is divided between a cold, rigid surface layer and a warm, ductile basal layer that may grow or shrink relative to each other. Depending on tidal heat, ice shell thickness may range from 5 to 30 km. However, the rigid upper layer remains at a constant geographic thickness of less than 8 km. We also find that the ice shell crystallizes much faster than previously thought, with crystallization times ranging from 200,000 years to 2 million years.

Automated summary

The study utilizes simulations to evaluate the effects of tidal heat on Europa's ice shell thickness and formation time. It finds that the thermal equilibrium thickness of the ice shell depends on the imposed tidal flexure strain rate, while the ice lithospheric thickness remains less than 8 km. The findings also suggest rapid whole-shell melting and recrystallization that may contribute to the young surface age of Europa's ice shell.