PLATE TECTONICS ON AN ICY MOON: EUROPA'S MOBILE LID EXAMINED IN THE TERRESTRIAL PLATE TECTONICS PARADIGM

Plate tectonics is an established paradigm for explaining Earth's mobile lid but is remarkably absent elsewhere in the Solar System. Europa, an icy moon of Jupiter similar in size to Earth's moon, exhibits a range of evidence in support of local plate motions and plate tectonic boundary processes. The surface of Europa is exceptionally geologically young (~90 Myr) compared to other Solar System bodies, implying it has been resurfaced or recycled in some manner, analogous to Earth's oceanic lithosphere. The total ice shell thickness is likely 20–30 km, consisting of a cold, brittle outer shell (a few km thick and fragmented into plates) and a thicker, convecting interior. Band-like dilational features (locally up to 40% of the surface area) reminiscent of mid-ocean ridge spreading are well documented, with spreading rates modeled to be similar to those on Earth (up to 40 mm/yr). Major strike-slip faults are present that rival the San Andreas fault in length. A recent 2014 study provided evidence for the removal of ~20,000 km² of Europa's surface along a tabular convergent zone (up to 1700 km long) that may be analogous to terrestrial subduction zones. We have since discovered numerous examples of plate motions that can be reconstructed sequentially in GPlates using multiple piercing point indicators on Europa's surface. The plates have abrupt definable boundaries that span the three terrestrial plate boundary types and their hybrids. Numerical modeling of convection within the ice shell over a range of viscosity contrasts, outer ice shell strength, Rayleigh numbers, and model dimensionality suggests a mobile lid is possible under certain conditions. The ice plates cannot subduct into the underlying global ocean due to buoyancy constraints, but rather into the convecting portion of the ice shell. A 2017 study implies that temperature, density, porosity, and salinity differences between the outer layer and the underlying convecting ice may result in non-buoyancy and hence subduction. Europa may thus be the only Solar System body other than Earth to have a system of plate tectonics; however, our work suggests this plate tectonic behavior is intermittent in our study areas and may be regionally confined at any particular time. Global imaging is needed to further test the similarities and differences to Earth’s plate tectonic system.