CRYOMAGMATISM ON EUROPA

The surface of Europa contains many elliptical features a few km to a few 10s of km across, called lenticulae. These features may have negative relief, positive relief, disrupt the surface, or appear dark. Respectively, they are categorized as pits, domes, chaos, and spots. We consider the possibility that all these features are the consequence of water intrusion within Europa’s ice shell. We model the thermal and mechanical processes that govern and accompany water intrusion and examine the resulting evolution of surface deformation. We assume that water spreads within the elastic part of the ice shell and show that the mechanical properties of ice exert a strong control on the lateral extent of the sill. At shallow depths, water makes room for itself by lifting the overlying ice layer and water weight promotes lateral spreading of the sill. In contrast, a deep sill bends the underlying elastic layer and its weight does not affect its spreading. As a result, the sill lateral extent is limited to about a few to a few tens of kilometers by the fracture toughness of ice. In that case, the sill can thicken substantially, until the feeder dyke closes, since downward flexure of the lower elastic layer provides a way of keeping the pressure high in the water source. Cooling of the sill after emplacement warms the surrounding ice and thins the overlying elastic ice layer. As a result, preexisting stresses in the elastic part of the ice shell increase locally to the point that they may disrupt the ice above the sill (to create small chaos). Furthermore, disruption of the surface also allows for partial isostatic compensation of water weight, leading to a topographic depression at the surface (to make pits), of the order of ∼10² m. Finally, complete water solidification causes expansion of the initial sill volume and results in an uplifted topography (to make domes) of ∼10² m. We map all elliptical features in Gallileo high-resolution images and quantify the relationship between lenticulae shape, size and type. These observations are not inconsistent with predictions from models of water intrusion. If correct, the implication is that large bodies of liquid water currently exist within the ice shell. Europa Clipper will test this model.