Injection and Subsequent Evolution of a Water Sill in an Ice Shell: Application to Europa's Lenticulae

We study the injection and subsequent evolution of a water sill into an ice shell overlying an ocean and examine the resulting successive surface deformations. 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. Pits, domes, and small chaos on Europa's surface are quasi-circular features a few to a few tens of kilometers in diameter whose morphology could be explained by the subsequent evolution of such a thick sill. Indeed, 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 (small chaos). Furthermore, disruption of the surface also allows for partial isostatic compensation of water weight, leading to a topographic depression at the surface (pit), of the order of ~102 m. Finally, complete water solidification causes expansion of the initial sill volume and results in an uplifted topography (dome) of ~102 m.