Domes, pits and small chaos on Europa produced by water sills

The surface of Europa is littered with quasi-circular features a few to a few tens of kilometers in diameter. They may be uplifted (domes) or depressed (pits) relative to the surroundings. In some cases, they also show evidence of resurfacing and disruption of the crust (small chaos), requiring liquid water to be present at depth. Here we examine if injection of horizontal bodies of liquid water, or sills, into Europa's ice shell and their subsequent evolution can induce successive surface deformations similar to the morphologies and characteristics of pits, domes and small chaos. We study the dynamics of water spreading within the elastic part of the ice shell and show that the mechanical properties of ice exerts a strong control on the sill morphology. At shallow depth, water makes room for itself by lifting the layer of ice above the sill, the 'roof'; the pressure due to water weight promotes spreading and the sill cannot reach large thicknesses, not more than a few m. In contrast, if emplaced deep into the elastic part of the ice shell, the sill bends the underlying elastic layer and the pressure due to water weight does not affect sill spreading. In that case the sill may reach large thicknesses of the order of a kilometer for diameters of a few km to a few tens of kilometers, depending on the intrusion depth. For such large water volumes, topographic relaxation at the bottom of the ice shell, and compensation of water weight might happen faster than sill solidification and would induce large topographic depressions at the surface, of the order of ~ 100 m. Eventual water solidification would cause expansion of the initial sill volume and result in an uplifted topography of ~100 m as well. Furthermore, stresses caused by compensation and/or solidification can be large enough to overcome the tensile strength of ice and explain ice disruption. Deep sill emplacement not only creates thick sills and thus large topographic variations, both positive and negative, at Europa's surface; it also provides for a way of transferring water to the ice shell without draining the ocean overpressure since its volume during emplacement is accommodated by downward flexure, into the ocean, of the underlying elastic layer.