SILL FORMATION IN EUROPA'S ICY SHELL

Jupiter’s icy moon Europa is covered with tectonic features such as the double ridge, characterized by a central trough bounded by two raised ridges. Models of ridge formation [1,2] imply the presence and distribution of liquid water in Europa’s ice shell at the time of ridge formation [3]. Recent work suggests that marginal troughs and flanking fractures observed in association with some double ridges can only be described by the presence of cryovolcanic sills ~1 km beneath the ridges [4]. Here we explore a mechanism of emplacing sills in the shallow subsurface via fracturing within Europa’s ice shell as it cools and thickens.

Establishing a sill in the near subsurface of Europa’s ice shell requires the generation of liquid water or access to a liquid water body. The former may be facilitated by a mechanism involving diapirism [e.g. 5], but producing the linear continuity of such double ridges would be difficult. The latter would involve fracturing Europa’s shell to the ice-ocean interface. Shell thickening has been shown to generate tensile stresses sufficient to overcome lithostatic loads [6] and enable fracture initiation and propagation [7-9]. Ice shell growth increases ocean pressure, which provides a means for water to rise through a fracture to form a sill. On Earth, sills tend to occur where a stiff layer overlies a weaker layer, at neutral buoyancy levels and/or where hydrostatic equilibrium occurs [10-12]. Assuming Europa’s ice shell is homogeneous, the latter two conditions are applied to determine sill emplacement depth. We also consider the time for fracture closure and sill cooling to determine the system lifetime. Results are then compared to the sill characteristics suggested by [4] for formation of double ridges.

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