Liquid water sill emplacement on Europa?

Recent work has suggested that lithospheric flexure and flanking fractures observed along some ridges on Europa are best explained by the initial presence of a shallow liquid water sill. The emplacement of a sill suggests certain conditions existed that were favorable to water flow from the ocean to the subsurface, stresses that allowed horizontal fracturing for sill emplacement, and liquid water replenishment to enable a sill lifetime of ~ 1000s of years. Here, we investigate whether these conditions could occur and result in sill formation. Previous models of the stresses resulting from ice shell thickening on Europa indicated that fractures can initiate within the shell and propagate both upward toward the surface and downward to the ice-ocean interface. For an ~10 km thick ice shell, we determined that flow velocities for ocean water driven up a vertical fracture by the release of lithostatic pressures are adequate for reaching the subsurface before freezing occurs (LPSC #3033). We propose the next step for sill emplacement could occur through horizontal fracturing. Nominally, the stress field in a material under lithostatic load is conducive to vertical crack propagation. However, factors exist that can cause the stress field to change and propagate cracks horizontally. Seismically imaged terrestrial sills beneath mid-ocean ridges often occur in areas with extensive cracking and/or faulting, suggesting crack interactions may play a key role. Through application of a finite element program, we modeled four stress changing mechanisms and the resulting fracture propagation in a 10 km thick ice shell on Europa: (1) mechanical layering, (2) shallow cracks to the surface, (3) deep cracks from the ocean-ice interface and (4) shallow and deep cracks combined. Results determined that all mechanisms cause some turn in propagation direction, with Model 4 (both shallow and deep cracks) enabling the greatest turn to ~ horizontal. The horizontal extent of the fracture propagation, however, only reaches a width of ~ 100s meters, whereas a sill of ~ 4 km width is necessary for formation of the flanking fractures at their observed locations on the ridges. Future work will explore the effect of crack spacing on fracture propagation and will study mechanical layering and lateral stress gradients in greater detail in an effort to enable wider sill emplacement. Assessment of the sill lifetime finds that a 10 - 100 m thick sill will convect and transfer its heat away over ~ hours to a few days, respectively. According to recent work, a liquid sill would need to exist for 1000s of years to enable the lithosphere flexure. One possible mechanism to extend the sill lifetime could involve liquid water replenishment from the ocean driven by brine migration, although the lifetime may still prove challenging to achieve. Overall, our analyses suggest sill emplacement may be possible by liquid ocean water flow up an open vertical fracture to the subsurface and fracture propagation turned horizontal by stress field change factors such as shallow and deep cracks. However, sill width and lifetime must both be extended to enable flexure and flanking fracture formation.