EXPLORING MECHANISMS FOR SHALLOW SILL FORMATION BENEATH DOUBLE RIDGES ON EUROPA

One of the most prevalent structures on Jupiter’s icy moon Europa is the double ridge, which can span distances of up to more than 1000 km. Double ridges may provide an avenue for water to reach the surface also making them an astrobiological target of interest and a possible source for the recent possible plume observation [1]. By increasing our understanding of how ridges form, we can better constrain the structure of Europa’s ice shell including the plumbing contained within. Recent work by [2] explored proposed double ridge formation mechanisms through thermal-mechanical modeling and found that the lithospheric flexure and flanking fractures associated with some double ridges is best explained by the presence of a shallow, positive thermal anomaly of sizable horizontal extent. A water sill could provide the temperature increase and horizontal width needed for ridge flexure.

In previous work, we performed 2-D, finite element fracture models that show promising results for forming a shallow water sill at Europa. Results indicate sill formation can begin where a stress field change occurs as a result of proximal shallow and deep cracks, causing a vertical fracture to change propagation direction towards horizontal [3-5]. However, instead of the ~ 2 km half-width estimated necessary by [2] for the thermal anomaly, only about 400 m of horizontal propagation occurred. Here we report our results for simulations of mechanisms considered for increasing fracture propagation horizontal extent. Mechanisms we consider include: (a)the effects of the brittle-elastic interface expected in the ice shell, (b)different stress fields due to spacing variations between deep and shallow cracks, and (c)pressures caused by freezing water in the sill. Mechanism (a) is based on terrestrial analog studies performed on lateral dike propagation beneath terrestrial rifts and mid-ocean ridges [e.g. 6] and mechanism (b) is based on Antarctic basal fracture studies [e.g. 7].

References: [1] Roth et al. (2014), Science, 343, 171-4; [2] Dombard et al. (2013), Icarus, 223, 74-81; [3] Craft et al. (2012), GSA, abstract #131-3, Charlotte; [4] Craft et al. (2013), LPSC XXXXIV, abstract #3033, LPI, Houston; [5] Craft et al., in prep; [6] Grandin et al. (2012), Earth and Planet. Sci. Lett., 319-20, 83-95; [7] Luckman et al. (2012), The Cryosphere, 6, 113-23.