Multi-Ring Basins on Europa: Implications for Subsurface Structure
E.P. Turtle, C.B. Phillips (LPL)
Two of the most circular Europan maculae are surrounded by multiple rings (e.g., Moore et al, LPSC XXVIII, 1997, Thomas, LPSC XXVIII, 1997) Although debate still exists as to whether these features are exogenic or endogenic in origin (Moore et al, LPSC XXVIII, 1997), in this study we have assumed that they are analogous to multi-ring impact basins seen elsewhere in the solar system. The rings around these structures are thought to be formed as a result of inward flow in a fluid soft ice or water layer located beneath the rigid icy crust (McKinnon and Melosh, Icarus 44, 1980). This flow occurs as the crater relaxes and induces radial tension, which is relieved by the formation of concentric normal faults around the crater (Allemand and Thomas, JGR 96, 1991).
We are currently using finite-element methods to model the formation of multi-ring basins. Three candidate multi-ring basin systems exist on Europa: Tyre Macula (34 N, 146 W); the provisionally-named Callanish Macula (16 S, 334 W); and the much smaller crater Taliesin (23 S, 137 W). An interesting fourth data point is the relatively young crater Pwyll (Greeley et al., LPSC XXVIII, 1997) which has no visible concentric rings.
Preliminary results from our modeling indicate that relaxation of an impact basin emplaced in a layered surface, with a thin ( 10 km) liquid water layer sandwiched between a thin rigid upper crust of comparable thickness and a thick rigid lower layer, induces sufficient stresses to cause fracturing of the overlying ice (5-10 bars, Parmerter and Coon, JGR 77, 1972). For thicker rigid surface or liquid water layers, the radial extent of the zone of sufficient extensional stress is inconsistent with the observed fractures around Callanish. We are currently varying the thicknesses of the rigid and liquid layers, as well as the transient crater size, to constrain the crustal structure necessary for the formation of the observed craters.