Kattenhorn, S.A (2000)
Evidence For Two Complete Nonsynchronous Rotations of Europa's Outer Crust.
GSA Abstracts with Programs 32(7), Paper #A-40
Kattenhorn, S.A (2000)
Evidence For Two Complete Nonsynchronous Rotations of Europa's Outer Crust.
GSA Abstracts with Programs 32(7), Paper #A-40
A detailed analysis of Galileo SSI images of the Bright Plains region of Europa's trailing hemisphere (~15N/273W) has enabled the unraveling of a deformation sequence involving faults, fractures, double ridges, and smooth bands. These structures developed perpendicular to regional maximum principal tensile stresses of sufficient magnitude to break the ice and provide direct evidence of the principal stress orientation history. Tensile stresses can be induced by "diurnal" tidal flexing of Europa's crust in response to the variable gravitational pull of Jupiter during Europa's elliptical orbit. However, the range of fracture orientations at Bright Plains is inconsistent with the diurnal stress field that currently exists at this longitudinal position. A plausible explanation for these observations is rotation of a decoupled crust with respect to Europa's interior in response to the spin rate being slightly faster than synchronous. Consequently, a nonsynchronous rotation stress field is superimposed on the diurnal stress field. Nonsynchronous rotation causes the global stress pattern to migrate relatively westwards across the Europan surface through time, resulting in a progressive rotation of fracture lineaments during the fracture sequence (clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere). Successively older fracture sets can thus be placed into a time sequence of older stress states that existed when the Bright Plains region was positioned at successively more westerly longitudes. Accordingly, principal stress axes in the Bright Plains rotated consistently in a clockwise sense, and imply a minimum of two complete nonsynchronous rotations (720 degrees) of Europa's crust during the visible fracture history. Recent surface fracturing suggests an additional 180 degrees of rotation for a total of 900 degrees. The amount of determinable nonsynchronous rotation of Europa's crust is thus significantly higher than previously documented (25-360 degrees), attributable to a relatively higher fracture density and thus a better preserved stress history in the Bright Plains compared to previously mapped areas. The fracture sequence implies intermittent fracture development with intervening time intervals of perhaps several thousands of years. In the interim, tectonic activity is seemingly restricted to shear reactivation of existing lineaments.
