CONSTRAINTS ON THE SUBSURFACE STRUCTURE OF EUROPA

The identification of anti-Jovian wedge-shaped bands as tension cracks separating thin lithospheric blocks that have rotated apart is used with accepted failure criteria and ductile flow laws to quantitatively derive conductive thermal gradients in the brittle crust and minimum amounts of differentiation of water on Europa. The interpretation of wedge-shaped bands as tension cracks that have allowed the rotation and translation of large blocks of lithosphere is based on offsets of preexisting lineations and the ability to fit blocks of Europa's surface back together without gap or overlap. The size of the blocks and their undeformed state suggest the lithosphere was greater than a few kilometers thick but less than a few tens of kilometers thick to have allowed the rotation to occur and that it was decoupled from the silicate interior at the time of rotation. Analysis of the Griffith criterion for tensile and shear failure of intact materials and Byerlee's law for the shear failure of prefractured materials shows that the slip on preexisting fractures with dips greater than 25° will occur at lower stress differences than those required to break intact ice or rock. The formation of tension cracks on Europa requires a intact lithosphere with no preexisting fractures or subsurface mechanical discontinuities. Confining pressure and the tensile strength of ice limit the depth to which tension cracks can form, and thus the thickness of the brittle lithosphere under Europan conditions, to ∼6 km with a corresponding maximum stress difference of 10 MPa. Below the brittle lithosphere (brittle-ductile tr transition) the ice deforms by ductile creep, which is thermally activated. The ductile yield stress curve must intersect the tensile failure curve no deeper than ∼6 km or tension cracks would not have formed (shear failure occurs below this depth). This tensile/brittle-ductile intersection depth constrains thermal gradients on Europa to 3-7°/km for tensile lithospheres of 6-3 km thick, respectively, for the assumed ductile flow laws and material constants. The estimate of 3°/km is a minimum because faster strain rates than assumed and strengthening of ice by inclusion of silicates each raises gradients by about a third and possible (though unlikely) fluid pressure can raise gradients by about a degree per kilometer for tensile lithospheres less than 6 km thick (given a fluid pressure ratio of 0.5). Our results limit the possible effects of surface warming processes such as insulating regoliths or a solid-state greenhouse to less than 10-30° on Europa at the time of fracturing. A minimum of about one-quarter of Europa's water is required to have differentiated to have allowed complete decoupling of the ice lithosphere from the underlying silicate mantle. © 1990.