Evidence for shear failure in forming near-equatorial lineae on Europa

[1] Global stress models for Europa are unable to readily explain the orientations of lineae in the equatorial region of Europa’s trailing hemisphere if lineae originate as tension cracks. Our analysis of two equatorial, trailing regions reveals that lineae are predominantly oriented NE and NW, and E-W lineae are rare, contrary to predictions of stress and formation models. The measured orientations are consistent with an origin by shear failure. The studied regions are located near the point of maximum differential stress and minimum surface tensile stresses, where shear faulting may surmount tension fracturing. Several types of lineae are recognized; their relative abundance is inferred to have changed with time, consistent with formation models suggesting lineae evolve from simple troughs to complex ridges. The opening of crevasse-like tensile fractures is not required for generation of all lineae; the observations are consistent with ridge formation models where troughs (formed in tension or shear) experience shear heating from tidal deformation, allowing warm ice to buoyantly uplift, creating ridges. The stratigraphic relationships indicate the following: ridged plains formed first, followed by continued formation of a wide range of lineae, and lastly emplacement of lenticulae and continued linea formation. This sequence is consistent with an early thin, brittle lithosphere that thickened with time and was subject to repetitive diurnal tides during 30 –90 of nonsynchronous rotation. Ultimately, the thickened shell underwent thermally induced solid-state convection, producing lenticulae. The young surface age of Europa implies that this entire stratigraphic sequence was emplaced in the geologically recent past.