Comparison of Global Stress Models with Geographical Features on Europa

COMPARISON OF GLOBAL STRESS MODELS WITH GEOGRAPHICAL FEATURES ON EUROPA.Gregory Hoppa, Richard Greenberg, Paul Geissler, B. R. Tufts, C. B. Phillips, Daniel Durda, and the Galileo Imaging TeamLunar and Planetary Lab, University of Arizona, Tucson, Arizona. Tidal stresses and possible non-synchronous rotation maybe the primary source of crack formation and ridgedevelopment on Europa. Greenberg et al. [1,2] have shownthat the combination of diurnal tides and the tides due to non-synchronous rotation can generate sufficient stress in thecorrect orientation to match the incipient cracks observed in theCadmus-Minos region. The stress trajectory is also consistentwith the age relationship of lineaments observed in this region[3]. These global stress models have been used as a tool tolook at the principle stresses of regional features with respectto their formation and evolution.Agenor Linea is a 1500 km feature located between 40oS,180oW and 40oS, 250oW on Europa. Figure 1 shows thecombination of diurnal stress plus 1o non-synchronousrotation for Agenor Linea at a quarter orbit after Europareaches apocenter. The axis of maximum tension isperpendicular to Agenor across the entire feature. Agenor'sgeographic position on Europa is such that the regions to theeast and west are dominated by zones of compression andlower tension. Unlike Agave and Asterius Linea, Agenorappears to stop before it can turn and cross the equator.Diurnal tides could also affect the evolution of AgenorLinea. Every 3.5 days Agenor goes through a period ofextension perpendicular to its orientation. Half an orbit later,compression is then perpendicular to Agenor. Photometricobservations of Agenor suggest that it may be an active regionon Europa [4]. If Agenor is currently active then ridgeformation may be taking place in accordance to the Greenberget al. model [1,5]. In the southern hemisphere diurnal stressesrotate clockwise by 180o over each orbit. Any activation alongAgenor during the diurnal cycle would begin in the west andmove eastward as tension increases within this region.Astypalaea Linea is a 810 km strike-slip feature with 35km of offset is located near Europa's south pole [6]. As withAgenor Linea, Astypalaea may have formed as a tension crackdue to a combination of 1o non-synchronous and diurnal stress[1,2]. After the crack formed, Astypalaea Linea's orientationwith respect to the diurnal tides would result in the strike-slipoffset observed: At apocenter the diurnal tides are aligned suchthat tension is perpendicular to this feature, and the fault opens(Fig. 2a). A quarter of an orbit later the stresses are aligned atapproximately 45o with respect to the fault resulting in right-lateral motion (Fig. 2b). At apocenter the fault closes back upbecause the compressive component of stress is nowperpendicular to Astypalaea. As Europa completes its orbit,the fault experiences shear in the left-lateral direction, butbecause the fault has just been closed friction prevents the faultfrom moving back to its original position. This net motion"walks" [1,7] the fault, creating shear displacement. In generalthis model predicts that shear motion in the southernhemisphere of Europa should be predominately right-lateral(as for Astypalaea Linea) and left-lateral motion in thenorthern hemisphere. Additional features parallel toAstypalaea Linea also show evidence of right-lateral motion.In the northern hemisphere Agave Linea shows evidence ofleft-lateral motion possibly from the same effect.Libya and Thynia Linea, two gray bands, are possiblyassociated with Astypalaea Linea, but show evidence of lateralextension [8]. The combination of the stresses due to non-synchronous and diurnal motion align in these regions toprovide maximum tension across these features. Gray bandshave been shown to be relatively young [8], and they mayhave formed by the same stresses that created AstypalaeaLinea.Higher resolution images of the Agave and Asteriusregion (10oN, 270o W) reveal a series of east-west cracks atleast 300 km in length. Stresses from 1o non-synchronousrotation plus diurnal stress can provide sufficient tensionperpendicular to these features to result in cracking. Additionaltidal flexing could eventually build a network of double ridgesalong these cracks according to the Greenberg et al. model[1,5]. Additionally, once a crack forms, regardless of itslatitude, longitude, or orientation, it will feel both tension andcompression every 3.5 days. The amplitude of these stressesmay vary with geographic location, but oscillation betweentension and compression may be the source of the ridgenetwork observed today [1,5].There are, however, prominent features which cannot beexplained by these global stress models. The maximumtension in the wedges or fracture zone to the west of the anti-Jupiter hemisphere is never perpendicular to the youngestwedges as reconstructed by Tufts et al [9]. These models havedifficulty explaining the formation of features at angles >45owith respect to the equator at low latitudes. As with AgenorLinea the diurnal stresses rotate through 180o during one orbitthrough the wedges region. However, due to the lower latitudeof the wedges area, the rotation occurs at a non-uniform rate.In the wedges region, the principle stresses spend the timebetween pericenter and apocenter (and vice versa) with theirorientation fixed, but their amplitude changes. Approachingpericenter, the stresses in this region quickly rotate by ~90o,but the amplitude (near maximum tension for both principleaxes) is constant. This rapid rotation of the diurnal stresses inlow latitude zones could significantly alter the evolution offeatures within this region.The combination of diurnal and non-synchronous stressworks remarkably well for the production of east-westtrending features near the equator, but cannot explain whylinea like Agave and Asterius cross the equator at 45o. Perhapsthese features form at higher latitudes. Once the crack beginsto form it propagates ahead over a great circle that crosses theequator at high angles. Polar wander models [10] couldproduce the Agave and Asterius intersection although not intheir current orientation. Polar wander would also generatenorth-south cracks and features near the equator, howeverpolar wander poorly fits the other features described above.These models are limited by poor global coverage of theentire surface of Europa, and the uncertainty of the chronologyof formation for some of the larger features. Attempts to match