Why aren't all ridges on Europa cycloidal?

Europa's arcuate cycloidal ridges formed in response to the rotating diurnal tidal stress field. However, most ridges are not cycloidal but are linear over great distances (>100s of km). Their orientations adequately fit a global stress field induced by a few degrees of nonsynchronous rotation (NSR) superimposed on the diurnal field, but only at the point in the orbit when tensile stress is maximized. At other times, linear ridges do not fit the pattern of stresses, unlike cycloids, which grow throughout a good portion of an orbit and are therefore very curved. Why does the ever-present diurnal stress field only sometimes dominate, creating cycloids? When do linear ridges grow? Was there an "age of cycloids" that heralded a fundamental change in europan tectonics, or simply a temporal oscillation between the two crack styles? We consider the relative merits of scenarios in which linear ridges: (1) only grow when stresses are maximized in the orbit, lengthening incrementally over time; (2) develop almost instantaneously by dynamic growth during a "snapshot in time" stress field; or (3) form when some other source of global stress temporarily drowns out diurnal stresses. Scenario (3) is considered most likely. A temporally variable global stress history, perhaps related to ice shell thickness variability, may have controlled crack growth style. Linear ridges dominated whenever the NSR stress exceeded some threshold level. Early in Europa's history, a thinner ice shell may have been associated with a faster NSR rate, allowing NSR stresses to consistently overwhelm the diurnal component, suppressing the formation of cycloids. As the shell thickened, NSR stress accumulation slowed. Linear ridge growth episodes relieved NSR stress, creating periods (perhaps many 1000s of years in duration) during which diurnal stresses dominated, conducive to cycloid development. Hence, there was overlap in the timing of cycloidal and linear ridge development.

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