Obliquity, Precession, And Fracture Mechanics: Implications Of Europa'S Global Cycloid Population

We catalog the global inventory of Europa's cycloids, arcuate fractures whose paths have been linked to diurnal tidal stress, and use the locations and orientations of their cusps to further test the formation mechanism of cycloids and constrain Europa's rotation state. We find that the global distribution of cycloids is better explained by a precessing spin pole than longitude translation due to nonsynchronous rotation, which is consistent with studies of Europa's strike-slip faults and linear fractures. We also find that a small obliquity can reproduce the orientations of most cycloid cusps at their current locations, outperforming a model using stress from eccentricity alone. Matching cycloid locations and cusp orientations under these conditions implies a variable failure threshold and that most cusps form at stresses that are 50%-90% of the local peak tidal stress, suggesting that periodic deformation is causing failure through fatigue. The fact that cycloids are not observed in some regions, despite the low failure stresses implied by some cusp orientations, suggests an additional control on cusp formation that was not included in our simplified model. We hypothesize that the larger amplitude of tidal stress in some regions leads to fatigue-induced failure after fewer tidal cycles, leading to a build-up of cycloids in those regions. Why more linear fractures do not seem limited in location or density is still unknown. More physically informed constraints on how ice fails under low, periodic deformation and across generations of fractures would be helpful in interpreting Europa's cycloids.