Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p31d..08g&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P31D-08
Physics
6218 Jovian Satellites, 6221 Europa, 8149 Planetary Tectonics (5475)
Scientific paper
The ice shell of Jupiter's moon, Europa, is pervasively fractured in response to stresses induced by oscillating tidal bulges in an underlying ocean. Principal stresses in the ice shell rotate through 180° of azimuth each orbit, and are referred to as diurnal stresses. Hence, any tensile fracture growing continuously during the ongoing rotation of the diurnal stresses would be expected to follow a curved crack path. Such fractures exist on Europa and are called cycloids. They form chains of arcuate segments linked at V-shaped cusps, producing a scalloped appearance. The ice shell also appears to have undergone nonsynchronous rotation (NSR) with respect to the interior. Although orders of magnitude slower than diurnal oscillations, this reorientation of the tidal bulge contributes a component of stress to the ice shell called the NSR stress. The resultant state of stress on Europa is dependent on latitude, longitude, and the point in Europa's orbit around Jupiter. Two models exist to describe the growth of cycloids on Europa: (1) In the tensile crack model, cycloids grow as consecutive curved segments in response to the rotating diurnal stresses. Each segment represents one Europan orbit. The crack only grows during portions of the orbit where the stresses are sufficiently high, so the cycloid segments do not trace out a full 180° of arc. Cusps form when a new segment propagates at an angle to the previous segment during a later orbit. (2) The tailcrack model states that cycloid cusps form through the creation of a tailcrack fracture at the tip of the older cycloid segment that undergoes strike-slip motion due to the rotating stress field. Once the tailcrack has formed, the cycloid segment so initiated continues to grow in the diurnal stress field. The tailcrack model thus uniquely accounts for resolved normal and shear stresses at the instant of cusp formation and predicts cusps as being due to locally perturbed stresses at the tip of a shearing fracture, not diurnal stress. However, the purely conceptual tailcrack model has not been tested quantitatively until now. We examined a progressive sequence of cycloid chains in the E15RegMap01 region and selected two examples that formed 30° and 90° of NSR ago, respectively. We then calculated the stress fields that would have existed at these locations at the time of development of each cycloid chain. We considered three stress scenarios: diurnal only, and diurnal plus NSR stress (accumulating elastically over either 0.5° or 1° of NSR). In each case, we calculated the ratio of resolved shear to normal stress that would have existed at each cusp throughout a Europa orbit. We used this stress ratio and principles of linear elastic fracture mechanics to predict the tailcrack angles that could theoretically have developed at the tip of a slipping cycloid segment. In 100% of the cases, we found a predicted tailcrack angle that exactly matched the measured cusp angle, confirming the viability of the tailcrack model for cycloids. Our results also indicate that a cycloid cusp is not initiated at the point in the orbit when the principal tensile stress is maximized (the limiting point in the orbit in the tensile growth model), but may form earlier or later in the orbit when a critical ratio of shear to normal stress is attained, typically when the tensile normal stress on the slipping cycloid is approximately maximized. Furthermore, the point in the orbit when cusps develop is similar along any one cycloid chain but is different for different chains. Finally, the tailcrack model provides a driving mechanism for the development of N-S oriented chains at low latitudes, which are not predicted by the tensile growth model.
Groenleer Julie M.
Kattenhorn Simon A.
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