[Previous] | [Session 66] | [Next]
G. V. Hoppa, B.R. Tufts, R. Greenberg, P.E. Geissler (LPL University of Arizona)
The formation of cycloid shaped ridges on Europa has been a mystery dating back to the Voyager encounter, and Galileo spacecraft data has confirmed that cracking in cycloidal patterns has been widely distributed. We now find that such cracks may develop in response to diurnal variations in tidal stress in Europa’s outer ice shell. When the tensile strength of the ice is reached, a crack can form perpendicular to the local tensile stress. Crack propagation crosses an ever-changing stress field, thus, propagation following a curving path until it reaches a place and time where the tensile stress is insufficient to continue the propagation. The propagation may be dormant until a few hours later when the stress at the end of the crack once again exceeds the strength and propagation continues, thus forming a cusp because the orientation of the tidal stress has significantly changed during the period that the crack was inactive. In addition to explaining the formation of arcuate segments and cusps, this model also reproduces many of the other major characteristics associated with cycloidal features including: (a) the distances between cusps, (b) the orientation of the cusps, (c) the skewness along cycloids, (f) the termination of cycloidal features, and (g) the large-scale curvature of the cycloidal chains. For this model to work, Europa’s ice must have a tensile strength of <100 kPa (< 1 bar), which is a plausible value for weak ice, and a thick fluid layer must have been present under the ice to allow sufficient tidal amplitude on a daily basis to provide that much stress.