Cycloidal crack

Figure 4 Some Cycloids have been modified by considerable dilation as the crack opened. The dark band that runs from the bottom center to the upper left is an example of a cycloidal crack that has dilated. This example is in a region called the Wedges , because it contains many dilational bands with this same wedged-shaped geometry. In fact, most of the dilational bands in the wedges originated as cycloidal cracks.

The mechanism by which tides create the cycloidal crack patterns was discovered by B.R. Tufts and G.V. Hoppa (Hoppa et al. 1999c). Due to the orbital eccentricity, which is pumped and maintained by the Laplace resonance, the tidal stress on Europas ice crust changes periodically with each orbit. We call this variation the diurnal tide, because the 85 hr orbital period is approximately the length of a Europan day, and is comparable to the length of a day on Earth. 

Figure 1. A Voyager image of a far southern region on Europa, showing many distinctive cycloidal ridges, which mark the paths of cracks in Europa s crust. These linear trajectories are the result of the Laplace orbital resonance among the Galilean satellites. Three of the Cycloids have IAU assigned names, as shown. Cycloids are chains of arcs, each typically 100 km long, often with a dozen or so arcs in each chain.

Figure 2. A Galileo image of cycloid arcs that run roughly north-south in the northern hemisphere. By chance, a rare crater is also seen in this region. That the ridges are double can be seen in this image. Also, the Cycloids appearance is somewhat complicated here by a small amount of dilation (opening of the crack) along a couple of arcs.

Figure 3. These high-resolution Galileo images show the pointy cusps of some Cycloids, where the double ridges make sharp turns. The crack in the crust lies between the ridges in each ridge-pair. The two examples to the left are details of cusps that appear in Figure 1.

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