Figure 10. Ridges present on Europa. Courtesy NASA/JPL.

One of the most common theories for double ridge formation was proposed by Greenberg et al. (1998) and illustrated in Figure 12. They suggest that diurnal stress causes fractures in the ice which separate as the stress increases. As the two sections of ice move away from each other, liquid water under the ice shell begins to rise. The top layer of the water freezes, making a thin "lead" ice layer above the rising water. As Europa continues in its orbit, the diurnal stress will begin to decrease and the crack will begin to close, crushing the lead ice. As the crack closes, the crushed ice is squeezed and deposited on the surface of the ice shell in a mount, as seen in Figure 12c. This will repeat on the next orbit, building up higher ridges. They estimate that if 10% of the crushed lead ice is squeezed onto the surface of Europa, a 10 km long ridge 100 m high could be formed in 30,000 yrs, making ridges very young features. These types of ridges are considered simple, but more complex ridges are observed.

Figure 11. Craters on EuropaDouble ridge. Courtesy NASA/JPL.

Figure 12. Double ridge formation. Greenbery et al. 1998.

Greenberg et al. (1998) list 3 classes of ridges which are shown in Figure 13. Class 1 are the simplest and described above. Class 2 ridges are assumed to be a more advanced stage of class 1. These likely have experienced longer periods of diurnal stress which allowed more material to be brought to the surface, resulting in higher ridges and wider troughs. If enough material is deposited on the top of the ice shell the weight of the material will cause the ice shell to warp, leading to lateral cracking as seen in Figure 13b (Pappalardo and Coon 1996). These weak points in the ice may lead to larger cracks that expand, forming new ridges that can braid their way around older ridges (Figure 12c). These formations are classified as class 3 ridges. Examples of these are see in Agave Linea in Figure 14. As material builds higher, the ice shell will further warp to a point where liquid beneath the shell can seep through the porous crushed-ice ridges creating flood regions. These areas are darker than the surrounds material and appear reddish. Geissler (1998b) proposes that the inflowing liquid or ice likely contains impurities such as silicates or organic material which leads to this discoloration.

Figure 13. Classes of craters. Greenberg et al. 1998.

Figure 14. Braided ridges. Greenberg et al. 1998.

Overview
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