Convergence

Europa’s crust is highly mobile. Tides created the cracks that allow plates of crust to slide around, and tides formed the ridges that mark those cracks. Thanks to those markers, and to the fact that there are no trees or other annoying manifestations of life to get in the way of our views, we can reconstruct the past motions of those plates.

Even the limited Voyager images showed clear evidence of both strike-slip displacement and dilation along cracks. Although Schenk and McKinnon had encountered resistance to the seemingly radical notion that a surface could be so mobile, Galileo images confirmed that dilation had been common. Large crustal plates (often hundreds of kilometers in scale) have separated, with the space between them filled by new surface material, creating the dilational bands discussed in Chapter 8. Similarly, new surface area has been created along strike-slip faults, where bends in the fault lines have resulted in pull-apart zones (Chapter 9). Whether dilation is caused by separating plates or by slip-strike pull-aparts, it increases the surface area of Europa’s ice crust as new material comes up to fill the gaps.