Signs of Europan plate tectonics

In the 11 years since the Galileo spacecraft transmitted its last images of Europa, those who study the icy moon have been left with a puzzle. Portions of the Europan surface appear to have expanded over time, but no part of the surface shows any clear sign of contracting. Researchers have advanced several ideas for mechanisms that might accommodate the extra surface area, but Galileo‘s incomplete, low-resolution images have offered no definitive verification of any of them. Simon Kattenhorn (then at the University of Idaho) and Louise Prockter (of Johns Hopkins University’s Applied Physics Laboratory) make the case that Europa could host a system of tectonic plates that move around—and subduct underneath—one another as Earth’s do. Revisiting images of a 134 000-km² region, slightly less than 0.5% of Europa’s total surface, the pair identified sharp discontinuities in numerous geological structures (shown as colored bands in the left panel of the figure). They chopped up the region into 16 putative plates, which they then meticulously translated and rotated, undoing the effects of a possible tectonic process, until all the structures were aligned. The resulting tectonic reconstruction, shown in the panel on the right, features a 99-km gap (the extended white area), which Kattenhorn and Prockter see as a sign that one icy plate has partially disappeared beneath another. If that process turns out to occur across the moon’s surface, Europa would be the first body other than Earth known to have tectonic plates. (S. A. Kattenhorn, L. M. Prockter, Nat. Geosci., in press, http://dx.doi.org/10.1038/ngeo2245 .)