Europa, that icy moon of Jupiter, is famous for its odd surface features  long lines, pits, domes, spots and the occasional crater.

Though the origin of some of these features has been explained, many mysteries remain. Among the puzzles are several arc-shaped troughs, first noticed in images from the Voyager missions. Three of them have been seen. Two appear to form parts of a circle roughly 1,400 miles across, centered near the equator. One is part of a circle of the same diameter on the moon’s opposite side.

Europa has plenty of troughs, but these are unusual because they are so circular and concentric, said Paul M. Schenk of the Lunar and Planetary Institute in Houston. “Nature is not supposed to work that way,” he said.

Image Large, circular markings on Europa may be from a surface shift.

Now Dr. Schenk and his colleagues have come up with an explanation for why these troughs formed. At some point, they say in a paper in Nature, the surface of Europa shifted, so that the poles neared the equator, and vice versa.