Global mapping of unusual large circular features on the ice-covered ocean world of Europa has revealed that Jupiter's curious icy moon is even more unstable than previously thought.

The features, arcuate troughs up to 40 kilometers (25 miles) wide and several hundred kilometers long, are some of the most enigmatic on the ice-shrouded moon. One of these troughs was glimpsed by Voyager during its thrilling pass through the Jupiter system in 1979.

When the Galileo spacecraft arrived in 1996, new images revealed several more of these strange troughs and New Horizons helped complete mapping for one trough when it passed the giant planet in 2007.

In results published in the May 15 issue of Nature magazine, a team of scientists led by Dr. Paul Schenk of the Lunar and Planetary Institute of Houston, Texas, and joined by Dr. Isamu Matsuyama of the Carnegie Institution of Washington and Dr. Francis Nimmo of the University of California, Santa Cruz, has discovered that these features form two huge nearly identical circular patterns several thousand kilometers across.

These circles are on exactly opposite sides of Europa but are oddly displaced from the equator and the Europa-Jupiter axis, forming one of the most unusual global patterns in the solar system.

The search for the origin of these features was not easy. Nonsynchronous rotation of the icy shell and daily tidal distortions as high as 30 meters (100 feet) have been proposed as explanations for many of Europa's faults and ridges, but none of these stresses matched the strange circular troughs. A third global stress force, called true polar wander, had also been proposed, although previous efforts to find geologic features that matched this prediction have not met with success.

In the polar wander mechanism, proposed by Greg Ojakangas and David Stevenson in 1989, the outer ice shell of Europa can slowly reorient, or "flop over," while the huge rocky core of Europa continues to rotate normally. Cold polar temperatures can thicken the icy shell and drive polar wander.

When the team found a match to the polar wander stress field for the concentric circles, they were surprised by the degree to which many of Europa's other bands and linear features matched the polar wander stress fields too.

"I have long suspected that Europa's fracture patterns were more symmetric than they appear on our maps," says Schenk. "When we found the polar wander scenario that best fit the concentric circles, other large tectonic features suddenly fit into a globally symmetric pattern that lined up with this stress field."

The findings published in Nature are the first direct evidence for polar wander on Europa, a mechanism predicted 18 years earlier. The large reorientation required to explain the observed tectonic pattern is very difficult to achieve if the outer shell is not decoupled from the core by a liquid layer.

Geophysical and geologic evidence from Galileo has already indicated the probable existence of a water ocean. This new evidence for polar wander provides an independent confirmation that Europa possesses a water ocean not far beneath its icy shell.

Europa is not the only planet or moon to have undergone polar wander. Mars has probably tilted over at least once, due to the formation of the Tharsis volcanos. Earth's outer layers have done so, as apparently have Enceladus, and possibly Miranda. Polar wander may be a common occurrence across the solar system, suggesting that planets in general are less stable than we have thought.

The troughs and depressions are up to 500 meters (almost half a mile) deep, the deepest known features on Europa. Deep depressions would be unstable unless the ice shell is relatively strong and thick. Thermal models and studies of impact craters suggest a thickness of 10 to 20 kilometers (6 to 12 miles), although determining the true thickness and depth to the ocean remains the job of a future Europa orbiter.

The concentric circles are the "smoking gun" in the polar wander story, but polar wander could explain a lot of Europa's complex tectonic history. Work to test this has already begun but only a new mission to map Europa and unravel these tectonic patterns will give us a true understanding of this ocean world and its evolution.

The concentric circles at least have shown that true polar wander was a significant event that may have fundamentally reshaped Europa's surface. Concludes Schenk, "Polar wander will change how we look at Europa's geologic history."