WANDERING POLEThis image of a section of Jupiter’s scarred moon Europa shows a long arc-shaped trough just to the right of center. The trough, some 25 kilometers wide and 500 kilometers long, is evidence that the moon’s icy shell has undergone true polar wander and that an ocean may lie beneath. Schenk et al., Nature, JPL/NASA

Imagine a shift in the position of Earth’s continents so extreme that Alaska would move to the equator. Astronomers have now found evidence that such a shift actually happened on Jupiter’s large icy moon Europa. The sliding of the moon’s icy surface provides further evidence that an ocean lies beneath the ice, upping the odds that Europa has a subterranean habitat that could support some kind of life.

Paul Schenk of the Lunar and Planetary Institute in Houston and his colleagues arrived at those conclusions after using data from three spacecraft — Voyager, Galileo and New Horizons — to map several arc-shaped depressions spread far apart on Europa. Each of the depressions is about 500 kilometers long, and two of them lie exactly on opposite sides of the planet. The depressions are shallow enough that “you would not notice them if you were walking on Europa,” Schenk says.

According to models analyzed by Schenk’s team, these surface scars have just the right shape, size and location to be the fractures generated if the moon’s icy shell had sometime in the past rotated by 80°— nearly a quarter-turn.

Schenk and his colleagues, Isamu Matsuyama of the Carnegie Institution of Washington (D.C.) and Francis Nimmo of the University of California, Santa Cruz, report their findings in the May 15 Nature.

A buildup of ice at the poles may have prompted the migration, notes Matsuyama. Because a spinning body is most stable when its mass is farthest from its spin axis, the concentration of polar ice could have triggered the polar wander.

Planetary scientists first suggested two decades ago that Europa’s shell may have undergone such a rotation, known as true polar wander. The new findings are “the best evidence so far” that the shell of Europa, a moon only slightly smaller than Earth’s, has indeed flopped over, comments Bill McKinnon of Washington University in St. Louis.

The movement was gradual, over a period of years to decades, but it involved Europa’s entire shell, some 300,000 trillion metric tons of ice, Schenk notes. If the shell were sitting directly on a rocky core, moving that much ice would be “an exceptionally difficult thing to do because of the huge amount of drag,” he adds. In contrast, the movement “is very easy if the shell floats on water.”

Adds Nimmo: “We prefer the model of an ice shell moving over an ocean because it is much easier to reorient the shell than the entire body and there is a plausible published mechanism for causing the shell to reorient by roughly 90 degrees.” The evidence of true polar wander supports previous findings suggesting Europa has a subterranean ocean, notes Bob Pappalardo of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

Both Earth and Mars are also likely to have undergone true polar wander, and researchers now have hints that Enceladus, the Saturnian moon that vents geysers of water vapor, has also reoriented.

True polar wander, says Pappalardo, “is becoming a new planetary stress mechanism that we really have to seriously consider.”

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