San Andreas-like faults in the crust of Jupiter's icy moon Europa provide evidence that the crust, floating on a liquid water ocean, has slipped over the globe so that the poles recently have wandered hundreds of miles.
A University of Arizona undergraduate student reported the discovery on Novermber 29.
Alyssa R. Sarid also reported a second discovery from her survey of Europa's strike-slip faults seen in Voyager and Galileo spacecraft images: She has identified zones where surface ice has converged and disappeared entirely.
Sarid reported both results last week at the 33rd annual meeting of the American Astronomical Society Division of Planetary Sciences in New Orleans. Her talk was titled "Polar wander and surface convergence on Europa."
In a project suggested by UA planetary sciences Professor Richard Greenberg, her research advisor, Sarid meticulously mapped all strike-slip faults visible in two vertical swaths of Europa, one in the leading hemisphere and the other in the trailing hemisphere.
(The leading hemisphere is the forward hemisphere of the moon in its journey around Jupiter; the trailing hemisphere is its opposite.)
In strike-slip, opposite sides of a fault shear apart, much as on Earth the California coast shears southward relative to inland parts of the state.
Greenberg's research group, including Greg Hoppa, Randy Tufts and Paul Geissler, had earlier shown how tides on Europa could stress the surface and in that way drive strike-slip motion. The theory predicted that strike-slips south of the equator would shift right; that strike-slips north of the equator would shift left; and that the orientation of strike-slips at the equator itself would be mixed.
When Sarid compared her maps of strike-slip faults with the predictions of the theory, she found that faults on one hemisphere of Europa were systematically too far south to match the predictions, but on the other side, they were too far north.
Crust that a few million years ago had formed at the poles by now is reoriented relative to the north and south poles by roughly 30 degrees, Sarid and Greenberg conclude.
"What this suggests is that nothing keeps the thin ice shell of ice covering Europa from sliding around -- it is free to slip and slide over the underlying ocean," Greenberg said. "The result is the first confirmation that the crust does wander relative to the poles of rotation."
Such polar wander had been predicted in 1989 by Gregory Ojakangas (then at the University of Arizona) and David Stevenson of Caltech, Greenberg added. They theorized that Europa's poles would "wander" because tidal friction heats ice at the equator more than ice at the poles, so ice at the equator is thinner than ice at the poles. And as Europa spins on its axis, centrifugal force pushes thicker, more massive polar ice toward the equator, so the poles shift.
Sarid and Greenberg also addressed a puzzle of the budget of Europa's surface area, the mystery of where existing surface goes as new surface is created.
Images from the Voyager and Galileo spacecraft had shown substantial dilation, or pulling apart, along widely distributed tectonic bands, sites where new surface area had been created, analogous to the widening of the Atlantic Ocean with continental drift.
But researchers have failed to find specific sites on Europa where comparable amounts of surface were being lost.
"Researchers have predicted there must be 'convergence zones' on Europa," Sarid said, "but no one knew what they would look like. We haven't known what to look for until now."
By cut-and-paste reconstruction of strike-slip motion backwards in time, Sarid found two locations where substantial surface convergence has occurred. She found "convergence bands" in both locations. These features are different from compression features on other bodies, which may explain why they had previously been difficult to identify.
"These features are subtle. It was difficult to see them. The only reason we could spot them was to go back in time," Sarid said.
"With its thin ice shell over a global ocean, Europa is unique. The strike-slip motion provided the key to what has been going on," she added.
Sarid's survey covers the two broad surface swaths that run from the far north to far south where Galileo spacecraft images at 200m/pixel resolution were obtained for regional mapping purposes.
The work was supported in part by an undergraduate research assistantship from the Arizona Space Grant program, funded by NASA. Greenberg was Sarid's research advisor in the space grant program, and she now is part of his research group.
[Contact: Alyssa R. Sarid, Richard Greenberg]