Structures on Jupiter's moon Europa that resemble California's earthquake faults have provided clues indicating that the moon's frozen crust slides over its liquid innards, according to a new study.
By noting the position and appearance of various faults, scientists found that while the moon's rotational axis remains fixed, its crust wanders over a presumed underground ocean. Material that used to be at the poles has migrated hundreds of miles away.
Alyssa Sarid, an undergraduate student at the University of Arizona, made the discovery. Sarid, who worked with Professor Richard Greenberg, studied images from the Galileo spacecraft.
In a telephone interview, Greenberg explained that Europa's insides are being heated by tidal forces. Jupiter and its other large moons tug on Europa, which is about the same size as Earth's moon. The effect is somewhat like squeezing a stress ball in multiple ways, and resulting friction heats Europa from within.
But scientists expect that the moon is heated more at its equator than at the poles. So the ice is relatively thin at the equator and thicker at the poles.
"This extra thick layer of ice near the poles wants to spin out toward the equator," Greenberg explained. But because the ice is locked into a global crust, it cannot simply spin out in all directions. Instead, the entire shell of the moon wanders.
This effect was originally predicted by Gregory Ojakangas and David Stevenson in 1989.
Disappearing ice
In a separate finding from the same study, Sarid found bands of surface features that may represent locations where plates of ice converge and dive into the moon, disappearing and leaving little trace. Similar convergence zones, where oceanic plates dive under the continental plate, cause large earthquakes along the coast of California.
The convergence zones might help balance the ice budget on Europa, Greenberg said. Previous studies have found regions of the surface where ice appeared to spread out, allowing water to come up from below and freeze into new surface features.
"But then there must be areas disappearing somewhere," Greenberg said. "There's got to be a sink where land goes away. On Europa, it's been a big puzzle for a number of years."
By reconstructing the motions of faults going back in time, Sarid found two locations where this convergence seems to have occurred.
"These features are subtle," she said. "It was difficult to see them."
The convergence phenomenon on Europa is unrelated to the polar wander, the researchers say.
The University of Arizona's Greg Hoppa, Randy Tufts, and Paul Geissler also worked on the study, which was detailed Thursday at a meeting of the American Astronomical Society's Division for Planetary Sciences in New Orleans.
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