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Cracks in Europa reveal a tilted past

Jenny Winder, News Writer
Sep 25, 2013, 7:00 UTC

Sen—By analyzing the distinctive cracks crisscrossing the icy surface of Europa, NASA scientists have found evidence that this moon of Jupiter likely spun around a tilted axis at some point.

This tilt could influence calculations of how much of Europa's history is recorded in its frozen shell, how much heat is generated by tides in its underground ocean, and even how long the ocean has been liquid.

"One of the mysteries of Europa is why the orientations of the long, straight cracks called lineaments have changed over time. It turns out that a small tilt, or obliquity, in the spin axis, sometime in the past, can explain a lot of what we see," said Alyssa Rhoden, a postdoctoral fellow with Oak Ridge Associated Universities who is working at NASA's Goddard Space Flight Center.

Europa's network of cracks is a record of the stresses caused by massive tides in the moon's global ocean. Europa travels around Jupiter in a slightly oval-shaped orbit, when it comes closer to the planet, the moon gets stretched, with the ocean height at the long ends rising nearly 100 feet (30 metres). When Europa moves farther from Jupiter, it relaxes back into the shape of a ball.

The moon's ice layer flexes to accommodate these changes, but when the stresses become too great, it cracks. The puzzle is why the cracks point in different directions over time, even though the same side of Europa always faces Jupiter.

Europa's frozen outer shell may rotate slightly faster than the moon orbits Jupiter. If this out-of-sync rotation does occur, the same part of the ice shell would not always face Jupiter.

Close-up of the Bright Plains near the equator of Europa reveal layers of cracks (left). The directions of the lineaments (right) Image Credit: NASA/JPL-Caltech/University of Arizona/Goddard.

Rhoden and her team used images taken by NASA's Galileo spacecraft during its nearly eight-year mission, which began in 1995. They compared the pattern of cracks near Europa's equator to predictions based on three different explanations. The first was based on the rotation of the ice shell. The second assumed that Europa was spinning around a tilted axis, which made the orientation of the pole change over time, an effect called precession. The third was that the cracks were laid out in random directions. The best results came when they assumed that precession was caused by a tilt of about one degree, combined with some random cracks.

The existence of tilt would not rule out the out-of-sync rotation, but it does suggest that Europa's cracks may be much more recent than previously thought. That's because the spin pole direction may change by as much as a few degrees per day, completing one precession period over several months, while one full rotation of the ice sheet would take roughly 250,000 years. In either case, several rotations would be needed to explain the crack patterns.

A tilt could also affect the age of Europa's ocean, suggesting that more heat is generated by tidal forces to keep the ocean liquid longer.

"One of the fascinating open questions is how active Europa still is. If researchers pin down Europa's current spin axis, then our findings would allow us to assess whether the clues we are finding on the moon's surface are consistent with the present-day conditions," said Rhoden.