![]() ![]() Out-of-sync rotation was surprisingly unsuccessful, in part because Rhoden found an oversight in the original calculations for this model. The researchers got the best performance when they assumed that precession had occurred, caused by a tilt of about one degree, and combined this effect with some random cracks, said Rhoden. The third explanation was that the cracks were laid out in random directions. This effect, called precession, looks very much like what happens when a spinning toy top has started to slow down and wobble. The second set assumed that Europa was spinning around a tilted axis, which, in turn, made the orientation of the pole change over time. The first set of predictions was based on the rotation of the ice shell. ![]() Rhoden and Hurford compared the pattern of cracks in a key area near Europa’s equator to predictions based on three different explanations. “Galileo produced many paradigm shifts in our understanding of Europa, one of which was the phenomena of out of sync rotation,” said Claudia Alexander of NASA’s Jet Propulsion Laboratory in Pasadena, California, who was the project manager when the Galileo mission ended. Rhoden and her Goddard co-author Terry Hurford put that idea to the test using images taken by NASA’s Galileo spacecraft during its nearly eight-year mission, which began in 1995. ![]() If this out-of-sync rotation does occur, the same part of the ice shell would not always face Jupiter. The puzzling part is why the cracks point in different directions over time, even though the same side of Europa always faces Jupiter.Ī leading explanation has been that Europa’s frozen outer shell might rotate slightly faster than the moon orbits Jupiter. The moon’s ice layer has to stretch and flex to accommodate these changes, but when the stresses become too great, it cracks. When Europa moves farther from Jupiter, it relaxes back into the shape of a ball. That’s roughly as high as the 2004 tsunami in the Indian Ocean, but it happens on a body that measures only about one-quarter of Earth’s diameter. When Europa comes closer to the planet, the moon gets stretched like a rubber band, with the ocean height at the long ends rising nearly 100 feet (30 meters). These tides occur because Europa travels around Jupiter in a slightly oval-shaped orbit. She is the lead author of a paper in the September–October issue of Icarus that describes the results.Įuropa’s network of crisscrossing cracks serves as a record of the stresses caused by massive tides in the moon’s global ocean. 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 in Greenbelt, Maryland. “One of the mysteries of Europa is why the orientations of the long, straight cracks called lineaments have changed over time. 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 ocean, and even how long the ocean has been liquid. Credit: NASA/JPL-Caltech/University of Arizonaīy using data from NASA’s Galileo spacecraft to analyze the distinctive cracks lining the surface of Europa, NASA scientists determined that Europa likely rotated around a tilted axis at some point. This mosaic image was taken by NASA’s Galileo satellite, which flew past this moon of Jupiter six times between 19. The distinctive cracks crisscrossing Europa’s icy surface are clues to the stresses that this moon of Jupiter has experienced. ![]()
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