By analyzing the distinctive cracks lining the icy face of Europa, NASA scientists found evidence that this moon of Jupiter likely spun around a tilted axis at some point.
Europa’s tilt could influence calculations of how much of the moon’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.
“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 in Greenbelt, Md. She is the lead author of a paper in the September-October issue of Icarus that describes the results.
Europa’s network of crisscrossing cracks serves as a record of the stresses caused by massive tides in the moon’s global ocean. These tides occur because Europa travels around Jupiter in a slightly oval-shaped orbit. 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). 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 moves farther from Jupiter, it relaxes back into the shape of a ball.
The moon’s ice layer has to stretch and flex to accommodate these changes, but when the stresses become too great, it cracks. The puzzling part is why the cracks in Europa’s icy layer point in different directions over time, even though the same side of Europa always faces Jupiter.
A leading explanation has been that Europa’s frozen outer shell might 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.