Jupiter's Frozen Moon Europa Likely Spun Around Tilted Axis to Form Cracks

Jupiter's frozen moon, Europa, is known for the long, straight cracks found on its surface. Now, NASA scientists may have found a reason for these cracks. It turns out that the moon might have once spun around a tilted axis, revealing the reason behind its unusual surface.

"One of the mysteries of Europa is why the orientations of the long, straight cracks called lineaments have changed over time," said Alyssa Rhoden, one of the researchers, in a news release. "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."

Europa has a global ocean with massive tides that can result in these crisscrossing cracks across its icy shell. These tides occur because Europa travels around Jupiter in a slightly oval-shaped orbit. When Europa comes closer to the planet, the moon becomes stretched like a rubber band. This causes the ocean height at the long ends of the moon to rise nearly 100 feet. When it moves back away from Jupiter, Europa relaxes back into the shape of a ball.

The moon's ice layer has to stretch and flex in order to accommodate these massive changes. When the stresses become too great, though, this ice cracks. Yet this doesn't explain why the cracks point in different directions over time, even though the same side of Europa always faces Jupiter.

In order to find that out, researchers used images taken by NASA's Galileo spacecraft during its eight-year mission. They then compared the pattern of cracks in a key area near Europa's equator to predictions based on three different explanations. The first set of predictions was based on the rotation of the ice shell. The second assumed that Europa was spinning around a tilted axis and the third was that the cracks were laid out in random directions.

So what did they find? It turns out that Europa is tilted, which explains the unusual cracks. Called precession, this effect looks very much like what happens when a spinning toy top has started to slow down and wobble.

The findings are crucial for better understanding this icy moon. In fact, a tilt could affect the estimates of the age of Europa's ocean. Because tidal forces are thought to generate the heat that keeps Europa's ocean liquid, a tilt in the spin axis might suggest that more heat is generated by tidal forces. This, in turn, might keep the ocean liquid longer.

Currently, researchers aren't sure exactly when this tilt could have occurred. So far, measurements have not been made of the tilt of Europa's axis. Future research, however, will find the answer.

The findings are published in the journal Icarus.