NASA Scientists Offer New Explanation of Europa's Puzzling Ice Cracks

Europa, the icy moon orbiting Jupiter, likely revolved around a tilted axis at some point, according to NASA scientist who analyzed the moon's distinctive ice cracks.

The researchers say the tilt could influence calculations of how much of the moon's history is entombed in its frozen shell, offer insights into how much heat is generated by tides in the moon's ocean and perhaps even tell how long Europa's ocean has been in liquid form.

"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, the lead author of the study, which was published in the journal Icarus.

The crisscrossing network of Europa's ice cracks serves as a record of the environmental stresses caused by massive tides in the moon's global ocean. The huge tides occur because Europa orbits around Jupiter in an ovular shape.

Rhoden and her colleagues say that when Europa comes close to Jupiter, the moon gets stretched out like a rubber band, creating a fluctuation of ocean water levels of nearly 100 feet. When the moon orbits farther away from Jupiter, it returns to its spherical shape. The moon's ice layer must accommodate to these changes, but as it does, the moon succumbs to stress, which creates the ice cracks.

But why the cracks point in different directions over time, even though the same side of the moon always faces Jupiter, has long been a puzzle.

A prominent explanation for why is that the moon's frozen outer shell might rotate slightly faster than the moon itself orbits Jupiter. If this is true, it would mean that the same part of the ice shell would not always face Jupiter.

But after running three models simulating various hypothesis to explain the cracks, Rhoden and co-author Terry Hurford contend that out-of-sync rotation is most likely not the cause and that the cracks are better explained by a tilted axis

Rhoden and Hurtford ran simulations and tests based on images taken by NASA's Galileo spacecraft during its nearly eight-year mission, which began in 1995.

After presenting their findings to their colleagues, the evidence was enough that Richard Greenberg, the University of Arizona professor who proposed the idea of out-of sync rotation, was satisfied that his theory could no longer hold up.

"By extracting new information from the Galileo data, this work refines and improves our understanding of the very unusual geology of Europa," said Greenberg, who was an academic advisor to both Rhoden and Hurford in the past.

The analysis does not, however, explain or specify when the moon's tilting would have occurred. Currently, no measurements of the tilt of Europa's axis have been done. Doing so will be a primary goal of future missions to the distant moon.

"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.