This tilt can 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.
Lead author Alyssa Rhoden, a postdoctoral fellow with Oak Ridge Associated Universities who is working at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said that one of the mysteries of Europa is why the orientations of the long, straight cracks called lineaments have changed over time.
She said that 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'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 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 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.
Rhoden and Goddard co-author Terry Hurford 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.
The paper has been published in Icarus.
--ANI (Posted on 19-09-2013)