Rotation of Europa: Constraints from Terminator and Limb Positions

Tidal torques on Europa due to Jupiter would tend to drive Europa's rotation to a rate slightly faster than synchronous, unless synchroneity is maintained by a permanent asymmetry in Europa's mass distribution. Moreover, global fracture patterns on Europa's surface, as well as the complex fine-scale tectonics revealed by Galileo high-resolution imagery, may be evidence of stress due to nonsynchronous rotation. A direct measurement of Europa's rotation rate is made from the positions of surface features relative to the terminator in a Galileo image and comparing the results with similar measurements of the positions of the same features relative to the terminator in a Voyager 2 image taken 17 years earlier. Any nonsynchronous rotation must be slower than a few tenths of a degree in 17 years (a complete rotation in >10⁴years) relative to the direction of Jupiter.