Rotation of Europa: Constraints from terminator and limb positions

Tidal torques on Europa due to Jupiter would tend to drive Europa's rotatio n to a rate slightly faster than synchronous, unless synchroneity is mainta ined by a permanent asymmetry in Europa's mass distribution. Moreover, glob al 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 rot ation rate is made from the positions of surface features relative to the t erminator in a Galileo image and comparing the results with similar measure ments of the positions of the same features relative to the terminator in a Voyager 2 image taken 17 years earlier. Any nonsynchronous rotation must b e slower than a few tenths of a degree in 17 years (a complete rotation in >10(4) years) relative to the direction of Jupiter. (C) 1999 Academic Press .