Rotation of Europa: Constraints from Terminator and Limb Positions
G.V. Hoppa, R. Greenberg, P.E. Geissler, J. Plassmann, B.R. Tufts (U. Arizona), Galileo SSI Team
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
[Greenberg and Weidenschilling (1984) Icarus 58,186]. 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 non-synchronous rotation (e.g. Geissler et al. (1997),
LPSC XXVIII,401.). A direct measurement of Europa's rotation rate has
been made by measuring 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 seventeen years earlier. These
measurements reveal that any prograde non-synchronous rotation (as
predicted by Greenberg et al.) is less than a few tenths of a degree in
17 yr. (a complete rotation in > 
yr.). Additional Galileo &
Voyager images of Europa have included the limb, permitting an
independent (albeit less precise) measurement of the rotation rate of
Europa by measuring the position of features with respect to the limb.
The position of features in these images is consistent with the rotation
rate limit given above.