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 > tex2html_wrap_inline11 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.