EVIDENCE FOR NONSYNCHRONOUS ROTATION OF EUROPA

Non-synchronous rotation of Europa was predicted on theoretical ground s(1), by considering the orbitally averaged torque exerted by Jupiter on the satellite's tidal bulges, If Europa's orbit were circular, or t he satellite were comprised of a frictionless fluid without tidal diss ipation, this torque would average to zero. However, Europa has a smal l forced eccentricity e approximate to 0.01 (ref. 2), generated by its dynamical interaction with Io and Ganymede, which should cause the eq uilibrium spin rate of the satellite to be slightly faster than synchr onous, Recent gravity data(3) suggest that there may be a permanent as ymmetry in Europa's interior mass distribution which is large enough t o offset the tidal torque; hence, if non-synchronous rotation is obser ved, the surface is probably decoupled from the interior by a subsurfa ce layer of liquid(4) or ductile ice(1), Non-synchronous rotation was invoked to explain Europa's global system of lineaments and an equator ial region of rifting seen in Voyager images(5,6). Here we report an a nalysis of the orientation and distribution of these surface features, based on initial observations made by the Galileo spacecraft. We find evidence that Europa spins faster than the synchronous rate (or did s o in the past), consistent with the possibility of a global subsurface ocean.