Evidence for non-synchronous rotation of Europa

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Abstract

Non-synchronous rotation of Europa was predicted on theoretical grounds1, by considering the orbitally averaged torque exerted by Jupiter on the satellite's tidal bulges. If Europa's orbit were circular, or the satellite were comprised of a frictionless fluid without tidal dissipation, this torque would average to zero. However, Europa has a small forced eccentricity e ≈ 0.01 (ref. 2), generated by its dynamical interaction with Io and Ganymede, which should cause the equilibrium spin rate of the satellite to be slightly faster than synchronous. Recent gravity data3 suggest that there may be a permanent asymmetry in Europa's interior mass distribution which is large enough to offset the tidal torque; hence, if non-synchronous rotation is observed, the surface is probably decoupled from the interior by a subsurface layer of liquid4 or ductile ice1. Non-synchronous rotation was invoked to explain Europa's global system of lineaments and an equatorial region of rifting seen in Voyager images5,6. Here we report an analysis 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 so in the past), consistent with the possibility of a global subsurface ocean.

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Figure 1: False-colour composite of northern high-latitude region of Europa, produced from images images taken through the 968-nm, 756-nm and green filters.
Figure 2: Distribution of ancient bands and bright wedges (a), intermediate-aged triple-bands and similarly coloured materials (b), and young fractures (c) which cross-cut the triple bands.
Figure 3: Principal stress directions for an incremental eastward shift in the orientation of the surface with respect to the tidal figure of Europa (coloured lines), overlain on a combined Galileo/Voyager mosaic of the anti-jovian hemisphere.

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Acknowledgements

We thank W. McKinnon and M. Golombek for reviews, and C. Phillips for providing the combined Galileo/Voyager mosaic used for Fig. 3. We also thank D. Senske and K. Klaasen for contributing to the acquisition of the data used in this study.

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Correspondence to P. E. Geissler.

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