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G Schubert, W. B. Moore (Dept. of Earth and Space Sciences and IGPP, UCLA)
The magnitude and phase of Europa's response to Jupiter's tidal forcing provide information on both the rheology and density as a function of depth within the satellite. Europa experiences both a large, static tide which does not depend on the orbital phase, and two time-varying tides arising from Europa's significant forced eccentricity. The gravitational signature of the static tide has been measured by Galileo, and, with the hydrostatic assumption, shows that Europa is differentiated into a metallic core, silicate mantle, and ~100 km thick water and/or ice layer. The time-varying tides provide information on the frequency-dependent rheology of Europa. Since much work has already been done using a model Europa with a thin elastic sheet over a fluid planet, it is important to determine the effects of a solid, viscoelastic mantle and a potentially thick viscoelastic ice layer. We present calculations of radial deflection, gravity anomaly, and surface stresses as a function of orbital phase for a variety of models of Europa's rheological structure. The response of a thin elastic sheet over a viscoelastic layer of solid ice becomes indistinguishable from that of a thin sheet over a fluid for viscosities less than 1010 Pa s. This value is extremely low for solid ice. Therefore any observations indicative of a decoupled shell would not be compatible with a low-viscosity solid, but rather with a mostly, if not completely, liquid layer.