The effects of laterally varying icy shell structure on the tidal response of Ganymede and Europa

Abstract

Abstract We use a finite‐element model to solve for the response of Ganymede and Europa to tidal forcing from Jupiter, using various icy shell models with laterally variable (3‐D) structure. In all cases, the shell is assumed to be underlain by a liquid‐water ocean. Icy shells with laterally varying thickness are derived from a thermal conduction model. Three‐dimensional shear modulus profiles for the shell are built either from a conduction model or, for Europa, by assuming a hemispherical difference in composition. Icy shell structures with a nonglobal ocean are built for Ganymede. Using these shell structures to calculate the tidal response of Ganymede and Europa, we conclude the following: (1) the presence of lateral variations in thickness or in shear modulus would not degrade future attempts to use tidal observations to decide on the existence or absence of a liquid ocean and to determine the mean icy shell thickness. (2) Given accurate enough observations, the presence of lateral variations in thickness or in shear modulus could be determined by searching for nondegree‐2 components in the tidal response. (3) In the absence of significant viscous convective flow in the shell, the effects of a laterally varying shear modulus on the tidal response would be smaller than those of a laterally varying shell thickness. (4) If the shell is partially grounded, tidal observations of either gravity or uplift would be able to roughly differentiate regions where the ice is grounded from those where it is floating.