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Tides on Europa: the membrane paradigm
Beuthe, M. (2015). Tides on Europa: the membrane paradigm. Icarus 248: 109-134. https://hdl.handle.net/10.1016/j.icarus.2014.10.027
In: Icarus. Elsevier. ISSN 0019-1035; e-ISSN 1090-2643, more
Peer reviewed article  

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Author keywords
    Europa; Tides, solid body; Tectonics; Planetary dynamics

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Abstract
    Jupiter's moon Europa has a thin icy crust which is decoupled from the mantle by a subsurface ocean. The crust thus responds to tidal forcing as a deformed membrane, cold at the top and near melting point at the bottom. In this paper I develop the membrane theory of viscoelastic shells with depth-dependent theology with the dual goal of predicting tidal tectonics and computing tidal dissipation. Two parameters characterize the tidal response of the membrane: the effective Poisson's ratio 9 and the membrane spring constant A, the latter being proportional to the crust thickness and effective shear modulus. I solve membrane theory in terms of tidal Love numbers, for which I derive analytical formulas depending on A, 9, the ocean-to-bulk density ratio and the number 14 representing the influence of the deep interior. Membrane formulas predict h(2) and k(2) with an accuracy of a few tenths of percent if the crust thickness is less than one hundred kilometers, whereas the error on 12 is a few percents. Benchmarking with the thick-shell software SatStress leads to the discovery of an error in the original, uncorrected version of the code that changes stress components by up to 40%. Regarding tectonics, I show that different stress-free states account for the conflicting predictions of thin and thick shell models about the magnitude of tensile stresses due to nonsynchronous rotation. Regarding dissipation, I prove that tidal heating in the crust is proportional to Im(A) and that it is equal to the global heat flow (proportional to Im(k(2))) minus the core-mantle heat flow (proportional to Im(k(2)degrees)). As an illustration, I compute the equilibrium thickness of a convecting crust. More generally, membrane formulas are useful in any application involving tidal Love numbers such as crust thickness estimates, despinning tectonics or true polar wander.

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