Coupled convection and tidal dissipation in Europa's ice shell using non-Newtonian grain-size-sensitive (GSS) creep rheology

摘要

We present self-consistent, fully coupled two-dimensional (2D) numerical models of thermal evolution and tidal heating to investigate how convection interacts with tidal dissipation under the influence of non-Newtonian grain-size-sensitive creep rheology (plausibly resulting from grain boundary sliding) in Europa's ice shell. To determine the thermal evolution, we solved the convection equations (using finite-element code ConMan) with the tidal dissipation as a heat source. For a given heterogeneous temperature field at a given time, we determined the tidal dissipation rate throughout the ice shell by solving for the tidal stresses and strains subject to Maxwell viscoelastic rheology (using finite-element code Tekton). In this way, the convection and tidal heating are fully coupled and evolve together. Our simulations show that the tidal dissipation rate can have a strong impact on the onset of thermal convection in Europa's ice shell under non-Newtonian GSS rheology. By varying the ice grain size (1-10mm), ice-shell thickness (20-120km), and tidal-strain amplitude (0-4×10~(-5)), we study the interrelationship of convection and conduction regimes in Europa's ice shell. Under non-Newtonian grain-size-sensitive creep rheology and ice grain size larger than 1mm, no thermal convection can initiate in Europa's ice shell (for thicknesses <100km) without tidal dissipation. However, thermal convection can start in thinner ice shells under the influence of tidal dissipation. The required tidal-strain amplitude for convection to occur decreases as the ice-shell thickness increases. For grain sizes of 1-10mm, convection can occur in ice shells as thin as 20-40km with the estimated tidal-strain amplitude of 2×10~(-5) on Europa.