Convection in ice I with non-Newtonian rheology: Application to the icy Galilean satellites.

摘要

Observations from the Galileo spacecraft suggest that the Jovian icy satellites Europa, Ganymede, and Callisto have liquid water oceans beneath their icy surfaces. The outer ice I shells of the satellites represent a barrier between their surfaces and their oceans and serve to decouple fluid motions in their deep interiors from their surfaces. Understanding heat and mass transport by convection within the outer ice I shells of the satellites is crucial to understanding their geophysical and astrobiological evolution.; Recent laboratory experiments suggest that deformation in ice I is accommodated by several different creep mechanisms. Newtonian deformation creep accommodates strain in warm ice with small grain sizes. However, deformation in ice with larger grain sizes is controlled by grain-size-sensitive and dislocation creep, which are non-Newtonian. Previous studies of convection have not considered this complex rheological behavior.; This thesis revisits basic geophysical questions regarding heat and mass transport in the ice I shells of the satellites using a composite Newtonian/non-Newtonian rheology for ice I. The composite rheology is implemented in a numerical convection model developed for Earth's mantle to study the behavior of an ice I shell during the onset of convection and in the stagnant lid convection regime. The conditions required to trigger convection in a conductive ice I shell depend on the grain size of the ice, and the amplitude and wavelength of temperature perturbation issued to the ice shell.; If convection occurs, the efficiency of heat and mass transport is dependent on the ice grain size as well. If convection occurs, fluid motions in the ice shells enhance the nutrient flux delivered to their oceans, and coupled with resurfacing events, may provide a sustainable biogeochemical cycle. The results of this thesis suggest that evolution of ice grain size in the satellites and the details of how tidal dissipation perturbs the ice shell to trigger convection are required to judge whether convection can begin in the satellites, and controls the efficiency of convection.