The Tectonic, Thermal and Magnetic Evolution of Icy Satellites

Abstract

Focusing on Ganymede and Enceladus, this work addresses a number of issues regarding icy satellite evolution, including the ultimate cause of Ganymede's tectonic and cryovolcanic resurfacing, the production of Ganymede's magnetic field, the formation of Ganymede's grooved terrain, and the tectonic and thermal evolution of Enceladus.Both Ganymede's resurfacing and the production of its magnetic field may be attributable to the Galilean satellites' passage through a Laplace-like resonance that excited Ganymede's orbital eccentricity. I examine how resonance passage effects Ganymede's thermal evolution using a coupled orbital-thermal model. Dissipation of tidal energy in Ganymede's ice shell permits high heat fluxes in its past, consistent with the formation of the grooved terrain; however, it also leads to the formation of a thin ice shell, which would have significant consequences for Ganymede's geologic history. In contrast, negligible tidal dissipation occurs in Ganymede's silicate mantle. Thus, passage through a Laplace-like resonance cannot reinvigorate Ganymede's metallic core or enable present-day magnetic field generation.Ganymede's thermal evolution has driven tectonic deformation on its surface, producing numerous swaths of ridges and troughs termed ``grooved terrain.'' Grooved terrain likely formed via unstable extension of Ganymede's lithosphere, but questions regarding instability growth at large strains remain unanswered. To address these questions, I use the finite-element model TEKTON to simulation the extension of an icy lithosphere to examine instability growth at finite strains. My results indicate that large-amplitude deformation requires lower thermal gradients than have been suggested by analytical models; however, the maximum deformation amplitudes produced by our numerical models are lower than typical observed groove amplitudes.Finally, I apply our finite-element modeling to the formation of ridges and troughs on Enceladus. Comparison between our models and photoclinometry profiles of Enceladus' topography indicate that the heat flux was high at the time of ridge and trough formation. Thus, the tectonic resurfacing and high heat fluxes currently observed at Enceladus' south pole may be only the latest episode in a long history of localized resurfacing and global reorientation.