Understanding the Ice-shell Evolution of Europa and Enceladus from Geomorphological Observations and Experimental-Analogue Modeling
- Author(s): Leonard, Erin Janelle
- Advisor(s): Yin, An
- et al.
Although sub-surface oceans have been detected on Europa and Enceladus, two of the best studied icy satellites in the outer solar system, the birth and evolution of these oceans and their impacts on the tectonic history of the overlying ice shells remain poorly understood. One of the fundamental questions regarding Europa and Enceladus is how the apparently young surfaces, evident from a lack of impact craters, are resurfaced and how this is related to the evolution of the underlying oceans. In this dissertation, I address this question by examining the cross-cutting relationships and spatial association of key morphological features on the two satellites. The results are expressed as sets of geomorphological maps that quantify the spatial and temporal relationships of the geologic features, which can then be related to the physical states of the ice shells.
For the Europa studies, I constructed a global geologic map at 1:15M scale and the first detailed structural-geology map using the highest-resolution (~14 m/pixel) Galileo SSI images with a corresponding regional geological map for context. The main mapping results of my work include: (1) establishment of a global three-stage evolutionary model of the features on Europa’s surface, and (2) recognition of a transition from distributed to discrete deformation potentially resulting from ice-shell cooling and thickening. In order to further understand how the observed morphological relationships have developed, I used a gravity-scaled physical analogue model to analyze the formation of the ridged plains, the oldest surface features visible on Europa. I performed a series of two-layer physical analogue experiments, with a brittle layer above a ductile layer. The model permits a systematic test of two competing hypotheses for the formation of the ridged plains: horizontal ice-shell compression versus horizontal ice-shell extension. With these experiments, I showed that the similarity between the experimentally generated landscape and the observed ridged plains morphology strongly favors the compression mechanism.
For the Enceladus study, I conducted a systematic investigation of high-resolution Cassini ISS images focusing on establishing regional structural relationships and the kinematics of major shear zones across the southern part of the Leading Hemisphere Terrain (LHT). The study area displays a unique morphological feature informally known as the “cratered islands” and referred to in this study as the relic cratered blocks. The blocks are characterized by isolated heavily cratered domains surrounded by younger ridged terrain that is essentially free of craters. The main finding of this work is that the relic cratered blocks have experienced vertical-axis rotation during distributed ductile strike-slip shear deformation. In order to understand how the ductile shear deformation evolved, I developed an analytical thermo-mechanical model that relates the shear strain rate of ductile deformation constrained by surface observations to the heat flow during the formation of the deformed features. The model results indicate that the ice shell of Enceladus likely experienced several periods of intense but local heating (> 0.3 W/m2), causing ice-shell thinning, ductile flow, and local resurfacing.
By evaluating the formation mechanisms of first-order surface morphological features on Europa and Enceladus, I show that the predicted past ice-shell thicknesses on Europa and Enceladus are considerably thinner than their current values. This in turn implies that the underlying oceans may have experienced episodic development caused by the evolution of the overlying ice shell. As a result, this may have an effect on the potential habitability of the underlying oceans if the thermal and chemical environment is not stable over geologic time.