Analyzing Surface Structures on Icy Satellites: a Physical Analogue

Introduction: The existence of global oceans on some icy satellites—Europa and Enceladus, for example—implies the presence of a ductile warm ice layer. However, the role of such a ductile layer in controlling icy-surface deformation has never been systematically investigated nor quantified. We aim to address this issue by combining previous observations from geomorphological mapping of surface features on icy bodies [1] with a unique two-layer analogue model containing an overlying brittle layer and a ductile creeping layer. Although analogue models have been widely used for tectonic studies on Earth [2], they have only rarely been adapted to the studies of the icy-surface deformation [cf., 3] Using the analogue experimental approach and analyzing the effects of a subsurface ductile layer, we will gain understanding of different formation mechanisms for surface features and aid in reconstructing the resurfacing history of icy satellites such as Europa. Procedure: The basic analogue model consists of a ductile, lower viscosity layer underlying a cohesive brittle layer. We initially use therapeutic putty with a measured viscosity of about 10 P•s for our ductile layer and fine-grained sand for our brittle layer. We chose these materials for our initial experiments because they will scale up properly to conditions on Europa. For example, if we scale with the cohesive strength of our sand (~60 Pa) and use well accepted values for Europa [4], we get a scale where about 1 km on Europa corresponds to 1 cm in our model, or 10 scale factor [5]. To set up the experiment, we first layer the putty into a low-walled box. Because the putty is ductile, but still fairly viscous, we let it relax to a flat surface over the course of a few days before adding the desired amount of sand. We also create a 1 cm x 1 cm grid of blue marker sand on top of the experiment to aid us in keeping track of how much extension or contraction is occurring. For experiments where we want to simulate extensional processes, we eliminate one of the box walls causing the ductile material to flow out, creating uniform extension in the surface brittle layer. To simulate compressional processes, we tilt the box at an angle causing the putty to flow towards one end of the box, creating uniform contraction in the brittle surface layer. Initial Results: We began the analogue experiments by varying the thickness of the brittle layer, 0.25 cm, 0.5 cm, and 1.0 cm, to study the effects on the surface features. The experiments are run over the course of about 6 hours, scaling to 10 yrs on Europa [2]. By varying the thickness of the brittle layer, we are effectively studying the effect of varying heat flux on the surface morphology. For example, an icy body with a higher heat flux would have a thinner ice shell and therefore a thinner brittle layer. Extension. In the extension experiments, we see the spacing of resulting normal faults in the brittle layer increase with the increasing thickness of the brittle layer (Fig. 1). We expected this result based on past work on normal fault spacing on Earth [6, 7]; thus this