The Freezing and Fracture of Icy Satellites: Experimental Analog and Stress Analysis

The icy worlds of the outer solar system are a compelling class of bodies to study for their intriguing and diverse geology as well as their habitability potential. Many icy satellites have or have had oceans, some of which are likely to be out of equilibrium and freezing. Capping these oceans, ice shells are covered by extensional features and fractures. In this project, we investigate how the surface features observed on icy satellites are controlled by thermal stresses and the stresses associated with the expansion of water upon freezing. We address this question with a series of analog experiments, freezing spheres of water with various radii. Using a nitrogen-cooled freezing chamber, spheres of water are frozen from the outside in, undergoing cyclic fracture events until a critical fracture splits the ice sphere entirely. Our experiments reveal that the ice shell thickness at which critical fracture occurs is independent of freezing temperature and sphere radius, but controlled by the compressibility of the water. Additionally, the evolution of the pressure in the water shows a regime change upon exsolution of volatiles. This process controls the time between fracture events and the number of events which occur before the critical final fracture. Remarkably, we find that our freezing water spheres generate similar surface features to those observed on icy satellites, including curvilinear fractures with normal or strike-slip motion, tilted ice blocks, and flow features. The abundance of these features depends on the dissolved volatile content in the water, which suggests that the dissolved volatile content in the subsurface ocean can influence the abundance of surface features on icy satellites.