Breaking the ice: Widespread fracturing and fragmentation processes in Europa's ice shell

The fracture of the icy shells remains a relatively poorly understood phenomenon, despite the fact that observations have shown it to be a ubiquitous process in the outer solar system satellites. Like Enceladus and some others, Europa has a relatively young surface and exhibits signatures of activity - if not current, it is at least geologically recent. For interior material to erupt onto or resurface the surfaces of these satellites, fractures have to vertically penetrate the ice shell to the depth of a subsurface reservoir or ocean. With this particular work, we focus on the fact that Europa's ice shell is highly fractured, and investigate the effects of such structural heterogeneity in two ways. First, we use a linear elastic fracture mechanics model (LEFM) to show that when surface fractures are closely spaced, the stress required for these fractures to penetrate the entire icy shell thickness is at least an order of magnitude higher than previously estimated by studies that considered only isolated fractures. We also show that fractures that initiate from the base of the icy shell penetrate a larger fraction of the ice shell thickness than closely-spaced surface fractures, providing a more promising avenue to permit full thickness fracture penetration. Moreover, basal fractures are prevalent beneath terrestrial ice shelves, perhaps providing an apt analogy. Second, we use a statistical approach to investigate whether or not there exists a quantifiable critical fracture density, or 'critical mass' of closely-spaced fractures in a given area, beyond which dynamic fragmentation of the shell occurs in localized regions. This is a technique that for the most part has been employed in civil and weapons engineering in the past. By studying the size distribution of fragments in Europa's chaos regions, we seek to back out physical properties of the ice, such as material strength and cohesion properties, and the energy necessary to create a fragmentation event. Characterizing fracture density at different locations and quantifying a critical factor to describe the transition from highly-fractured to collapse/fragmentation region may help to understand the distribution of fracture patterns observed on the surface of Europa and underlying differences within regions of the ice shell.