Defining the Contribution of Fragmentation to Icy World Geology

Abstract:

The fracture and failure of the outer planet satellites' icy shells are not only an observable record of stress and activity throughout their evolution, they are also key mechanisms in the interaction of surface and subsurface material and thus crucial aspects of the study of crustal overturn and ice shell habitability. Recent work suggests that Europa’s chaos terrain formation may include a collapse phase, with the eventual appearance of the chaos terrain determined in part by fracture density in the ice. Similarly, Enceladus' subdued south polar terrain is now known to exist above a trapped south polar sea. With this in mind, the work to be presented combines fractal analysis and a novel fragmentation physics model to characterize the transition from fractured ice to fragmented/collapsed ice and thus, implications for crustal overturn in Europa’s ice shell using terrestrial glaciological studies to develop and test plausible processes. While intuition may suggest that ice "explosions" and simple elastic crack propagation are different, a fast-propagating fracture or other abrupt shift to the status quo (e.g., thinned shell) of the system is in effect very similar, as damage radiates through the system. In studying the size distribution of fragments in Europa’s chaos regions, it is possible to back out physical properties of the ice, e.g., material strength, cohesion properties and most importantly, energy necessary to create such a fragmentation event. Fragmentation theory describes the breakage of a body into several pieces. When integrated, our models of crack propagation and fragmentation account for the three modes of fracturing: initiation, propagation, and interaction, and allows for characterization of the energy necessary for fragmented topography (e.g., chaos) to form within an ice shell to better understand the implications for surface overturn. We have modeled fracture formation and coalescence into fragments that results in a similar distribution to that of the fragments in Europa’s chaos regions. We have also recreated the observed “sunken” topography in Europa’s chaos regions over water pockets within the shell and, in a similar but altered setting, the depression at South Pole of Enceladus. We present comparisons of styles of failure to the activity and geology of these icy worlds.