Chaos formation by porous compaction on Europa

Abstract:

The Galileo Mission provided evidence for a saltwater ocean beneath Europa’s thick outer ice shell. Key analyses have provided significant insight to the properties of the ice shell and its surface, including the characterization of reddish-brown broken, uneven landscapes. Portions of this irregular terrain known as chaos have been found to cover approximately one-quarter of Europa’s surface. Many previous studies have geologically mapped the morphological elements of chaotic terrains, describing broken, rotated, blocky rafts separated by fine matrix material. An important question that remains debated is, how exactly is chaos formed? Current theories that invoke liquid water reservoirs in the shallow subsurface of the ice shell are difficult to support energetically, and solid-state formation mechanisms fail to predict all of the observed characteristics of chaos. Therefore, this study numerically models Europa’s outer ice shell by developing a pseudo-two-dimensional (1.5-D) finite element model that quantifies and characterizes heat transfer and mechanical deformation related to chaos terrain formation. The modeled properties of interest include temperature, strain, porosity, viscosity, thermal conductivity, heat capacity, density, thermal diffusivity, and thermal expansivity. Where applicable, properties are dependent on depth, porosity, and/or temperature. The model is initialized to represent the present-day surface age of the ice shell (~ 65 Myr). We track the dynamic evolution of the ice shell with depth in response to regional and temporal changes in basal heating and tidal heat production. The results of these initial models suggest that long before melting occurs (at energies an order of magnitude lower), significant porous compaction at depth results in the local failure and collapse of overriding icy lithosphere. We therefore hypothesize that geologic interpretations of the formation of chaos through surface collapse and the fluidization of debris are correct, but suggest that the process most readily occurs in response to the viscous relaxation of pores within solid ice. This has implications for the role of chaos formation in conveying surface material to depth, thus affecting habitability.