ENCELADUS' ICY LITHOSPHERE: CONSTRAINTS FROM GEOLOGICAL MAPPING AND 3D MODELING OF CRATERED TERRAIN

Enceladus, an icy moon of Saturn, is small in size but riddled with complexity. The understanding of Enceladus’ interior structure — a rocky silicate core, a subsurface ocean, and a dynamic ice shell — has shifted several times and remains uncertain. Current estimates put the total average ice shell thickness anywhere between 14 km and 41 km, with the thinnest portion being in the South Polar Terrain. Numerous studies produced these estimates based largely on gravity, topography, and libration (or astronomical oscillation) observations with relatively large uncertainties. Further, only a handful of studies have sought to constrain the thickness of the brittle layer, each using a combination of the above mentioned methods. These studies also yielded an order of magnitude range of values for this parameter, suggesting that not only is more work necessary but that a numerical model together with visual observations may provide a new perspective on constraining the nature of Enceladus’ ice shell.

Previous work by coauthors included mapping and analyzing the orientations of fractures in the equatorial cratered terrains to characterize the relationship between surface features and brittle layer thickness. There are instances where young, fresh fractures appear to change orientation proximal to large craters, while passing through small craters unchanged, suggesting a controlling factor on stress orientations that has been unaccounted for. We propose that large craters are acting as stress concentrations, with this effect being a function of the thickness of the brittle layer. Here we present the next steps in understanding this relationship: 1) analytical approximations of the thermal and mechanical structure of Enceladus’ ice shell, and 2) preliminary models of the local shallow subsurface stress regime. Based on published assumptions of Enceladus’ min/max heat flux and strain rate, and assuming an isotropic polycrystalline ice structure, we construct a range of depths of the brittle-ductile transition (BDT) in the cratered terrain which are fed into our 3D finite element model to constrain the most realistic scenarios. Testing analytical solutions for the BDT depth with numerical models offers new insight into the nature of Enceladus’ ice shell and the mechanisms that drive tectonic deformation on the surface.