Stress Field Above an Ice Cauldron on Europa

Abstract:

Chaos and Macula are geological features on Europa where the pre-existing surface material has been broken into discrete blocks surrounded by matrix materials and stand above the surrounding terrain with a dome like topography. Schmidt et al. (2011) recently proposed that Maculae and Chaos are analogue to terrestrial ice cauldron. A melt body would form in the ice shelf above an ascending warm ice diapir, with briny water erupting along the resulting cracks, and doming a result of crystallization. We study here the deformation and stress fields generated by plume ascent, ice melting, and subsequent crystallization to test whether the ice cauldron analogy can explain the morphology of cracks and the topographic signature of maculae.

We consider an axisymmetric finite element model of an ice shell built with ABAQUS. Ice is regarded as an non-associated elastic-plastic solid with Mohr-Coulomb failure criterion. We determine the stress field and in the ice shell and discuss the resulting topography and ice failure mode. Uplift is initiated by reducing the weight of a predefined diapir at the base of the crust. Melting is implemented by increasing in density and reducing the shear strength of a predefined underground lens. The material inside the lens remains solid but its modified material properties are more fluid-like. The stress fields generated in our models give a first indication of the failure mode and direction of cracks in the ice. To explain the observation of impure ice in the matrix of maculae, tensile cracks must propagate from the melt lens to the surface and allow extraction of brine from the subsurface water body. However, compressive stresses dominate in our models, even under the combined effect of diapiric uplift and melting, so that shear failure is common but tensile cracks propagating from the melt lens unlikely. Expansion of the ice when the lens crystallizes has little effect beyond negating stresses generated by melting. However, plastic strain is non-recoverable, so that a circumferential graben forms along the periphery of the melt lens and may join the underground melt lens. Shallow radial crack develop but do not propagate to the melt lens. Our models explain the observations of radial cracks and circumferential graben at Thera Macula but predict the emplacement of impure ice only along the graben.