ENCELADUS’S TECTONIC STRESS FIELD

An Yin was an extraordinary geologist and planetary scientist. He was creative, insightful, and rigorous. His infectious enthusiasm inspired us to start work on background tectonic stresses on Enceladus, an icy satellite of Saturn. Water-rich plumes erupt in the South Polar Terrain (SPT) along four parallel tiger-stripe fractures. There are cyclic variations in the plume flux, which match the orbital period around Saturn, but with a phase delay of ~56 degrees. This ~5-hour delay has been observed between the predicted maximum fracture opening due to expected tidal stresses, and the timing of peak eruptions. An unconsidered possibility is the additional effect of tectonic stresses due to variations in the thickness, density, and elevation of the icy shell when superposed on the stress field generated by tidal effects, which vary spatially and temporally. We calculate the global tectonic stress of Enceladus driven by the lateral variations of icy shell properties. We use a publicly available global digital topographic model and assume Airy isostasy to model the icy-shell structure of the moon. We calculate the resulting gravitational potential energy (GPE) and the mean outward traction of each icy shell column on each other at .25° resolution. We use the commercial finite element package ABAQUS to compute the stress field resulting from the global GPE variations. We test variations in icy shell thickness and density. Tectonic stresses are comparable in magnitude to tidal stress. The most compressive horizontal stress is subparallel to the tiger stripe fracture zones. Our results support the hypothesis that the gravitational spreading of the ice shell complemented the tidal stresses in creating and maintaining motion along the tiger stripe fractures. Our next steps will include more sophisticated models of icy shell structures and the combined effects of tidal and tectonic stresses to compute the effects on the phase delay.