FIRST TECTONIC STRESS MAP ACROSS ENCELADUS’ SPT AND POSSIBLE DYNAMIC CAUSES

Enceladus is a small moon of Saturn (radius ~250 km), distinguished by its uniquely active south pole. The moon’s geological activity is expressed as regularly erupting plumes sourced from a series of parallel ‘‘tiger-stripe” fractures (TSF) (Porco et al., 2006). The cyclic nature of these eruptions are attributed to diurnal variations of tidal stress (Hurford et al., 2007). There is, however, a mismatch in timing between Cassini’s observations of peak eruption and what is predicted by the theory of tidally modulated cracks (Nimmo et al., 2014). Existing models have attempted to reconcile the plume timing discrepancy by invoking stress relaxation in a viscoelastic ice sell (Běhounková et al., 2015). However, such an approach assumes the stress in the ice shell to be entirely induced by tidal stress, neglecting the role tectonically induced stress play in order to support the high (>1 km) topographic relief around the moon’s south pole (Schenk and McKinnon, 2009). We propose to address this by relaxing the assumption of a tectonic stress-free ice shell and offering an analytical tensor analysis decomposing tidal and tectonic stresses in the ice shell. We thus investigate the total stress as a result of three sources: tidal stress from a bulged figure, stress induced by physical libration (Hurford et al., 2009), and tectonic stress. With the aforementioned framework, we derive the magnitude and direction of tectonic stress at numerous points along the active TSFs, resulting in a comprehensive tectonic stress map of Enceladus’ South Polar Terrain (SPT). We find tectonically derived stresses to be non-trivial; while these results are perhaps not surprising, the need for a more comprehensive plume eruption model, one that includes both tidal and tectonic stresses in its mathematical framework, is evident. We posit that perhaps the fundamental question of eruption timing may lie in cultivating this understanding.