STRESSING OUT: A LOOK AT THE STRESS REGIME OF ENCELADUS’ SOUTH POLE

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 cyclic eruption of plumes sourced from a series of parallel ‘‘tiger-stripe” fractures (TSF) (Porco et al., 2006). The cyclic nature of the plume’s eruption has been attributed to diurnal variations of tidal stresses acting on the moon. 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 as driven by orbital eccentricity (Nimmo et al., 2014). Previous studies introduce physical libration of the moon (Hurford et al. 2009; Nimmo et al. 2014), as well as rheological modeling (Běhounková et al. 2015), to reconcile the observed time delay. In general, these prior models assume a tectonically stress free ice shell when modeling the opening and closing of the TSF’s. The work that we present here is a first order attempt to tackle the validity of assuming a tectonically stress free ice shell when modeling the opening and closing of the TSFs. We investigate the total stress as a result of three sources: tidal stress creating an averaged bulge figure, stress induced by physical libration, and tectonic stresses. We postulate that the observed delay in eruption is a result of the relative difference in magnitude of the three stress sources. The mathematical framework we employ to calculate tidal stress and libration-induced stress follows that of Hurford et al. 2009. The derivation and determination of the tectonic stress tensor in the South Polar Terrain is our own work. 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.