Curtain-Based Maps of Eruptive Activity in Enceladus’ South-Polar Terrain at Fifteen Cassini Epochs

We map surface eruptive activity in Cassini images of Enceladus’ south-polar terrain (SPT) at fifteen epochs spanning late 2009 to late 2015 using a refined curtain approach derived from that of Spitale et al. (2015; Nature 521; 57–60) (S2015). This refined approach gives a better representation of the uncertainties in identifying source fractures by allowing for multiple candidate sources to be identified for an observed eruption. Our analysis includes the five epochs studied by S2015, so those epochs were independently re-examined here. Differences between the common epochs in the two analyses are primarily due to the refined approach, and a more conservative philosophy used in this analysis. About 80% of the currently active fracture system in the SPT (by length) was observed to be erupting at a level detectable in Cassini images during every epoch. On/off variability is observed almost exclusively at the fringes of the fractures system. These results support a model where the surface and the ocean are in direct contact via fractures within an ice shell that thickens away from the pole. No definitive connection between surface activity and the roughly factor-of-three plume optical depth variation with mean anomaly (Hedman et al. 2013, Nimmo et al. 2014) is made, but our results do not rule out such a correlation. Indeed, there is weak support for a scenario in which the eruptive activity is modulated by tides, with only the fractures at the fringe of the SPT ever turning off completely during the tidal cycle.