New global topography of Enceladus: Hypsometry, basins, spherical harmonics, shell thickness, and true polar wander revisited

A new global high-resolution stereogrammetric and photogrammetric topographic map and updated 3-color global mosaic of Enceladus reveal that the shape of this active icy ocean world is distorted from a simple triaxial ellipsoid. Although the topographic range (high-to-low) of Enceladus, when referenced to its global triaxial figure, is the narrowest among icy bodies mapped globally thus far, the global DEM indicates a pronounced zonal latitudinal zoning of topography, with the north polar region and mid-southern latitudes being similar to 300-400 m higher and south polar and mid-northern latitudes being similar to 400 m lower on average than the triaxial figure. Dynamically, both north and south poles are low, but the south remarkably so, by >1 km. Spherical harmonic analysis also indicates the tidal axis of the body is located similar to 4 degrees W of the current IAU-defined prime meridian. Superposed on this global pattern are topographic features associated with geologic units within each terrain, as well as several prominent regional 100-km-scale basins, 1-to-1.8 km deep, previously reported. The shapes of these large basins depend on the terrains they formed in; those in older cratered terrains are relatively simple (i.e., ovate) in shape, but those in resurfaced terrains are more complex in shape, often correlating with geologic features and units within those terrains. Thus, while these basins exhibit no explicit local-scale geologic deformation or disruption of the icy shell in association with their elevations/formation, they are partially controlled by the physical characteristics of the terrains they formed in, suggesting that the icy shell can be heterogenous in character. Most of these basins occur on the trailing hemisphere and although a few of these are aligned, we find that they do not form a global great circle pattern, weakening previous arguments for true polar wander. Our preferred hypothesis is that these basins are due to isostatic adjustment of the ice shell in response to thermal anomalies from below and suggest that they may be surface manifestations of diffused hydrothermal plumes originating at the surface of Enceladus' rocky core. If so, similar patterns in core activity might be revealed once global topographic mapping is complete on Europa.