One‐hundred‐km‐scale basins on Enceladus: Evidence for an active ice shell

Stereo‐derived topographic mapping of ∼50% of Enceladus reveals at least 6 large‐scale, ovoid depressions (basins) 90–175 km across and 800‐to‐1500 m deep and uncorrelated with geologic boundaries. In contrast, the south polar depression is larger and apparently shallower and correlates with active resurfacing. The shape and scale of the basins is inconsistent with impact, geoid surface deflections, or with dynamically supported topography. Isostatic thinning of Enceladus' ice shell associated with upwellings (and tidally‐driven ice melting) can plausibly account for these basins. Thinning implies upwarping of the base of the shell of ∼10–20 km beneath the depressions, depending on total shell thickness; loss of near‐surface porosity due to enhanced heat flow may also contribute to basin lows. Alternatively, the basins may overly cold, inactive, and hence denser ice, but thermal isostasy alone requires thermal expansion more consistent with clathrate hydrate than water ice.