Constraining the spatial variability of the thickness of the ice shell of Enceladus (i.e., the crust) is central to our understanding of its thermodynamics and habitability. In this study, we develop a new methodology to infer regional variations in crustal thickness using measurements of tidally-driven elastic strain. As proof of concept, we recover thickness variations from synthetic finite-element models of the crust subjected to diurnal eccentricity tides. We demonstrate recovery of crustal thickness to within ~2 km of true values with < 0.2 km error over spherical harmonic degrees l ≤ 12 (corresponding to half-wavelengths ≥ 60 km). Our computed uncertainty is significantly smaller than the inherent ~10 km ambiguity associated with inferring variations in crustal thickness solely from gravity and topography measurements. We therefore conclude that measuring elastic strain provides a relatively robust approach for probing crustal structure at Enceladus.

The thickness of the outer ice shell plays an important role in several geodynamical processes at ocean worlds. Here we show that observations of tidally-driven diurnal surface displacements can constrain the mean effective elastic thickness, ˜del, of the ice shell. Such estimates are sensitive to any significant structural features that break spherical symmetry such as faults and lateral variation in ice shell thickness and structure. We develop a finite-element model of Enceladus to calculate diurnal tidal displacements for a range of ˜del values in the presence of such structural heterogeneities. We find that the presence of variations in ice shell thickness can significantly amplify deformation in thinned regions. If major faults are also activated by tidal forcing—such as Tiger Stripes on Enceladus—their characteristic surface displacement patterns could easily be measured using modern geodetic methods. Within the family of Enceladus models explored, estimates of ˜del that assume spherical symmetry a priori can deviate from the true value by as much as ~ 20% when structural heterogeneities are present. Such uncertainty is smaller than that found with approaches that rely on static gravity and topography (~ 250%) or analyzing diurnal libration amplitudes (~ 25%) to infer ˜del at Enceladus. As such, despite the impact of structural heterogeneities, we find that analysis of diurnal tidal deformation is a relatively robust approach to inferring ˜del.

Ceres’ regolith contains water ice that has receded in response to insolation-driven sublimation. Specially targeted, high spatial-resolution measurements of hydrogen by Dawn’s Gamma Ray and Neutron Detector reveal elevated hydrogen concentrations in and around Occator, a young, 90-km diameter, complex crater located at 19.82N where near-surface ice is not expected. The excess hydrogen is explained by impact excavation of water-rich outer crustal materials and their emplacement in the crater floor and ejecta blanket. This is supported by thermophysical models that show water ice could survive at sub-meter depths, given Occator’s relatively young age (~20 Myr). We hypothesize that the regolith can be replenished with ice from large impacts and that this process partially controls the distribution and depth of near surface ice. This is supported by results from Occator and similarities in the global distribution of hydrogen and the pattern of large craters (20-100 km diameter).