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.

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.

Evidence from Arrokoth and comets strongly suggests a very low density for this and similar small Kuiper belt objects. Plausible compositions imply very high porosities, in excess of 70%, and low compaction crush strengths. If so, craters on Arrokoth (especially Sky, its largest) formed largely by compaction of pore space and material displacement. This is consistent with geological evidence from New Horizons imaging. High porosity reduces cratering efficiency in the gravity regime whereas compaction moves it towards a crush strength scaling. Compaction also guarantees that most impactor kinetic energy is taken up as waste heat near the impact point, with momentum transferred to the rest of the body by elastic waves only. Monte Carlo simulations of Sky-forming conditions indicate that the momentum imparted likely separated Arrokoth’s two lobes, but displacement was limited by dissipation at the neck. Unusual strength properties are not required to preserve Arrokoth’s bilobate configuration.