Inferring the Mean Effective Elastic Thickness of the Outer Ice Shell of
Enceladus from Diurnal Crustal Deformation
Abstract
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.