Plume Activity and Tidal Deformation on Enceladus Influenced by Faults and Variable Ice Shell Thickness

Abstract

We investigated the effect of variations in ice shell thickness and of the tiger stripe fractures crossing Enceladus' south polar terrain on the moon's tidal deformation by performing finite element calculations in three-dimensional geometry. The combination of thinning in the polar region and the presence of faults has a synergistic effect that leads to an increase of both the displacement and stress in the south polar terrain by an order of magnitude compared to that of the traditional model with a uniform shell thickness and without faults. Assuming a simplified conductive heat transfer and neglecting the heat sources below the ice shell, we computed the global heat budget of the ice shell. For the inelastic properties of the shell described by a Maxwell viscoelastic model, we show that unrealistically low average viscosity of the order of 1013 Pa s is necessary for preserving the volume of the ocean, suggesting the important role of the heat sources in the deep interior. Similarly, low viscosity is required to predict the observed delay of the plume activity, which hints at other delaying mechanisms than just the viscoelasticity of the ice shell. The presence of faults results in large spatial and temporal heterogeneity of geysering activity compared to the traditional models without faults. Our model contributes to understanding the physical mechanisms that control the fault activity, and it provides potentially useful information for future missions that will sample the plume for evidence of life. Key Words: Enceladus—Tidal deformation—Faults—Variable ice shell thickness—Tidal heating—Plume activity and timing. Astrobiology 17, 941–954.

Enceladus's crust as a non-uniform thin shell: II tidal dissipation

“…Thus, including the 1:1 forced libration increases tidal dissipation (by about 28%, see Eq. (49)), as already shown for a homogeneous body [Wisdom, 2004], instead of decreasing it as concluded by Běhounková et al [2017]. Introducing the forced libration into the tidal potential, as above, makes sense for a completely solid body, in which the differential rotation between layers is negligible.…”

“…In a previous study of the topic, Běhounková et al [2017] used the finite-element method (FEM) of Souček et al [2016] to solve for the deformations of an elastic shell with non-uniform thickness, and predicted several tens of GW of tidal heating by assuming a shell of uniform viscosity. This last assumption is not realistic for a conductive shell because viscosity increases by orders of magnitude from the bottom of the shell to the surface; the total power dissipated in the shell is actually much lower.…”

Internal Energy Dissipation in Enceladus's Subsurface Ocean From Tides and Libration and the Role of Inertial Waves

“…Tidal dissipation is the most likely source of power for the observed geological activity (Nimmo et al, ). Current models, however, have so far been unable to account for the observed ∼10 GW of endogenic heat flow emanating from the south polar region (Howett et al, ; Spencer et al, ) without invoking the presence of a highly porous (“fluffy”) solid inner core (Choblet et al, ; Roberts, ) or a convecting icy crust (Běhounková et al, , ). As suggested by Barr and McKinnon (), this latter hypothesis appears unlikely in view of the relatively small thickness of the crust (∼20 km) deduced from geodetic observations (Beuthe et al, ; Čadek et al, ).…”

Near/far side asymmetry in the tidally heated Moon

“…Even if the early tidal heating asymmetry found here does not directly induce asymmetric crustal growth through variations in basal heat flux, it might serve as a trigger for continued asymmetric crustal growth. If crustal growth is unstable (as discussed by Garrick-Bethell et al 2010), with thicker and stronger regions tending to grow faster, as suggested by the stress amplification scenario for hard-shells (Behounkova et al, 2017;Beuthe, 2018), then early asymmetric tidal heating might be amplified via later crustal growth, after tidal heating becomes predominantly quadrupolar. Alternatively, we could also conclude that an early episode of asymmetric tidal heating does not affect the later development of lunar crustal thickness variations.…”

“…Recent analytical studies of elastic shells with variable thickness predict a phenomenon called stress amplification where stress is inversely proportional to the shell thickness (Behounkova et al, 2017;Beuthe, 2018). Physically, thinner and weaker areas of the shell deform and dissipate more than thicker areas which are stiffer.…”