Dynamic ice shell evolution of Enceladus

Many icy bodies in the outer solar system have a cryosphere of solid ice covering a liquid ocean. Studies of thermal and compositional evolution in these ice shells are often modeled in a static domain in which thickness of the shell is constant and assumes thermal equilibrium. However, this may not be a good assumption for Saturns moon Enceladus, as it may have experienced variable tidal heating and resurfacing events that could have resulted in evolving ice shell thickness. We investigated a dynamic boundary between the ocean and ice shell throughout the thermal evolution of Enceladus. We found that maximum growth in the ice shell from freezing of the subsurface ocean is limited by the water available due to the presence of the rocky core, which provides a solid lower boundary. Based on this lower boundary, Enceladus can only accommodate a total ice shell thickness of ~66 km if frozen completely. For tidal heat ranges tested from 3.021 x 10-7 1.561 x 10-6 W m-3, the ice shell develops a convective region in the lower shell. Thickness of this convective layer decreases as tidally dissipated energy increases. Trends for ocean pressure are generally analogous to shell thickness seen in ranges of 10s 100s of MPa when not accounting for pressure loss from other mechanisms. An evolving ice shell may have implications for features found in south polar terrain, such as the array of four tiger stripes or linear troughs from which jets of vapor and ice escape. As water freezes, the density contrast between ice and water causes the total volume of combined liquid and ice layers to increase, thus increasing the pressure [1]. A mechanism to relieve pressure in the ocean is necessary on Enceladus, such as increasing satellite radius or removing water. Otherwise, these pressures could exceed ice strength and induce fracturing in the ice shell. However, continued pressurization after fracturing of the ice shell could also drive geyser activity consistent with the observed jets. Next, we will model initiation and propagation of fractures caused by ocean pressures using finite-element modeling of a pressurized ice shell. The feedback of pressure reduction is incorporated into the thermal model to improve fidelity of long-term thermal and mechanical evolution of Enceladus. [1] Manga M. and Wang C.-Y. (2007) Geophysical Research Letters, 34, L07202