A Recent Ocean or Sea on Enceladus

Abstract:

Enceladus is observed to emit several GW of heat from the south polar terrain, as well as plumes of vapor and ice. This activity, along with recent gravity measurements imply the presence of a subsurface ocean or south polar sea. However, models of the internal dynamics suggest that heat is removed from the interior faster than it can be produced, resulting in the geologically rapid freezing of any global subsurface ocean, although a regional sea may be longer-lived. Tidal dissipation in the ice shell is severely restricted if it is mechanically coupled to the rigid silicate core. Here, we consider mechanisms that may keep the interior of Enceladus warm at the present day and produce a geologically recent liquid ocean or sea: sustained tidal heating within a completely frozen Enceladus, and impact heating of the ice shell.

The low central pressure on Enceladus suggests that rock fragments are not easily compressed, and that the core may be a rubble pile with pore space filled by ice or water. If the core has 20-30% ice-filled porosity, ice rheology dominates the deformation. If the core is thus disaggregated, the tidal heating can be significant even when completely frozen (as long as the ice is relatively warm), up to a factor of ~20 higher than if the core were monolithic. The heating rates obtained for the more unconsolidated cases are broadly consistent with the long-term sustainable level of tidal dissipation. While a subsurface ocean is thus not required for continued tidal dissipation, the ability of a frozen Enceladus to dissipate heat may permit late formation of such an ocean.

Large impacts may also provide an external source of heat to regionally melt the ice shell. The denser meltwater may percolate to the base of the ice shell, forming a south polar sea. We use the CTH hydrocode to simulate an impact by either an icy or a rocky projectile into a 40-km thick ice shell, and track the resulting temperature increase. For example, an impact by an icy impactor with a diameter of 4 km and a velocity of 20 km/s increases the temperature of the ice shell by 100-400K down to a depth of ~12 km, and laterally ~7 km; the highest temperatures are a few km below the surface. Melting 12 km of ice corresponds to a reduction in elevation of ~1 km. Larger projectiles will produce greater melting, more consistent with the observed depression of the south polar terrain.