WARMING THE FROZEN HEART OF ENCELADUS

Enceladus is well-known for its young south polar terrain, observed by Cassini to emit several GW of heat as well as plumes of vapor and ice. The source of this energy is believed to be tidal dissipation. 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. In such a scenario, if the ocean freezes entirely, tidal heating is predicted to drop precipitously.

Here, I consider an alternative interior structure for Enceladus in which the core of Enceladus is fragmented, and relatively weak. Enceladus is most likely differentiated due to long-lived radioactive heating, although the size and density of the core are not well known. Unless Enceladus formed within 1.6 My of CAI formation, there will be insufficient ²⁶Al to melt the silicates. Furthermore, the central pressure on Enceladus is only ≈20 MPa, well below the compressive strength of most rocks. The core therefore may be a rubble pile with pore space filled by ice or water; the ice rheology will dominate the deformation as the rock fragments slide past each other with minimal contact. If the core is fully disaggregated (30% ice-filled porosity), tidal heating can be significant, up a factor of ~20 higher than for a monolithic core, but only if the interior starts out warm and tidal heating is strong from the beginning. If this is not the case, radioactive heating will be insufficient to prevent the interior from cooling.

The heating rates obtained for the more unconsolidated cases are broadly consistent with the long-term sustainable level of tidal dissipation. Although this level is short of the observed infrared flux, the models here apply to the rate of heat production rather than the observable rate of heat loss. Heat may be produced at a lower rate and released episodically without violating constraints of orbital mechanics. While the existence of a subsurface ocean is not a requirement for continued tidal dissipation, the ability of a frozen Enceladus to dissipate heat may permit the formation of such an ocean at later times.