The evolution of Enceladus

Saturn's small icy moon Enceladus shows evidence of several episodes of geologic resurfacing and extensional tectonism spread over much of its history. Freezing of liquid in the interior caused global expansion and fracturing of the crust. Resurfacing took place by eruption of fresh material, perhaps containing NH 3, to the surface. Solid-state convection could take place in Enceladus for a crustal thickness greater than about 30 km, assuming thermal properties like those of pure H 2O ice. Melting in the interior may have been made more likely by the presence of NH 3, as the H 2ONH 3 system has a eutectic point at 173°K. Tidal dissipation seems to be the only heating mechanism capable of melting Enceladus. For the thermal properties of pure H 2O, the orbital eccentricity would have to be higher by a factor of 5–7 than the present value of 0.0044 to maintain a molten interior, and may have to be greater by as much as a factor of 20 to cause melting in an initially frozen body. If the thermal conductivity is diminished by inclusion of clathrate hydrates, a significant enhancement over the present eccentricity would still be required to initiate melting, but it might be possible to maintain a molten interior and allow geologic activity with the present eccentricity. Removal of eccentricity forcing would result in rapid eccentricity damping, freezing, and cessation of tectonic activity.