Thermal-orbital evolution of a librating Enceladus

Since being first visited by the spacecraft Cassini in 2005, the Saturnian moon Enceladus has attracted far more attention than could be guessed from its modest size and lack of atmosphere, compared to its giant neighbour Titan. The reason for this state of affairs is the surprising geophysical activity observed at Enceladus’s south pole: a large thermal anomaly crossed by parallel faults ejecting huge plumes of water vapour and icy particles. From the start, it has been suspected that this anomalous activity results from solid tides caused by the eccentricity of the orbit, although the localisation of the heat source remains a puzzle. The simplest explanation consists in dissipating energy either within the icy shell or in the subsurface ocean, but it fails by more than one order of magnitude to match the measured power or to account for the 20 km-thickness of the icy shell. Dissipation within the core could provide enough power if the core is very soft, but this assumption is hardly consistent with laboratory measurements of material properties.

Alternatively, power could be episodically released if the orbit undergoes periodic variations. Enceladus and the neighbouring moon Dione are currently in a 2:1 mean motion resonance. This mechanism protects their orbital eccentricities against damping due to dissipation within the satellites, while it does not forbid periodic variations of the eccentricity around a long-term equilibrium value. Coupling orbital evolution to dissipative eccentricity tides is however not sufficient to induce significant variations of the orbit and thermal output. Besides eccentricity tides, libration is another cause of viscoelastic deformations. Although libration now contributes little to Enceladus’s energy budget, its amplitude is enhanced by a free libration mode as the icy shell decreases in thickness, and could thus become the major contributor to dissipation if the shell is thin enough. We report here the results of a thermal-orbital evolution model including eccentricity tides and libration of a viscoelastic icy shell above a subsurface ocean.