An Investigation of Libration Heating and the Thermal State of Enceladus's Ice Shell

TL;DR

This study models the effect of forced librations on Enceladus's ice shell heating and finds libration heating alone is insufficient to explain observed heat loss, suggesting additional heat sources or non-steady state conditions.

Contribution

It introduces a model including ice shell elasticity to assess libration heating's impact on Enceladus's thermal state, challenging previous runaway melting hypotheses.

Findings

Libration heating is insufficient to match Enceladus's heat loss.

Enceladus likely has additional heat sources beneath the ice shell.

Thermal state of Enceladus's ice shell is stabilized by shell thickness.

Abstract

Tidal dissipation is thought to be responsible for the observed high heat loss on Enceladus. Forced librations can enhance tidal dissipation in the ice shell, but how such librations affect the thermal state of Enceladus has not been investigated. Here we investigate the heating effect of forced librations using the model of Van Hoolst et al. (2013), which includes the elasticity of the ice shell. We find that libration heating in the ice shell is insufficient to match the inferred conductive heat loss of Enceladus. This suggests that either Enceladus is not in a thermal steady state, or additional heating mechanisms beneath the ice shell are contributing the bulk of the power. In the presence of such an additional heat source, Enceladus resides in a stable thermal equilibrium, resisting small perturbations to the shell thickness. Our results do not support the occurrence of a runaway melting process proposed by Luan and Goldreich (2017). In our study, the strong dependence of conductive loss on shell thickness stabilizes the thermal state of Enceladus's ice shell. Our study implies that thermal runaway (if it occurred) or episodic heating on Enceladus is unlikely to originate from librations of the ice shell.