Internal Structure of Enceladus and Dione from Orbital Constraints

Introduction Enceladus is emitting measurable heat (37 GW) from a region centered on its South Pole [1]. Radioactive and accretional heating are expected to be minimal for such a small, low-density body, implying that the observed heat flow is due to past or present tidal dissipation [2]. The tidal heat production for which Enceladus’ eccentricity is in steady state, however, cannot exceed 1.1 GW [3]. This estimate depends on Saturn’s tidal properties and is independent of Enceladus’ present-day eccentricity and internal structure. It therefore seems likely that either Enceladus’ eccentricity is not in steady-state at the current time, or that heat was generated at an earlier time and is now being released. Enceladus’ current eccentric orbit is forced by Dione through a 2:1 mean-motion resonance trapping. Orbital resonances, either current or ancient, have the potential to increase a satellite’s eccentricity, and thus generate heating and deformation via tidal dissipation [4]. Prior to the current e-Enceladus resonance, the two satellites have passed through a few other resonances near the 2:1 commensurability [5]. These resonances, although not able to generate enough heat in Enceladus to account for the current outflow, provide clue to the amount of dissipation inside each satellites. Here, we constrain the internal structure of Enceladus and Dione by modeling their orbital evolution through the 2:1 resonance numerically. We use a N-body code in our modeling. The connection between orbital evolution and internal structure is established by including perturbations from tidal deformation in the N-body integration.