DDA 2015 – Rotational and interior models for Enceladus II

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Radwan Tajeddine (Cornell)

Abstract

We will discuss the underlying dynamical models and the consequent interior models that pertain to our discovery of a forced rotational libration for Saturn’s moon Enceladus (Thomas et al. 2015).

Despite orbital variations that change the mean motion on timescales of several years owing to mutual satellite interactions, the rotation state of Enceladus should remain synchronous with the varying mean motion, as long as damping is as expected (Tiscareno et al. 2009, Icarus). Taking that dynamically synchronous rotation as the ground state, we construct a model that naturally focuses on the physically interesting librations about the synchronous state that occur on orbital timescales. We will discuss the differences between the method used here and other dynamical methods (e.g., Rambaux et al. 2010, GRL; cf. Tajeddine et al. 2014, Science), and we will review the rotation states (whether known or predicted) of other moons of Saturn.

We will also describe our measurements of the control point network on the surface of Enceladus using Cassini images, which was then used to obtain its physical forced libration amplitude at the orbital frequency. The fit of Cassini data results in a libration amplitude too large to be consistent with a rigid connection between the surface and the core, ruling out the simplest interior models (e.g., homogeneous, two-layer, two-layer with south polar anomaly). Alternatively, we suggest an interior model of Enceladus involving a global ocean that decouples the shell from the core, with a thinner icy layer in the south polar region as an explanation for both the libration (Thomas et al. 2015) and the gravity (Iess et al. 2014, Science) measurements.

Notes

  • Libration measurement
    • 3D reconstruction of coords of a network of control point (fiducial satellite surface points — e.g. craters)
    • most of Enceladus’s orbit was covered
    • Thomas et al. 2015
    • minimize RMS residual $\rightarrow 0.120 \pm 0.014$ deg
  • Solid models
    • core plus two-layer in hydro.equilib. plus south polar sea
      • measured libration amplitude rules this out
    • decoupled shell from the core (indep.librations)
      • consistent with observed libration amplitude if shell thickness 21-26 km and ocean thickness 26-31 km
  • Gravity data
    • suggests a local mass anomaly — interpreted as ocean thicker under south pole

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