Measurement of Physical Libration of Enceladus and its Interpretation

Abstract:

Enceladus’ interior structure [1] plays a critical role in understanding the composition and activity of the satellite’s south polar geysers [2,3] and in interpreting its tectonic activity. Enceladus’ physical rotational libration, a periodic wobble superposed upon its synchronous rotation and driven by the torques resulting from the satellite’s slightly elliptical orbit, provides a means of constraining the interior structure. We use stereogrammetric solutions of control points of Enceladus in Cassini images to test rotation states, calculating the change in the control-point model’s goodness of fit as we vary the predicted physical libration amplitude, and thus constraining Enceladus’ moments of inertia. Our analysis of over 5300 measurements in 340 images indicates a physical libration amplitude of 0.11 deg. This libration amplitude is not consistent with a solid hydrostatic body (for which the amplitude should be about 0.04 deg), but would be consistent with some versions of recent models incorporating global subsurface oceans or non-hydrostatic cores. We will report on our continuing analysis and on the consistency of interior models with the observed physical libration.

References. [1] Iess, L. et al. (2014) Science, 344, 78-80. [2] Porco, C. et al. (2014) Astron. J. 148,45. [3] Nimmo, F. et al. (2014) Astron. J. 148,46.