Measurement Methods for Ganymede's Librations from Laser Altimetry Observations

Abstract

We investigate the potential for the Ganymede Laser Altimeter (GALA) on board the JUICE (JUpiter ICy moons Explorer) mission to measure the tidal deformations and the rotational state of Ganymede, in particular physical librations. A subsurface ocean would cause the upper ice shell to librate decoupled from its interior. Two distinct iterative least-squares inversion routines have been implemented to estimate simultaneously for the topography, the orientation of the rotation pole, the mean rotation and the libration amplitudes and tidal deformations using synthetic laser altimetry data. We follow the approach of a global expansion of spherical harmonics and as a second method the parametrization in 2D B-splines. Results indicatet hat a global ocean could only be found for a very thin ice shell thickness, if the ice shell is elastic and fully decoupled of the interior. However, to prove the non-existence of an ocean is difficult due to small libration amplitudes and their possibly ambiguous interpretation. In this case independent observations, such as the Love numbers, provide more unique constraints on the interior structure.