Measuring Ganymede's Tidal Deformation by Laser Altimetry: A performance Analysis for the GALA Experiment

Abstract:

Since measurements of Ganymede’s induced magnetic field suggest a salty water layer underneath the satellite’s icy crust [1], we studied the perspectives for the Ganymede Laser Altimeter (GALA) onboard ESA’s JUICE mission to infer the ice-I thickness by measuring the dynamic response of Ganymede's ice shell to tidal forces exerted by Jupiter. We found that the uncertainties in the determination of the tidal Lover number h₂ is 2% and we will demonstrate that this will constrain the ice-I thickness to ± 20 km.

Our model also includes a detailed analysis of the instrument performance taking account of Ganymede's rough small-scale topography. The model is combined with the current mission scenario and spacecraft performance expectation of the JUICE mission giving an estimate for the accuracy of the range measurements in Ganymede orbit.

However, the determination of the resulting ice thickness further depends on the tidally effective rheology of the outer ice shell [2]. Laboratory measurements of ice I (e.g. [3]) suggest that the rigidity can be constrained, however, the less well known viscosity can play a major role when inferring the thickness of the outer ice shell. As a consequence we further investigated under which conditions the ice I viscosity could be constrained by measuring the phase-lag of Ganymede’s tidal response using laser altimetry.

[1] Kivelson et al. 2002.

[2] Moore and Schubert, 2003.

[3] Sotin et al., 1998