Frequency-dependent Ganymede's tidal Love number k2 detection by JUICE's 3GM experiment and implications for the subsurface ocean thickness

Ganymede will be the first icy satellite in the Solar System orbited by a spacecraft, ESA's JUpiter ICy moons Explorer (JUICE). JUICE launch is scheduled for April 2023 and the arrival at Ganymede is foreseen in 2035. Thanks to the advanced Ka-band radio tracking system, the Geodesy and Geophysics of Jupiter and Galilean Moons (3GM) experiment aboard the mission will provide range and range-rate measurements that will be used to infer the static (up to degree 35-45) and time-varying gravity field and the internal structure of the moon. Ganymede is subject to tidal deformation, which periodically modifies its gravity field. The larger contribution to the time-varying gravity is due to the tidal interaction with Jupiter, modulated by the eccentricity and the inclination of the moon's orbit. In addition, Ganymede also experiences a lower amplitude time-varying tidal interaction with Io, Europa and Callisto. To a good approximation, the corresponding gravitational signals are periodic and they contain several harmonics of the fundamental synodic frequencies. The elastic response of Ganymede is expected to be frequency-dependent as well. In this work we describe a procedure to estimate the Ganymede's tidal Love number k2 at different frequencies in the orbit determination process of the 3GM experiment, supported by numerical simulations. Finally, we show that 3GM measurements can be used to characterize the interior structure of Ganymede, providing a tight constraint on the subsurface ocean thickness. The ocean density can be also constrained, but with a lower precision.