Frequency-dependent Ganymede’s tidal Love number k2 detection by JUICE’s 3GM experiment and implications regarding the subsurface ocean characterization

Ganymede will be the first icy satellite orbited by a spacecraft: ESA's JUpiter ICy moons Explorer (JUICE). The launch is scheduled for September 2022 and the arrival at Ganymede is foreseen in 2035.

Precise range and range-rate data (Ka-band) will be provided by the tracking system of the onboard Geodesy and Geophysics of Jupiter and Galilean Moons experiment (3GM).

These measurements will be used to infer, among others, the static gravity field of the moon up to degree 35-45.

Tidal stresses generate the time-varying part of the Ganymede’s gravitational field and the largest contribution is due to the interaction with Jupiter, modulated by the eccentricity of the Ganymede’s orbit.

However, our work is focused on the lower amplitude time-varying components: those generated by the tidal interactions with Io, Europa and Callisto.

To a good approximation, the corresponding gravitational signals are periodic functions composed by several harmonics of the fundamental synodic frequencies Io-Ganymede, Europa-Ganymede and Ganymede-Callisto. The elastic response of Ganymede is expected to be frequency-dependent as well.

Therefore, we modeled Ganymede’s k2 as a set of coefficients, one for each frequency, to be estimated.

In this work we describe a procedure, supported by numerical simulations and a covariance analysis, to estimate these coefficients in the standard orbit determination framework of the 3GM experiment during the orbital phase at Ganymede.

Finally, we show how the measured coefficients and their accuracies, supported by a viscoelastic model of Ganymede, can be used to provide constraints on the outer ice shell thickness, and on the subsurface ocean density and thickness.