Overview of the 3GM experiment on board the JUICE mission

The ESA’s Jupiter Icy moons Explorer (JUICE) L-class mission is devoted to the study of the Jovian system.

It will be launched in 2023 and, after an 8-year cruise phase (with 3 gravity assists to Earth and 1 to Venus), will start a tour of the Galilean moons that will last 3.2 years.

The onboard Geodesy and Geophysics of Jupiter and the Galilean Moons (3GM) radio science experiment will accomplish a detailed study of Europa, Ganymede and Callisto thanks to a state-of-the-art radio tacking system. 3GM will rely on a multi-frequency link enabled by two onboard units: the Ka-band Transponder (KaT) payload (establishing a full 2-way link in Ka band) and the Deep Space Transponder (DST), enabling 2-way coherent X/X and X/Ka link used for telemetry and telecommand. The multi-frequency link allows accurate measurements of range (≈1-4 cm @60s) and range rate (≈0.003 mm/s @1000s) at nearly all Sun-probe-Earth angles.

The data achieved during the tour phase (2 flybys at Europa and 21 flybys at Callisto) will be used to estimate the Europa’s quadrupole gravity field and Callisto’s static gravity field to at least degree and order 7 and its tidal Love number k₂ with an accuracy of ~0.06 [1]. This will allow 3GM to detect the presence or absence of a subsurface ocean underneath the ice shell of Callisto.

At the end of the tour phase, JUICE will be the first spacecraft to orbit around and icy satellite, allowing a comprehensive study of the moon. The Ganymede 9-month orbital phase is composed of a 5-month elliptical orbit followed by a 4-month circular orbit at 500 km of altitude (GCO-500). The mission could be extended for a 200 km altitude campaign if the residual propellant will be sufficient to decrease the orbital radius.

Range and Doppler data achieved by 3GM during the GCO-500 will be used to infer the static (up to degree 35-45) gravity field, the rotational state, and the tidal response of Ganymede.

Ganymede’s k₂ is subject to time-varying tides due to Jupiter, Io, Europa and Callisto. In particular, the Ganymede’s gravitational perturbations due to the satellites has a high spectral content. This signal can be used to estimate k₂ at a set of frequencies, up to 4d^-1. The profile of k₂ as a function of the frequency, due to the subsurface ocean, is expected to show a peak at a certain resonance frequency, its value being strictly related to the ocean’s depth. We show that the accuracy of the 3GM radio science data is sufficient to detect the peak, if present, and measure its amplitude. In this case the ocean thickness can be estimated with a 7% uncertainty [2].

References:

[1] Cappuccio, P.; Di Benedetto, M.; Durante, D.; Iess, L. (2022) Planet. Sci. J. 3 199

[2] De Marchi, F.; Cappuccio, P.; Mitri, G.; Iess, L. (2022)  Icarus 386 115150