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Probing the interior of Ganymede and Callisto with 3GM

by Gabriel Tobie, 18/08/2023

Ganymede and Callisto have similar sizes and masses, and both possess an internal ocean underneath their cold icy surface. However, the lack of surface activity on Callisto indicates a geological history very different form that of Ganymede.  What are the processes in their interior at the origin of this dichotomy still remain to be unveiled. By measuring very accurately the gravity field of these two moons, 3GM (Geodesy and Geophysics of Jupiter and the Galilean Moons) will investigate in detail their interior structure.

3GM is one of the geophysical instruments on JUICE. The 3GM instrument relies on the precise measurements of the radio link between the spacecraft and ESA’s tracking stations. By using a combination of specific radio frequencies (Ka/Ka, X/X and X/Ka link) needed to correct plasma noise between the spacecraft and the ground, 3GM will be able to determine the position of the spacecraft and its velocity relative to the Earth with an accuracy as good as 20 cm and 3 micron/s, respectively. 3GM can use either the main antenna (High-Gain Antenna) when it is pointed to the Earth during data transfer, or a secondary dedicated antenna (Medium-Gain Antenna) to track the spacecraft.

During close flybys of Callisto and Ganymede and once in orbit around Ganymede, any spatial and temporal variations in the gravity field result in a perturbation in the spacecraft trajectory. Thus, by monitoring accurately the position and velocity of the spacecraft, 3GM will be able to reconstruct the gravity field of the moons and determine any variations with time. JUICE will also perform two flybys of Europa, which will not be sufficient to provide useful constraints on its gravity field. Europa’s gravity field will be measured much more precisely by the NASA Europa Clipper mission which will perform more than 50 close flybys.

The gravity field is sensitive to the mass distribution inside the moons. Precise determination of the main components of the gravity field thus provides information on the degree of differentiation, i.e. the way the mass is distributed from the center to the surface. From these measurements, we will better know what is the nature of Callisto’s interior and why it is so different from Ganymede. On Ganymede, we will constrain the size and density of the metallic core, of the rocky mantle and the hydrosphere (ice + water) providing key constraints on its composition and its past evolution.

By monitoring the time variations of the gravity field, 3GM will also quantify how the tides generated by Jupiter deform the moons interior (see Tides not only induce the periodic surface motions but also a periodic displacement of internal mass resulting in periodic variations of the gravity field.  3GM will also provide support for the altimetric measurements performed by GALA by providing accurate position of the spacecraft, essential to get the correct reference to determine the surface topography and any time change.  The amplitude of tidal deformation is at first order controlled by the presence of an internal ocean. For Callisto, the number of flybys will not be sufficient to measure very accurately the tidal amplitude, but it will still be sufficient to confirm if Callisto has really an ocean.

Figure 2: Possible internal structures of Europa, Ganymede and Callisto (credits: R. Pappalardo). Ganymede is the only moon for which we have clear evidence of a metallic core and strong mass concentration to the center. For Europa, the existence of a metallic core is still debated, while for Callisto, the interior seems to still contain a large fraction of water ice mixed with rocks. The relative thicknesses of ice and water layers is totally unknown and requires future tidal measurements to be constrained.

For Ganymede, a dedicated campaign at only 500 km altitude during about 4 months at the end of the JUICE mission will permit 3GM to measure the gravity field with extremely high accuracy. Every day during 8 hours the High Gain Antenna will point to the Earth allowing very accurate spacecraft radio tracking. Such a precise tracking will allow 3GM to determine very subtle changes in space and time of the gravity field, providing key constraints on the depth of the ocean, on its composition,  on any density anomalies associated to different geological terrains at the surface and to deep mass anomalies revealing potential activities in the rocky core.

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