Observability of Ganymede's gravity anomalies related to surface features by the 3GM experiment onboard ESA's JUpiter ICy moons Explorer (JUICE) mission

The Geodesy and Geophysics of Jupiter and the Galilean Moons (3GM) experiment aboard the JUpiter ICy moons Explorer (JUICE) will measure the gravity fields of Europa, Callisto and Ganymede. For the first two moons the data will be acquired during flybys, while for Ganymede a 9-months orbital phase is planned. This latter phase is divided in a 5-month elliptical orbit phase (GEO) and a 4-month circular phase (GCO-500). The GCO-500 will provide the first gravity data from an orbiter for Ganymede, enabling the measurement of the Ganymede's gravity field up to the degree 35–45 of spherical harmonics expansion. The large amount of data collected by 3GM will provide enough details to potentially identify regional (hundreds of km) surface structures. Remarkably, Ganymede's outer ice shell is characterized by the presence of dark and bright terrains: evidence of older, dirty ice and younger, cleaner ice, respectively. In this work we investigate the possibility to detect gravity anomalies related to the surface distribution of bright and dark terrains using the 3GM data. By assuming a range of i) surface density contrasts for the dark and bright terrains based on an estimation of the impactors flux, ii) average surface topographies, iii) internal structure configurations, and comparing several models for the external gravity field of Ganymede, we simulate the expected gravity field as it would be reconstructed by JUICE. Our results show that 3GM data might allow to discriminate and separate the gravitational contributions from the deep interior and the surface distribution of dark and bright terrains.