Callisto and Europa Gravity Measurements from JUICE 3GM Experiment Simulation

The JUpiter Icy Moons Explorer is an ESA mission set for launch in 2023 April and arrival in the Jovian system in 2031 July to investigate Jupiter and its icy satellites with a suite of 10 instruments. The mission will execute several flybys of the icy moons Europa, Callisto, and Ganymede before ending the mission with a 9-month orbit around Ganymede. The 3GM experiment on board the spacecraft will exploit accurate range and Doppler (range-rate) measurements to determine the moons' orbit, gravity field, and tidal deformation. The focus of this paper is on the retrieval of Europa's and Callisto's gravity field, without delving into the modeling of their interior structures. By means of a covariance analysis of the data acquired during flybys, we assess the expected results from the 3GM gravity experiment. We find that the two Europa flybys will provide a determination of the J (2) and C (22) quadrupole gravity field coefficients with an accuracy of 3.8 x 10(-6) and 5.1 x 10(-7), respectively. The 21 Callisto flybys will provide a determination of the global gravity field to approximately degree and order 7, the moon ephemerides, and the time-variable component of the gravitational tide raised by Jupiter on the moon. The k (2) Love number, describing the Callisto tidal response at its orbital period, can be determined with an uncertainty sigma ( k2) similar to 0.06, allowing us to distinguish with good confidence between a moon with or without an internal ocean. The constraints derived by 3GM gravity measurements can then be used to develop interior models of the moon.