The gravity harmonics of a fluid, rotating planet can be decomposed into static components arising from solid-body rotation and dynamic components arising from flows. In the absence of internal dynamics, the gravity field is axially and hemispherically symmetric and is dominated by even zonal gravity harmonics J that are approximately proportional to q, where q is the ratio between centrifugal acceleration and gravity at the planet's equator. Any asymmetry in the gravity field is attributed to differential rotation and deep atmospheric flows. The odd harmonics, J, J, J, J and higher, are a measure of the depth of the winds in the different zones of the atmosphere. Here we report measurements of Jupiter's gravity harmonics (both even and odd) through precise Doppler tracking of the Juno spacecraft in its polar orbit around Jupiter. We find a north-south asymmetry, which is a signature of atmospheric and interior flows. Analysis of the harmonics, described in two accompanying papers, provides the vertical profile of the winds and precise constraints for the depth of Jupiter's dynamical atmosphere.

Tidal effects in planetary systems are the main driver in the orbital migration of natural satellites. They result from physical processes occurring deep inside celestial bodies, whose effects are rarely observable from surface imaging. For giant planet systems, the tidal migration rate is determined by poorly understood dissipative processes in the planet, and standard theories suggest an orbital expansion rate inversely proportional to the power 11/2 in distance 1 , implying little migration for outer moons such as Saturn's largest moon, Titan. Here, we use two independent measurements obtained with the Cassini spacecraft to measure Titans orbital expansion rate. We find Titan migrates away from Saturn at 11.3 ± 2.0 cm/year, corresponding to a tidal quality factor of Saturn of Q ' 100, and a migration timescale of roughly 10 Gyr. This rapid orbital expansion suggests Titan formed significantly closer to Saturn and has migrated outward to its current position. Our results for Titan and five other moons agree with the predictions of a resonance locking tidal theory 2 , sustained by excitation of inertial waves inside the planet. The associated tidal expansion is only weakly sensitive to orbital distance, motivating a revision of the evolutionary history of Saturns moon system. The resonance locking mechanism could operate in other systems such as stellar binaries and exoplanet systems, and it may allow for tidal dissipation to occur at larger orbital separations than previously believed.Saturn is orbited by 62 moons, and the intricate dynamics of this complex system provide clues about its formation and evolution. Of crucial importance are tidal interactions between the moons and the planet. Each moon raises a tidal bulge in the planet, and because Saturn rotates faster than the moons orbit, frictional processes within the planet cause the tidal bulge to lead in front of each moon. Each moon's tidal bulge pulls the moon forward such that it gains angular momentum and migrates outward, similar to the tidal evolution of the Earth-Moon system. However, in giant planets such as Saturn, the dissipative processes that determine the bulge lag 2

The Juno spacecraft reached the mid‐point of its nominal mission in December 2018, after completing 17 perijove passes. Ten of these were dedicated to the determination of the gravity field of the planet, with the aim of constraining its interior structure. We provide an update on Jupiter's gravity field, its tidal response and spin axis motion over time. The analysis of the Doppler data collected during the perijove passes hints to a non‐static and/or non‐axially symmetric field, possibly related to several different physical mechanisms, such as normal modes or localized atmospheric or deeply‐rooted dynamics.

The combination of the Doppler data from the first two Juno science orbits provides an improved estimate of the gravity field of Jupiter, crucial for interior modeling of giant planets. The low‐degree spherical harmonic coefficients, especially J4 and J6, are determined with accuracies better than previously published by a factor of 5 or more. In addition, the independent estimates of the Jovian gravity field, obtained by the orbits separately, agree within uncertainties, pointing to a good stability of the solution. The degree 2 sectoral and tesseral coefficients, C2,1, S2,1, C2,2, and S2,2, were determined to be statistically zero as expected for a fluid planet in equilibrium.

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The Galilean moons of Jupiter are known to have atmospheres and ionospheres, detected with both ground-based observations and spacecraft data. An oxygen-hydrogen atmosphere was discovered on Ganymede with observations by the Hubble Space Telescope (Hall et al., 1998). Ganymede is a unique object in the solar system in that it has its own intrinsic magnetic field which interacts with the Jovian magnetosphere (Kivelson et al., 1997). Within the open field line regions at higher latitudes, sputtering generates an atmosphere of molecular oxygen subject to ionization and dissociated excitation from the Jovian magnetosphere (Eviatar et al., 2001). Within closed field line regions, it is expected the atmosphere is produced by sublimation (Eviatar et al., 2001). It is thought the ionosphere is generated from the neutral atmosphere via photoionization and electron impact from the Jovian magnetosphere (Carnielli et al., 2019). Prior to Juno's encounter with Ganymede, the only direct measurements of Ganymede's ionosphere were those acquired in-situ measurements from the Galileo particle detectors and by the Galileo radio occultation experiment. Due to the flyby distance of the in-situ spacecraft measurements, radio occultation data provide valuable information about the electron densities near the surface of Ganymede.The Galileo spacecraft executed a total of eight S-band radio occultations of Ganymede throughout its mission, resulting in five non-detections, two weak detections, and one strong detection of an ionosphere (McGrath et al., 2004). To the best of our knowledge, the Galileo radio science data at Ganymede were never archived. In particular with respect to Ganymede, only occultation profiles from the G8 encounter were ever published in scientific literature. The strong ionosphere detection occurred during the Ganymede G8 egress occultation resulting in a peak electron density of ∼5,000 cm −3 near the surface (Kliore, 1998). Initially, the lack of detection was surprising, but it was hypothesized that positive detections occurred where the trailing hemisphere (where the

The jovian moon Io disperses about 1 ton/s of material in the planetary magnetosphere, mainly by sublimation of SO2 from the surface and by its intense volcanic activity. The ejected material supplies the plasma cloud surrounding Jupiter known as Io Plasma Torus (IPT). The radio communication between Juno and the Earth DSN station crosses the IPT near the closest approach. Being a dispersive medium, the IPT introduces a path delay in the signal, which can be analyzed to retrieve the density distribution of electrons. We used radio tracking data from the first 25 orbits to investigate the morphology of the IPT and its variability. We adopted a static and axisymmetric model for the electron density and we updated it including temporal and longitudinal variability. We found that our best fit model must include both variabilities, even though on average the morphology of the IPT agrees with previous analyses. Our results suggest that the density of the outer region of the IPT fluctuates over 50% the average value over a typical time scale of about 420 days.

The latest estimation of Jupiter’s gravitational field was obtained by processing the Doppler data from two gravity orbits of NASA’s Juno mission, using the Jet Propulsion Laboratory software monte. In this work, we present the results of the analysis of the same measurements employing the orbit determination software orbit14, developed at the University of Pisa, used here for the first time with real data. We found that the estimated values of Jupiter’s spherical harmonic coefficients from the two solutions are consistent within the formal uncertainty. The analysis is complemented with a discussion on the results obtained with alternative set-ups.