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 obliquity of a planet is the tilt between its equator and its orbital plane. Giant planets are expected to form with near-zero obliquities [1,2]. After its formation, some dynamical mechanism must therefore have tilted Saturn up to its current obliquity of 26.7 • . This event is traditionally thought to have happened more than 4 Gyrs ago during the late planetary migration [3,4,5] because of the crossing of a resonance between the spin-axis precession of Saturn and the nodal orbital precession mode of Neptune [6]. Here, we show that the fast tidal migration of Titan measured by [7] is incompatible with this scenario, and that it offers a new explanation for Saturn's current obliquity. A significant migration of Titan would prevent any early resonance, invalidating previous constraints on the late planetary migration set by the tilting of Saturn [8,9,10]. We propose instead that the resonance was encountered recently, about 1 Gyr ago, forcing Saturn's obliquity to increase from a small value (possibly less than 3 • ), up to its current state. This scenario suggests that Saturn's normalised polar moment of inertia lies between 0.224 and 0.237. Our findings bring out a new paradigm for the spin-axis evolution of Saturn, Jupiter [11], and possibly giant exoplanets in multiple systems, whereby obliquities are not settled once for all, but continuously evolve as a result of the migration of their satellites.We investigate whether the spin-axis dynamics of Saturn could have been influenced by the migration of its satellites. The torque applied by the sun on Saturn's equatorial bulge produces a precession of its spin axis with a mean frequency ψ = α (1−e 2 ) −3/2 cos ε, where e is the eccentricity of Saturn, ε is its obliquity, and α is called its precession constant [12]. The value of α incorporates the extra torques produced by Saturn's satellites [1,13]. It also depends on Saturn's orbital and physical parameters, among which all are well known except Saturn's normalised polar moment of inertia λ. Estimates found in the literature broadly range in λ ∈ [0.200, 0.240] but they are model-dependent and a proper uncertainty range is hard to define (see Methods). Exploring this whole interval gives a current precession constant α 0 ranging from 0.747 to 0.894 /yr.We take into account the migration of Saturn's satellites by changing their contributions to α. The measurements of [7] unveiled a fast migration for Titan, and a similar tidal timescale t tide = a/ ȧ for all six satellites of Saturn studied. Both these findings support the "resonance locking" tidal 1

Context. The Galilean satellites have very complex orbital dynamics due to the mean-motion resonances and the tidal forces acting in the system. The strong dissipation in the couple Jupiter–Io is spread to all the moons involved in the so-called Laplace resonance (Io, Europa, and Ganymede), leading to a migration of their orbits. Aims. We aim to characterize the future behavior of the Galilean satellites over the Solar System lifetime and to quantify the stability of the Laplace resonance. Tidal dissipation permits the satellites to exit from the current resonances or be captured into new ones, causing large variation in the moons’ orbital elements. In particular, we want to investigate the possible capture of Callisto into resonance. Methods. We performed hundreds of propagations using an improved version of a recent semi-analytical model. As Ganymede moves outwards, it approaches the 2:1 resonance with Callisto, inducing a temporary chaotic motion in the system. For this reason, we draw a statistical picture of the outcome of the resonant encounter. Results. The system can settle into two distinct outcomes: (A) a chain of three 2:1 two-body resonances (Io–Europa, Europa–Ganymede, and Ganymede–Callisto), or (B) a resonant chain involving the 2:1 two-body resonance Io–Europa plus at least one pure 4:2:1 three-body resonance, most frequently between Europa, Ganymede, and Callisto. In case A (56% of the simulations), the Laplace resonance is always preserved and the eccentricities remain confined to small values below 0.01. In case B (44% of the simulations), the Laplace resonance is generally disrupted and the eccentricities of Ganymede and Callisto can increase up to about 0.1, making this configuration unstable and driving the system into new resonances. In all cases, Callisto starts to migrate outward, pushed by the resonant action of the other moons. Conclusions. From our results, the capture of Callisto into resonance appears to be extremely likely (100% of our simulations). The exact timing of its entrance into resonance depends on the precise rate of energy dissipation in the system. Assuming the most recent estimate of the dissipation between Io and Jupiter, the resonant encounter happens at about 1.5 Gyr from now. Therefore, the stability of the Laplace resonance as we know it today is guaranteed at least up to about 1.5 Gyr.

Context. Giant planets are expected to form with near-zero obliquities. It has recently been shown that the fast migration of Titan could be responsible for the current 26.7°-tilt of Saturn’s spin axis. Aims. We aim to quantify the level of generality of this result by measuring the range of parameters allowing for this scenario to happen. Since Titan continues to migrate today, we also aim to determine the obliquity that Saturn will reach in the future. Methods. For a large variety of migration rates for Titan, we numerically propagated the orientation of Saturn’s spin axis both backwards and forwards in time. We explored a broad range of initial conditions after the late planetary migration, including both small and large obliquity values. Results. In the adiabatic regime, the likelihood of reproducing Saturn’s current spin-axis orientation is maximised for primordial obliquities between about 2° and 7°. For a slightly faster migration than expected from radio-science experiments, non-adiabatic effects even allow for exactly null primordial obliquities. Starting from such small tilts, Saturn’s spin axis can evolve up to its current state provided that: (i) the semi-major axis of Titan changed by more than 5% of its current value since the late planetary migration, and (ii) its migration rate does not exceed ten times the nominal measured rate. In comparison, observational data suggest that the increase in Titan’s semi-major axis exceeded 50% over 4 Gyr, and error bars imply that the current migration rate is unlikely to be larger than 1.5 times its nominal value. Conclusions. If Titan did migrate substantially before today, tilting Saturn from a small obliquity is not only possible, but it is the most likely scenario. Saturn’s obliquity is still expected to be increasing today and could exceed 65° in the future. Maximising the likelihood would also put strict constraints on Saturn’s polar moment of inertia. However, the possibility remains that Saturn’s primordial obliquity was already large, for instance as a result of a massive collision. The unambiguous distinction between these two scenarios would be given by a precise measure of Saturn’s polar moment of inertia.

Aims. We aim to determine whether Jupiter’s obliquity is bound to remain exceptionally small in the Solar System, or if it could grow in the future and reach values comparable to those of the other giant planets. Methods. The spin-axis of Jupiter is subject to the gravitational torques from its regular satellites and from the Sun. These torques evolve over time due to the long-term variations of its orbit and to the migration of its satellites. With numerical simulations, we explore the future evolution of Jupiter’s spin axis for different values of its moment of inertia and for different migration rates of its satellites. Analytical formulas show the location and properties of all relevant resonances. Results. Because of the migration of the Galilean satellites, Jupiter’s obliquity is currently increasing, as it adiabatically follows the drift of a secular spin-orbit resonance with the nodal precession mode of Uranus. Using the current estimates of the migration rate of the satellites, the obliquity of Jupiter can reach values ranging from 6° to 37° after 5 Gyr from now, according to the precise value of its polar moment of inertia. A faster migration for the satellites would produce a larger increase in obliquity, as long as the drift remains adiabatic. Conclusions. Despite its peculiarly small current value, the obliquity of Jupiter is no different from other obliquities in the Solar System: It is equally sensitive to secular spin-orbit resonances and it will probably reach comparable values in the future.