Carl D. Murray

The Jupiter and Icy Moons Explorer (JUICE) mission of the European Space Agency (ESA) will investigate the Jovian system with multiple instruments over several years, beginning in early 2031. This paper describes the historical context and state of knowledge, as well as JUICE’s scientific goals and measurement techniques of the satellites that will not be encountered in close flybys. These include the large volcanically active moon Io, the four small inner moons Metis, Adrastea, Amalthea, and Thebe, and the numerous small Irregular (outer) moons. JUICE will provide multiple opportunities to observe Io from relatively remote distances of hundreds of thousands of kilometers. These observations will enable monitoring of Io’s surface for changes, and for the study of its neutral clouds and plasma torus. Io observations will be performed with the four optical remote sensing instruments and with the Particle Environment Package. For the small inner moons it is planned to obtain complete geographic longitude (scales up to 8 km/px), solar-phase and multi-color coverage, oblique polar views, and UV to near-IR spectra. Astrometric measurements will also be performed. The Irregular moons will mostly appear unresolved to the JUICE instruments. Nonetheless, long-duration disk-integrated lightcurves will be acquired to derive rotation periods, object dimensions, pole-axis orientations, and colors for most objects for the first time. From these data, convex-shape models will be generated and phase curves determined. Furthermore, the precision of the orbital elements will be improved via accurate astrometry. UV and near-IR measurements will be attempted for the largest of these objects.

The origin of Saturn’s rings is a long standing mystery in planetary science, for which the age of this system is a critical constraint. After having clarified what the age of the rings may mean, this article reviews several aspects of this question. We discuss the exposure age, which was recently found to be on the order a few 100 Myrs based on the rings’ composition and their bombardment rate by micrometeoroids. The derivation of this age is explained, and the possible caveats are discussed, including the idea of a putative phenomenon of cleaning of the rings. We address the age of structures such as the Cassini division, plateaux, ramps, and how they are constrained by interactions with satellites and the effects of bombardment. We then address the dynamical evolution of the rings, due to viscosity, micrometeoroid bombardment, and satellite torques. Initially massive rings are found to viscously spread and lose mass quickly and then converge to the present mass in billions of years. However, the dynamical effects of micrometeoroid bombardment can take the reins over viscosity and may wipe the rings out efficiently. Naturally, consideration of ring origin also motivates us to review the ring formation models existing in the literature, and discuss their implications on the composition and age of the rings, in particular in the frame of large tidal dissipation inside Saturn. Finally, putting together and synthesizing these results, we conclude that a fully consistent picture for the origin and age of Saturn’s rings has yet to be established and more work remains to be done to resolve this important question.

One of the stranger planetary rings is Saturn’s narrow, clumpy F ring, lying just outside the main rings, in a region disturbed by chaotic orbital dynamics. We show that the F ring has a stable “true core” that dominates its mass and is confined into discontinuous short arcs of particles larger than a few millimeters in radius. The more obvious micron-size particles seen in images, outlining and obscuring the true core, contribute only a small fraction of its mass. We found that these arcs of large particles orbit Saturn in a specific corotational resonance with the nearby 100-kilometer diameter ringmoon Prometheus, which stabilizes the F ring material and allows it to persist within the disturbed region for decades or longer. Toward the end of the observing period, a small chaotic glitch in the orbit of Prometheus temporarily disrupted the confinement, but the arcs seem to be able to adapt.

Tidal interactions between moons and planets can have major effects on the orbits, spins, and thermal evolution of the moons. In the Saturn system, tidal dissipation in the planet transfers angular momentum from Saturn to the moons, causing them to migrate outwards. The rate of migration is determined by the mechanism of dissipation within the planet, which is closely tied to the planet’s uncertain structure. We review current knowledge of giant planet internal structure and evolution, which has improved thanks to data from the Juno and Cassini missions. We discuss general principles of tidal dissipation, describing both equilibrium and dynamical tides, and how dissipation can occur in a solid core or a fluid envelope. Finally, we discuss the possibility of resonance locking, whereby a moon can lock into resonance with a planetary oscillation mode, producing enhanced tidal migration relative to classical theories, and possibly explaining recent measurements of moon migration rates.

A determination of the dynamical evolution of the asteroid belt is difficult because the asteroid belt has evolved since the time of asteroid formation through mechanisms that include: (1) catastrophic collisions, (2) rotational disruption, (3) chaotic orbital evolution, and (4) orbital evolution driven by Yarkovsky radiation forces. The time-scales of these loss mechanisms are uncertain and there is a need for more observational constraints. In the inner main belt (IMB), the mean size of the non-family asteroids increases with increasing inclination. Here, we use that observation to show that all IMB asteroids originate from either the known families or from ghost families, that is, old families with dispersed orbital elements. We estimate that the average age of the asteroids in the ghost families is a factor of 1/3 less than the Yarkovsky orbital evolution time-scale. However, this orbital evolution time-scale is a long-term average that must allow for the collisional evolution of the asteroids and for stochastic changes in their spin directions. By applying these constraints on the orbital evolution time-scales to the evolution of the size-frequency distribution of the Vesta asteroid family, we estimate that the age of this family is greater than 1.3 Gyr and could be comparable with the age of the Solar system. By estimating the number of ghost families, we calculate that the number of asteroids that are the root sources of the meteorites and the near-Earth asteroids that originate from the IMB is about 20.

Cassini's last look at Saturn's rings: During the final stages of the Cassini mission, the spacecraft flew between the planet and its rings, providing a new view on this spectacular system (see the Perspective by Ida). Setting the scene, Spilker reviews the numerous discoveries made using Cassini during the 13 years it spent orbiting Saturn. Iess et al. measured the gravitational pull on Cassini, separating the contributions from the planet and the rings. This allowed them to determine the interior structure of Saturn and the mass of its rings. Buratti et al. present observations of five small moons located in and around the rings. The moons each have distinctive shapes and compositions, owing to accretion of ring material. Tiscareno et al. observed the rings directly at close range, finding complex features sculpted by the gravitational interactions between moons and ring particles. Together, these results show that Saturn's rings are substantially younger than the planet itself and constrain models of their origin. Science , this issue p. 1046 , p. eaat2965 , p. eaat2349 , p. eaau1017 ; see also p. 1028

Saturn’s narrow F ring exhibits several unusual features that vary on timescales of hours to years. These include transient clumps, a central core surrounded by a multistranded structure and a regular series of longitudinal channels associated with Prometheus, one of the ring’s two ‘shepherding’ satellites. Several smaller moonlets and clumps have been detected in the ring’s immediate vicinity, and a population of embedded objects has been inferred. Here we report direct evidence of moonlets embedded in the ring’s bright core, and show that most of the F ring’s morphology results from the continual gravitational and collisional effects of small satellites, often combined with the perturbing effect of Prometheus. The F-ring region is perhaps the only location in the Solar System where large-scale collisional processes are occurring on an almost daily basis.