The rotation rate of a planet is a fundamental parameter, no less than its mass or composition, and planetary investigators require this rate to assess various other phenomena such as planetary wind speeds, internal and atmospheric models, ring dynamics and so forth. Saturn presents a conundrum, however, because none of its various planetary periods indicates the “true” rotation of the planet. Thus, although the planet displays an abundance of periodicities near 10.7 hours, the exact rotation period of Saturn is unknown. In the magnetosphere, “planetary-period oscillations” (PPOs) appear in charged particles, magnetic fields, energetic neutral atoms, radio emissions and motions of the plasma sheet and magnetopause. In Saturn’s rings, the spoke phenomenon can exhibit periodicities near 10.7 hours, and ring phenomena themselves may be related to the interior rotation of the planet. In the high-latitude ionosphere, modulations near this period appear in auroral motions and intensities. In the upper atmosphere, some cloud features rotate near this period, although wind speeds are generally faster, and the well-known polar hexagon rotates with a period close to 10.7 hours. Some of the magnetospheric/ionospheric oscillations differ in the northern and southern hemispheres and their periods do not remain constant, sometimes varying on long time scales of a year or longer and sometimes on much shorter time scales. These variations in the period argue against a cause related to changes interior to Saturn, and because the magnetic and spin axes of Saturn are reported to be axisymmetric (unlike those of any other known planet), Saturn’s periodicities cannot be explained as “wobble” caused by a geometric tilt or by a nondipolar magnetic anomaly. Several models have been proposed to account for the observed periodicities, including rotating atmospheric vortices, periodic plasma releases and a flapping magnetodisk, but none can satisfactorily explain all of Saturn’s periodicities nor their common origin, and none can determine the exact rotation rate of the planet. This chapter reviews Saturn’s periodicities, theories thereof, and how they might be used to determine the elusive rotation rate of the planet.

We review our current understanding of the interior structure and thermal evolution of Saturn, with a focus on recent results in the Cassini era. There has been important progress in understanding physical inputs, including equations of state of planetary materials and their mixtures, physical parameters like the gravity field and rotation rate, and constraints on Saturnian free oscillations. At the same time, new methods of calculation, including work on the gravity field of rotating fluid bodies, and the role of interior composition gradients, should help to better constrain the state of Saturn’s interior, now and earlier in its history. However, a better appreciation of modeling uncertainties and degeneracies, along with a greater exploration of modeling phase space, still leave great uncertainties in our understanding of Saturn’s interior. Further analysis of Cassini data sets, as well as precise gravity field measurements from the Cassini Grand Finale orbits, will further revolutionize our understanding of Saturn’s interior over the next few years.

Despite the lack of another Flagship-class mission such as Cassini–Huygens, prospects for the future exploration of Saturn are nevertheless encouraging. Both NASA and the European Space Agency (ESA) are exploring the possibilities of focused interplanetary missions (1) to drop one or more in situ atmospheric entry probes into Saturn and (2) to explore the satellites Titan and Enceladus, which would provide opportunities for both in situ investigations of Saturn’s magnetosphere and detailed remote-sensing observations of Saturn’s atmosphere. Additionally, a new generation of powerful Earth-based and near-Earth telescopes with advanced instrumentation spanning the ultraviolet to the far-infrared promise to provide systematic observations of Saturn’s seasonally changing composition and thermal structure, cloud structures and wind fields. Finally, new advances in amateur telescopic observations brought on largely by the availability of low-cost, powerful computers, low-noise, large-format cameras, and attendant sophisticated software promise to provide regular, longterm observations of Saturn in remarkable detail.

Our understanding of Saturn’s magnetosphere has been drastically changed over the last decade, since the arrival of Cassini, the first spacecraft to go into orbit around the planet. The trajectory of Cassini allowed the Saturnian magnetosphere to be studied both in the equatorial plane and at high latitudes, in a wide range of radial distances and local time sectors. This chapter reviews the current picture of Saturn’s global magnetospheric configuration and describes the local fields and particle properties in key regions like the radiation belts and the inner, middle and outer magnetosphere. The moon Enceladus, deep in the magnetosphere, is the major source of neutrals and charged particles in the magnetosphere, and in this chapter we describe how the particles are generated, transported and lost within the highly dynamic magnetosphere. We also describe how both particles and fields in the Saturnian magnetosphere vary with time, both on shorter timescales and with Saturn’s seasons. We highlight some of the most recent findings and discoveries, including a formerly unknown electric field oriented in the noon-midnight direction. Finally, we discuss magnetospheric measurements planned for the final sequence of the Cassini mission in 2017, called the “Grand Finale,” along with a list of open questions to be solved by future missions.

The longevity of Cassini’s exploration of Saturn’s atmosphere (a third of a Saturnian year) means that we have been able to track the seasonal evolution of atmospheric temperatures, chemistry and cloud opacity over almost every season, from solstice to solstice and from perihelion to aphelion. Cassini has built upon the decades-long ground-based record to observe seasonal shifts in atmospheric temperature, finding a thermal response that lags behind the seasonal insolation with a lag time that increases with depth into the atmosphere, in agreement with radiative climate models. Seasonal hemispheric contrasts are perturbed at smaller scales by atmospheric circulation, such as belt/zone dynamics, the equatorial oscillations and the polar vortices. Temperature asymmetries are largest in the middle stratosphere and become insignificant near the radiative-convective boundary. Cassini has also measured southern-summertime asymmetries in atmospheric composition, including ammonia (the key species forming the topmost clouds), phosphine and para-hydrogen (both disequilibrium species) in the upper troposphere; and hydrocarbons deriving from the UV photolysis of methane in the stratosphere (principally ethane and acetylene). These chemical asymmetries are now altering in subtle ways due to (i) the changing chemical efficiencies with temperature and insolation and (ii) vertical motions associated with large-scale overturning in response to the seasonal temperature contrasts. Similarly, hemispheric contrasts in tropospheric aerosol opacity and coloration that were identified during the earliest phases of Cassini’s exploration have now reversed, suggesting an intricate link between the clouds and the temperatures. Finally, comparisons of observations between Voyager and Cassini (both observing in early northern spring, one Saturn year apart) show tantalizing suggestions of non-seasonal variability. Disentangling the competing effects of radiative balance, chemistry and dynamics in shaping the seasonal evolution of Saturn’s temperatures, clouds and composition remains the key challenge for the next generation of observations and numerical simulations.

This chapter summarizes our current understanding of the ionosphere of Saturn. We give an overview of Saturn ionospheric science from the Voyager era to the present, with a focus on the wealth of new data and discoveries enabled by Cassini, including a massive increase in the number of electron density altitude profiles. We discuss recent ground-based detections of the effect of “ring rain” on Saturn’s ionosphere, and present possible model interpretations of the observations. Finally, we outline current model-data discrepancies and indicate how future observations can help in advancing our understanding of the various controlling physical and chemical processes.

Saturn formed beyond the snow line in the primordial solar nebula, and that made it possible for it to accrete a large mass. Disk instability and core accretion models have been proposed for Saturn’s formation, but core accretion is favored on the basis of its volatile abundances, internal structure, hydrodynamic models, chemical characteristics of protoplanetary disk, etc. The observed frequency, properties, and models of exoplanets provide additional supporting evidence for core accretion. The heavy elements with mass greater than 4He make up the core of Saturn, but are presently poorly constrained, except for carbon. The C/H ratio is super-solar, and twice that in Jupiter. The enrichment of carbon and other heavy elements in Saturn and Jupiter requires special delivery mechanisms for volatiles to these planets. In this chapter we will review our current understanding of the origin and evolution of Saturn and its atmosphere, using a multi-faceted approach that combines diverse sets of observations on volatile composition and abundances, relevant properties of the moons and rings, comparison with the other gas giant planet, Jupiter, and analogies to the extrasolar giant planets, as well as pertinent theoretical models.

Our knowledge of Saturn’s neutral thermosphere is far superior to that of the other giant planets due to Cassini Ultraviolet Imaging Spectrograph (UVIS) observations of 15 solar occultations and 26 stellar occultations analyzed to date. These measurements yield H2 as the dominant species, with an upper limit on the H mole fraction of 5%. Inferred temperatures near the lower boundary are ~150 K, rising to an asymptotic value of ~400 K at equatorial latitudes and increasing with latitude to polar values in the range of 550–600 K. The latter is consistent with a total estimated auroral power input of ~10 TW generating Joule and energetic particle heating of ~5–6 TW that is more than an order of magnitude greater than solar EUV/FUV heating. This auroral heating would be sufficient to solve the “energy crisis” of Saturn’s thermospheric heating if it can be efficiently redistributed to low latitudes. The inferred structure of the thermosphere yields poleward-directed pressure gradients on equipotential surfaces consistent with auroral heating and poleward increasing temperatures. A gradient wind balance aloft with these pressure gradients implies westward, retrograde winds ~500 m s−1 or Mach number ~0.3 at mid-latitudes. The occultations reveal an expansion of the thermosphere peaking at or slightly after equinox, anti-correlated with solar activity, and apparently driven by lower thermospheric heating of unknown cause. The He mole fraction remains unconstrained, as no Cassini UVIS He 58.4 nm airglow measurements have been published.