Revealing Saturn’s deep interior for the first time with Cassini

Jupiter and Saturn are predominantly hydrogen and helium, and there has been steady progress in defining the thermodynamic properties of H/He mixtures, primarily through theory. Residual uncertainties at the percent level limit our ability to be certain about some aspects of planetary structure despite very high precision data provide by Juno and Cassini. In both planets, there is a clear gravity signal of the differential rotation and this is so large in Saturn that it limits our ability to improve interior models. These models are important because they can help us understand how these planets and our planetary system formed. Can we determine whether these planets are not simply adiabatic mixtures of hydrogen and helium, homogeneous or layered in a simple way, overlying a centrally concentrated “core” of heavier elements? The data suggest a non-uniform mixture of the heavy elements and a less well-defined core, plausibly a consequence of how the planet formed. Ring seismology for Saturn has pointed to the likely presence of compositional gradients and also offered new constraints on rotation rate. With some uncertainties still in the atmospheric abundances (water in both, He for Saturn) it is not yet possible to state with high confidence the extent to which these planets are understood. The path forward should focus on the promise of planetary seismology for both, and a future atmospheric probe for Saturn, while exoplanets may help guide us on formation.

Magnetosphere-ionosphere coupling is accomplished through the existence of large-scale field-aligned currents which are generated in a source region and end in a sink region, and which transfer momentum (and energy) between the regions. To move beyond a theoretical picture of how this works, we require in-situ measurements of these regions primarily in the form of magnetic field measurements, but combined with in-situ plasma, plasma wave data, and remote auroral observations. With Cassini at Saturn for 13 years we have had many opportunities to sample the high-latitude magnetosphere and aurora, and have learned a great deal about how this giant magnetosphere operates. In the case of Jupiter, we now also have new high-latitude observations from the Juno mission which are providing a wealth of new information on this topic. Observations at Saturn indicate that two main current systems are present. The first is associated with sub-corotation of the magnetospheric plasma relative to the neutral atmosphere flow, and this causes the magnetic field lines to bend out of meridian planes into a lagging field configuration. The associated torques are such that angular momentum is transferred from the planetary atmosphere to the magnetospheric plasma. This system is approximately axisymmetric (m=0). The second system has been revealed through observations of magnetic field oscillations near the planetary rotation period. Analysis of these magnetic perturbations show that the planetary period oscillations (PPOs) are due to two rotating large-scale current systems flowing between the ionosphere and the magnetosphere, one associated with the northern hemisphere and one associated with the south, and each with its own variable period. There is evidence for interhemispheric current flow in the PPO systems, and the physical origin of the currents is thought to be related to m=1 rotating twin-vortex perturbations in the planet’s neutral atmosphere. Curiously, these two main sub-corotation and PPO current systems at Saturn are located at similar ionospheric co-latitudes and are of similar amplitudes, and hence strongly interact as the non-axisymmetric PPO currents rotate over the axisymmetric sub-corotation system. In addition to these two main current systems, we also see a strong solar-wind effect at Saturn associated with shock compressions of the magnetosphere, whose effects have been interpreted as being due to the excitation of strong, tail reconnection that strongly modulates the field-aligned currents and the auroras. Recent work shows that, in addition, the system is strongly primed for tail reconnection when the combined phases of the rotating north and south PPO perturbations provide the perfect conditions.

Looking to Jupiter, it is widely accepted that the main auroral oval at Jupiter is related to corotation breakdown and associated magnetosphere-ionosphere coupling currents, similar to the sub-corotation system described above for Saturn. However, it is not evident whether there are also solar wind compression-related effects at Jupiter, and it does not seem likely that there is a PPO equivalent at Jupiter. Following a shock compression at Jupiter the aurora brightens but a simple application of corotation breakdown models would suggest that the field-aligned currents associated with the main auroral oval should weaken under compression (ie dimmer aurora), and brighten under rarefaction conditions. By comparison with Saturn, we might expect that the polar aurora (possibly related to the solar wind interaction) should be brightest under compression conditions. Professor Bunce will summarise both the theoretical framework for magnetosphere-ionosphere coupling, and the main findings of Cassini and Juno (so far) on this topic.

During the Cassini Grand Finale orbits at Saturn the focus of the magnetometer investigation was determination of the internal planetary magnetic field as well as the rotation rate of the deep interior. The unique geometry of these orbits provided an opportunity to measure the internal magnetic field at closer distances to the planet than ever encountered before. The surprising close alignment of Saturn’s magnetic axis with its spin axis (known about since the Pioneer 11 observations) has been confirmed, however external effects, observed even around periaspse are masking some of the magnetic field signals from the interior. The varying northern and southern magnetospheric planetary period oscillations and field aligned currents at both high and low latitudes are contributing to the magnetic signals observed. We report new features in the internal planetary magnetic field as well as the external planetary magnetic field, enhancing our view of the auroral current systems and including the discovery of inter-hemispheric currents flowing in the magnetospheric plasma near the inner edge of the D ring.

The gravity field measured by the spacecraft Cassini during the Grand Finale Orbits reveals a planet with many surprising features. During six carefully selected orbits, as Cassini was in free fall between the planet and its innermost ring, NASA’s Deep Space Network and ESA’s ESTRACK antennas measured the spacecraft range rate with accuracies of 0.02-0.08 mm/s (60 s integration time), providing an estimate of the zonal field till degree 10 and the mass of the rings. The zonal coefficients J6, J8 and J10 deviate from the theoretical predictions based on a uniformly rotating planet, showing the clear signature of a differential rotation extending deep below the cloud level. On Jupiter, the shallower differential rotation was inferred from the odd harmonics, a measurement made possible by Juno’s more advanced radio system. The zonal field measured by Cassini was also used in models of the interior structure to constrain the mass of the core. In a striking difference with Juno at Jupiter, a purely zonal field cannot account for Cassini’s accelerations near the pericenter. These quite significant, unexplained accelerations could be due to a time-variable gravity field, such as that generated by acoustic oscillations or convection associated with differential rotation.

This talk will discuss the recent gravity measurements of Saturn and Jupiter by the Cassini and Juno spacecrafts. During 13 years in orbit around Saturn, the Cassini spacecraft made many surprising discoveries. However, only during its Grand Finale phase when it traveled inside Saturn’s rings, it came close enough to measure the planet’s gravitational field with high precision. The measurements of Saturn’s even gravity harmonics led to highly unusual results. The magnitude of J6, J8, and J10 were so large that they could not be explained with models that assume uniform rotation. In this talk, Dr Militzer will introduce differential rotation on cylinders into the models that he has constructed with the concentric McLaurin spheroid method. He will show that the even gravity harmonics J2 through J10 can then be matched. Finally he will compare the findings for Saturn with recent measurements of Jupiter’s gravity field that have been made by the Juno spacecraft. Dr Militzer will conclude by discussing common features and differences between the interior models of the two planets.

The depth and structure to which the cloud-level east-west jet streams on Jupiter and Saturn extend has been a long-lasting mystery. The recent gravity results from Juno and the Cassini Grand Finale have shown that these flows must extend to great depths in order to match the gravity measurements, reaching about 3000 km on Jupiter and 9000 km on Saturn. Remarkably, on both planets this depth matches where the electrical conductivity rises so that there can be interaction between the flow and the magnetic field, providing a possible decay mechanism for the flows. In addition, the gravity results allow quantifying the vertical decay profile of the flows, and imply that nearly the same meridional profile of the zonal jets that is observed at the cloud-level, extends to those depths. This has important implications on our understanding of the dynamical mechanisms driving and maintaining the zonal jets on both planets. In this talk, we comparatively review these results from both missions, their implications and what new understanding can be gained about the mechanisms driving the zonal jets.

Magnetic fields are windows into planetary interiors. The existence and properties of the planetary magnetic fields reflect the interior structure, dynamics, and evolution of the host planets. Saturn’s magnetic field continues to offer surprises since its discovery during the Pioneer 11 flyby. Recent observations from the Cassini mission, in particular the Grand Finale phase, have revealed new features of Saturn’s magnetic field, including an extremely tight upper bound on the non-axisymmetry of the field and a rich axisymmetric magnetic spectrum extending to high spherical harmonic degrees.

In this talk, Dr Cao will discuss the constraints and implications from the magnetic field on deep zonal flows (differential rotation) and stable stratification inside Saturn. He will present the upper limit on the flow speed inside Saturn as a function of radial distance placed by the kinematic Ohmic dissipation constraints. Although the luminosities of Jupiter and Saturn are on the same order of magnitude, the internal magnetic field of Saturn are much weaker than that of Jupiter, which indicates faster flows are permitted inside Saturn. Dr Cao will then discuss the speed of deep zonal flow and the thickness of stable stratification needed to account for the observed level of magnetic axisymmetry, using the plane layer analytical formula derived by Stevenson (1982) and kinematic MHD simulations. He will then discuss how MAC (Magnetic-Archimedes-Coriolis) waves could produce time variations in Saturn’s axisymmetric magnetic field and inform us about the physical properties of the deep stable stratification.

In closing, Dr Cao will highlight a few open questions concerning the interior dynamics of giant planets, including the physical mechanism of (magnetic) truncation of deep zonal flows and how stable stratification and external forcing impact their thermal evolution pathways.

Seismology of the giant planets represents a major frontier for understanding their structures and origin, much as global helioseismology for was for Solar physics in the 1970s and asteroseismology has been for stellar physics in the 2010s.

After more than a decade of peering at bright stars through Saturn's translucent C ring, Cassini has characterized more than 20 ring waves excited at orbital resonances with Saturn’s nonradial oscillations. The ongoing characterization of these waves is filling out a stunningly precise power spectrum for Saturn's oscillations, making full-fledged normal mode seismology of a gas giant possible for the first time.

The earliest detected waves led to striking results about Saturn's deep interior, and the many subsequent wave detections have led to a rather complete set of frequencies that strongly constrains the planet's interior rotation. Dr Mankovich will present work interpreting these waves in terms of Saturn's fundamental-mode oscillations, including the seismological determination of an elusive quantity: Saturn's bulk spin period. He will also discuss what the seismology implies for Saturn's differential rotation in light of recent results from the Cassini Grand Finale gravity field experiment. Finally, Dr Mankovich will describe the peculiar subset of these frequencies that rather profoundly suggests an extended stable stratification in Saturn’s metallic interior, a configuration that may present a challenge in the context of Saturn’s dynamo generation and thermal evolution.