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

09:05-09:50 Giant planet interiors

Professor David Stevenson FRS, Caltech, USA

Abstract

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.

09:50-10:35 Magnetosphere-Ionosphere coupling at giant planets

Professor Emma Bunce, University of Leicester, UK

Abstract

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.

10:35-11:00 Coffee

11:00-11:45 Cassini Magnetic Field observations during the Grand Finale Orbits

Professor Michele Dougherty CBE FRS, Imperial College London, UK

Abstract

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.

11:45-12:30 Cassini Gravity Measurements

Professor Luciano Iess, University of Rome, Italy

Abstract

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.

13:30-02:15 Models for the interiors of Saturn and Jupiter that match gravity measurements of the Cassini and Juno spacecrafts

Dr Burkhard Militzer, Berkeley, US

Abstract

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.

14:15-15:00 Saturn's deep wind

Professor Yohai Kaspi, Weizmann Institute of Science, Israel

Abstract

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.

15:00-15:30 Tea

15:30-16:15 A magnetic perspective on Saturn’s interior

Dr Hao Cao, Harvard, USA

Abstract

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.

16:15-17:00 Ring seismology: sounding the interior of Saturn

Chris Mankovich, University of California Santa Cruz, USA

Abstract

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.

09:00-09:45 Modeling of giant planet zonal flows and vortices

Professor Moritz Heimpel, University of Alberta, Canada

Abstract

Zonal flow on Jupiter and Saturn consists of equatorial superrotation and alternating East-West jet streams at higher latitudes. Interacting with these zonal flows, numerous vortices occur with various sizes and lifetimes. The Juno mission has shown that Jupiter’s jets have origins deep within the molecular envelope. The vast majority of low and mid-latitude jovian vortices are anticyclonic, whereas cyclones appear at polar latitudes. Cassini mission observations revealed a similar pattern on Saturn; its North and South polar vortices are cyclonic, whereas anticyclones occur at mid-latitudes. We use numerical models to study rotating convection in 3D spherical shells. Many of these models result in dynamical flows that are comparable to those on the giant planets. We find that deep convective turbulence can explain the structure of jets. On the other hand, the strength and depth of stable stratification, and the latitude, can determine the formation and dynamics of vortices. Weak stability, a thin stable layer, and lower latitudes favour anticyclonic vortices that form due to upward and divergent flow near the outer boundary. These anticyclones are typically shielded by cyclonic filaments associated with downwelling return flow. In contrast, strong stability, a deep stable layer, and high latitudes favour cyclonic vortices. In our models a typical pattern emerges, with anticyclones in the first anticyclonic shear zone away from the equatorial jet (corresponding to the region of the Great Red Spot on Jupiter and Storm Alley on Saturn), cyclonic and anticyclonic vortices at higher mid-latitudes, and cyclones at both poles.    

09:45-10:30 Jupiter interior models in the context of Juno and Galileo: a template for solar and extrasolar gaseous planets

Professor Gilles Chabrier, Centre de Recherche Astrophysique de Lyon, France and University of Exeter, UK

Abstract

The combination of recent observational and theoretical significant advances, namely Juno’s determination of Jupiter’s high order gravitational moments and numerical calculations of dense matter equations of state, have drastically impacted our previous understanding of Jupiter in particular and thus of giant planets in general. In this talk, Professor Chabrier will briefly present the most recent models of Jupiter interior that fulfill both the Juno gravitational constraints and Galileo’s observed atmospheric adundances. he will highlight the particular density and temperature profiles required to fulfill both constraints and the physical properties necessary to ensure the viability of such models. Lastly, he will discuss the impact of these results on the structure and evolution of solar and extrasolar giant planets.

10:30-11:00 Coffee

11:00-11:45 Anelastic Dynamos: from Jupiter to Saturn

Professor Chris Jones, University of Leeds, UK

Abstract

A series of numerical simulations of the convection-driven dynamos of gas giant planets has been performed. We use an anelastic, fully nonlinear, three-dimensional, benchmarked MHD code to evolve the flow, the entropy and the magnetic field. Our models take into account the varying electrical conductivity, high in the ionised metallic hydrogen region, low in the molecular outer region. The anelastic reference state models include the variations of density, pressure and temperature from the deep interior to within 3000 km of the surface, scaled from a Jupiter model of French et al. 2012. Our suite of electrical conductivity models ranges from Jupiter-like, where the outer hydrodynamic region is quite thin, to Saturn-like, where there is a thick non-conducting shell. The rapid rotation leads to two distinct dynamical regimes forming which are separated by a magnetic tangent cylinder - mTC. Outside the mTC there are strong zonal flows, where Reynolds stress balances turbulent viscosity, but inside the mTC Lorentz force reduces the zonal flow. We find a rich diversity of magnetic field morphologies. There are Jupiter-like steady dipolar fields, but also a belt of quadrupolar dominated dynamos spanning the range of models between Jupiter-like and Saturn-like conductivity profiles. This diversity may be linked to the appearance of reversed sign helicity in the metallic regions of our dynamos. With Saturn-like conductivity profiles we find models with dipolar magnetic fields, whose axisymmetric components resemble those of Saturn, and which oscillate on a very long time-scale. However, the nonaxisymmetric field components of our models are at least ten times larger than those of Saturn. We conclude that Saturn's magnetic field cannot be explained by an isentropic interior model with the dynamo reaching up to the top of the metallic hydrogen region.

11:45-12:30 ’Saturn-like’ Dynamo Models

Professor Sabine Stanley, Johns Hopkins University, USA

Abstract

Cassini data have provided detailed characteristics of Saturn’s magnetic field. This includes the axisymmetric Gauss coefficients up to degree 10, as well as bounds on the non-axisymmetric magnetic field components. In this talk I will discuss numerical dynamo simulations that attempt to reproduce the most ‘Saturn-like’ magnetic field possible. Although these models simplify some of the dynamics in Saturn’s dynamo region, they are able to reproduce many salient features of Saturn’s magnetic field, including its axisymmetry and dominant features of the magnetic power spectrum. Several observed magnetic features are dependent on properties of Saturn’s interior, including thermal anomalies and stable stratification at the top of the metallic hydrogen region. This suggests that we may be able to use magnetic field characteristics as a sort of tomography of Saturn’s deep interior.

13:30-14:15 Results from the Cassini Grand Finale at Saturn using Cassini INMS, UVIS, CIRS, and RSS

Dr Hunter Waite, Southwest Research Institute, USA

Abstract

Waite, Perryman, Miller, Bell, Koskinen, Guerlet, Hubbard, Glein, and Stevenson

The Grand Finale phase of the Cassini-Huygens mission was completed in September of 2017. Both the Saturn bulk atmosphere (H₂ and He) and infalling material from the rings were measured. The most surprising result was the amount of methane, carbon dioxide, carbon monoxide, molecular nitrogen, water, ammonia and organic compounds falling from the rings into the atmosphere at a rate of over 10,000 kg s^-1. This rate of infall over the lifetime of the rings can have significant observable effects on the observed composition of the atmosphere and ionosphere. This will be discussed in the presentation.

The He/H₂ ratio in the well mixed atmospheres of Jupiter and Saturn serves as an important parameter for assessing formation and evolution models of the giant planets. The smaller size of Saturn relative to Jupiter provides different predictions for He rainout and in turn internal heat sources. The Galileo Probe used two independent techniques to determine the He/H₂ ratio at Jupiter. However, determining the He/H₂ ratio at Saturn has proven to be more elusive with values in the well mixed atmosphere ranging from highly depleted He to solar He abundance. This talk will concentrate on recent efforts to put together INMS mass spectrometry measurements of He/H₂ in the upper atmosphere during the Cassini Grand Finale with infrared (CIRS) and ultraviolet (UVIS) observations in the well mixed atmosphere to provide a consistent pressure temperature profile that can be stitched together with the help of a new shape model to provide a consistent picture of the He/H₂ ratio throughout the atmosphere.

14:15-15:00 The magnetic fields of the Giant Planets: more differences than similarities

Professor Jeremy Bloxham FRS, Harvard, USA

Abstract

Jupiter and Saturn are ostensibly similar as are Uranus and Neptune, yet their magnetic fields differ considerably. In particular, the magnetic fields of Jupiter and Saturn, as recently revealed in detail by the Juno and Cassini spacecraft, are quite dissimilar, suggesting that their magnetic fields are sensitive markers of the interior dynamics of these planets. We examine recent magnetic field observations from the Juno spacecraft, which is currently in a polar orbit around Jupiter. From the first phase of the Juno mission we find a magnetic field that is quite unlike any other: the field in Jupiter’s northern hemisphere is non-dipolar, with flux concentrated in a single band at mid-latitudes; in the southern hemisphere the field is nearly dipolar. In addition, we see a single, isolated intense flux spot at the equator. We consider possible explanations for this field morphology in terms of the interior of Jupiter, and contrast its magnetic field with that of Saturn.

15:00-15:30

15:30-16:15 Giant planet formation

Professor Ravit Helled, University of Zurich, Switzerland

Abstract

Giant planets are thought to have cores in their deep interiors, and the division into a heavy-element core and hydrogen-helium envelope is used in both formation and structure models. 

Proffesr Helled will briefly discuss the standard model for giant planet formation, and will show that the primordial internal structure of giant planets depends on their formation location and growth history. She will present a formation scenario for Jupiter that is consistent with cosmochemical constraints, and discuss the expected primordial internal structure of Jupiter from recent formation and evolution models (fuzzy core, inhomogeneous interior, planetesimal/pebble accretion). Professor Helled will also discuss mechanisms for heavy-element enrichments, and the challenges linked to enriched outer envelopes.  Finally, she will discuss the importance of these theoretical results for interpreting the measurements of the Juno and Cassini missions.

16:15-17:00 Lessons from Juno & Cassini: linking atmosphere and interior of Jupiter and Saturn

Professor Tristan Guillot, Observatoire de la Côte d'Azur, France

Abstract

In orbit since July 2016, Juno is changing the way we see Jupiter but also the other giant planets. The measurements of the gravity field of the planet, two orders of magnitude better than previous measurements (Folkner et al. 2017, Iess et al. 2018) have allowed to probe the deep interior in several ways. First it allowed for the first time to constrain the depth of the planet’s zonal jets to about 3000km below the clouds (Kaspi et al. 2018, Guillot et al. 2018). Second, it led to new interior models including the presence of a dilute core (Wahl et al. 2017) and a puzzling, still unsolved interior structure (Debras & Chabrier 2019). In parallel, similar measurements during the Cassini Grande Finale orbits led to a constraint on the depth of Saturn’s zonal flow, about 9000km (Iess et al. 2019, Galanti et al. 2019), in agreement with the Juno results for Jupiter. This tells us that differential rotation in the interior is suppressed where hydrogen becomes conductive and is dragged by the giant planet's powerful magnetic fields into a nearly-uniform rotation. Similarly, the complex interior structure of Jupiter is to be related to the likely presence of a deep extended stable region in Saturn which is required to explain the planets’ oscillations (Fuller 2014).

Adding to this complexity, measurements of Jupiter’s deep atmosphere by Juno’s microwave radiometer (Janssen et al. 2017) show that ammonia, but probably also temperature are not as uniform as one expected, down to pressures of tens of bars (Li et al. 2017). I will show that this may be explained by the interaction of water storms with ammonia, leading to a non-uniform and intrinsically variable distribution of abundances and temperatures both vertically and latitudinally. This appears to be a feature of hydrogen-dominated atmospheres, both due to the absence of a surface and to the fact that contrary to the Earth, condensates are much heavier than the surrounding air (see Guillot 1995, Li & Ingersoll 2015). The implications could be far-reaching for the understanding of giant planets dynamics and interiors.