Saturn's Gravitational Field, Internal Rotation, and Interior Structure

Observational constraints on the strength and depth of the zonal jets on Saturn

Saturn is characterized by strong zonal winds at the cloud level, with a wide equatorial west-to-east flow, two retrograde jets on its flanks, and a series of mid-to-high latitude jets in both hemispheres. Based on the Cassini mission gravity measurement up to zonal harmonic J10, it was shown that these winds penetrate to approximately 9000 km. However, these analyses relied fully on the low, even harmonics dominated by an interior structure that is not known to high accuracy. Here, we present a Cassini-based gravity analysis, resolving the gravity harmonics up to J20, by constraining the surface gravity in the polar regions. Using the extended harmonics and surface gravity, we show evidence that the wind structure poleward of latitude 45° must be shallower than 3000 km, while equatorward, the wind must extend along cylinders and decayed radially at a depth of approximately 11,000 km. We also find that nearly doubling the wind strength, while keeping its latitudinal structure, allows for an excellent fit to all 20 measured gravity harmonics, suggesting that the zonal jets reach their maximum strength below the observed cloud level.

Long-lasting, deep effect of Saturn's giant storms

Planetary-scale giant storms erupt on Saturn quasiperiodically. There have been at least six recorded occurrences of past eruptions, and the most recent one was in 2010, with its whole life span captured by the Cassini mission. In 2015, we used the Very Large Array to probe the deep response of Saturn's troposphere to the giant storms. In addition to the remnant effect of the storm in 2010, we have found long-lasting signatures of all mid-latitude giant storms, a mixture of equatorial storms up to hundreds of years old, and potentially an unreported older storm at 70°N. We derive an ammonia anomaly map that shows an extended meridional migration of the storm's aftermath and vertical transport of ammonia vapor by storm dynamics. Intriguingly, the last storm in 2010 splits into two distinct components that propagate in opposite meridional directions, leaving a gap at 43°N planetographic latitude.

Cassini Exploration of the Planet Saturn: A Comprehensive Review

Before Cassini, scientists viewed Saturn's unique features only from Earth and from three spacecraft flying by. During more than a decade orbiting the gas giant, Cassini studied the planet from its interior to the top of the atmosphere. It observed the changing seasons, provided up-close observations of Saturn's exotic storms and jet streams, and heard Saturn's lightning, which cannot be detected from Earth. During the Grand Finale orbits, it dove through the gap between the planet and its rings and gathered valuable data on Saturn's interior structure and rotation. Key discoveries and events include: watching the eruption of a planet-encircling storm, which is a 20- or 30-year event, detection of gravity perturbations from winds 9000 km below the tops of the clouds, demonstration that eddies are supplying energy to the zonal jets, which are remarkably steady over the 25-year interval since the Voyager encounters, re-discovery of the north polar hexagon after 25 years, determination of elemental abundance ratios He/H, C/H, N/H, P/H, and As/H, which are clues to planet formation and evolution, characterization of the semiannual oscillation of the equatorial stratosphere, documentation of the mysteriously high temperatures of the thermosphere outside the auroral zone, and seeing the strange intermittency of lightning, which typically ceases to exist on the planet between outbursts every 1-2 years. These results and results from the Jupiter flyby are all discussed in this review.

Deep rotating convection generates the polar hexagon on Saturn

Numerous land- and space-based observations have established that Saturn has a persistent hexagonal flow pattern near its north pole. While observations abound, the physics behind its formation is still uncertain. Although several phenomenological models have been able to reproduce this feature, a self-consistent model for how such a large-scale polygonal jet forms in the highly turbulent atmosphere of Saturn is lacking. Here, we present a three-dimensional (3D) fully nonlinear anelastic simulation of deep thermal convection in the outer layers of gas giant planets that spontaneously generates giant polar cyclones, fierce alternating zonal flows, and a high-latitude eastward jet with a polygonal pattern. The analysis of the simulation suggests that self-organized turbulence in the form of giant vortices pinches the eastward jet, forming polygonal shapes. We argue that a similar mechanism is responsible for exciting Saturn's hexagonal flow pattern.

How Well Do We Understand the Belt/Zone Circulation of Giant Planet Atmospheres?

The atmospheres of the four giant planets of our Solar System share a common and well-observed characteristic: they each display patterns of planetary banding, with regions of different temperatures, composition, aerosol properties and dynamics separated by strong meridional and vertical gradients in the zonal (i.e., east-west) winds. Remote sensing observations, from both visiting spacecraft and Earth-based astronomical facilities, have revealed the significant variation in environmental conditions from one band to the next. On Jupiter, the reflective white bands of low temperatures, elevated aerosol opacities, and enhancements of quasi-conserved chemical tracers are referred to as 'zones.' Conversely, the darker bands of warmer temperatures, depleted aerosols, and reductions of chemical tracers are known as 'belts.' On Saturn, we define cyclonic belts and anticyclonic zones via their temperature and wind characteristics, although their relation to Saturn's albedo is not as clear as on Jupiter. On distant Uranus and Neptune, the exact relationships between the banded albedo contrasts and the environmental properties is a topic of active study. This review is an attempt to reconcile the observed properties of belts and zones with (i) the meridional overturning inferred from the convergence of eddy angular momentum into the eastward zonal jets at the cloud level on Jupiter and Saturn and the prevalence of moist convective activity in belts; and (ii) the opposing meridional motions inferred from the upper tropospheric temperature structure, which implies decay and dissipation of the zonal jets with altitude above the clouds. These two scenarios suggest meridional circulations in opposing directions, the former suggesting upwelling in belts, the latter suggesting upwelling in zones. Numerical simulations successfully reproduce the former, whereas there is a wealth of observational evidence in support of the latter. This presents an unresolved paradox for our current understanding of the banded structure of giant planet atmospheres, that could be addressed via a multi-tiered vertical structure of "stacked circulation cells," with a natural transition from zonal jet pumping to dissipation as we move from the convectively-unstable mid-troposphere into the stably-stratified upper troposphere.

Saturn’s magnetic field revealed by the Cassini Grand Finale

During 2017, the Cassini fluxgate magnetometer made in situ measurements of Saturn’s magnetic field at distances ~2550 ± 1290 kilometers above the 1-bar surface during 22 highly inclined Grand Finale orbits. These observations refine the extreme axisymmetry of Saturn’s internal magnetic field and show displacement of the magnetic equator northward from the planet’s physical equator. Persistent small-scale magnetic structures, corresponding to high-degree (>3) axisymmetric magnetic moments, were observed. This suggests secondary shallow dynamo action in the semiconducting region of Saturn’s interior. Some high-degree magnetic moments could arise from strong high-latitude concentrations of magnetic flux within the planet’s deep dynamo. A strong field-aligned current (FAC) system is located between Saturn and the inner edge of its D-ring, with strength comparable to the high-latitude auroral FACs.

Dynamic transition of supercritical hydrogen: defining the boundary between interior and atmosphere in gas giants

Understanding the physics of gas giants requires knowledge about the behavior of hydrogen at extreme pressures and temperatures. Molecular hydrogen in these planets is supercritical, and has been considered as a physically homogeneous state where no differences can be made between a liquid and a gas and where all properties undergo no marked or distinct changes with pressure and temperature, the picture believed to hold below the dissociation and metallization transition. Here, we show that in Jupiter and Saturn, supercritical molecular hydrogen undergoes a dynamic transition around 10 GPa and 3000 K from the "rigid" liquid state to the "nonrigid" gas-like fluid state at the Frenkel line recently proposed, with the accompanying qualitative changes of all major physical properties. The consequences of this finding are discussed, including a physically justified way to demarcate the interior and the atmosphere in gas giants.