Research Article

Measurement and implications of Saturn’s gravity field and ring mass

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Science  14 Jun 2019:
Vol. 364, Issue 6445, eaat2965
DOI: 10.1126/science.aat2965

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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

Structured Abstract

INTRODUCTION

The interior structure of Saturn, the depth of its winds, and the mass and age of its rings have been open questions in planetary science. The mass distribution inside a fluid and rapidly rotating planet like Saturn is largely driven by the ratio between centrifugal and gravitational forces. In static conditions, the planet should rotate uniformly and its gravity field should be axially and hemispherically symmetric and thus described by even zonal harmonics. However, optical tracking of clouds in the atmospheres of gas giants indicates that their rotation is not uniform. If the velocity field seen at cloud level extends deep into the interior, then there is a redistribution of mass that modifies the gravity field. Gravity measurements therefore constrain both Saturn’s deep interior and the depth of its winds. They also provide the mass of its rings, which dynamical and compositional dating methods have shown is related to their age.

RATIONALE

In its final 22 orbits, the Cassini spacecraft passed between the rings and the atmosphere of Saturn. In six of these orbits, while the spacecraft was in free fall under the combined attraction of Saturn and its rings, radio tracking from an antenna on Earth was established to measure the radial velocity of the spacecraft with accuracies between 0.023 and 0.088 mm·s−1 at a 30 s integration time. Fitting these data with a dynamical model of the forces acting on the spacecraft allowed us to determine the separate signatures from each zonal harmonic and the ring mass.

RESULTS

The estimated quantities were a zonal gravity model for Saturn and the gravity from a uniformly distributed mass spanning the A, B, and C rings. Additional small accelerations of unknown origin (possibly related to a time-variable gravity field) were required to obtain a good fit of the data. The determination of Saturn’s static zonal coefficients and ring mass does not depend on the assumed source of these small effects. We found that the measured values of J6, J8, and J10 are so large that they cannot be matched with interior models relying on uniform rotation and plausible compositions, but they are in agreement with interior models that assume deep differential rotation, extending from the equator into the interior up to distances of 0.7 to 0.8 Saturn radii from the spin axis. The equatorial outer layers of Saturn must rotate at an angular velocity that is 4% faster than that of the deep interior, whereas regions at higher latitudes must rotate 1 to 2% slower, regardless of the assumed rotation period. A thermal-wind approach shows that flow profiles that are similar in general character to the observed one yield solutions within the uncertainty of the gravity measurements when extended to a depth of ~9000 km, confirming that the flows are very deep and likely extend down to the levels where magnetic dissipation occurs.

We also found that the mass of the rings is 0.41 times that of the Saturnian moon Mimas, which is at the lower end of the expected values.

CONCLUSION

Saturn’s gravity field measured by Cassini implies a strong and deep differential rotation, extending to a depth of ~9000 km. This differs from Jupiter, where winds are shallower (~3000 km). The gravity measurements are consistent with a mass of Saturn’s core of 15 to 18 Earth masses.

The low value of the ring mass suggests a scenario where the present rings of Saturn are young, probably just 10 million to 100 million years old, to be consistent with their pristine icy composition. Nevertheless, the rings may have evolved substantially since their formation and were perhaps once more massive than they are today. Models for a young ring system invoke the chance capture and tidal disruption of a comet or an icy outer Solar System body, suggesting that catastrophic events continued to occur in the Solar System long after its formation 4.6 billion years ago.

During the Grand Finale phase of its mission, Cassini passed between the inner edge of Saturn’s D ring and the cloud top, enabling the measurement of the ring mass.

This orbit allowed gravitational acceleration of the rings to be disentangled from the much larger acceleration resulting from Saturn’s oblateness, as they pull the spacecraft in opposite directions. The curves show the spacecraft velocity variation due to the rings and the differential rotation.

Abstract

The interior structure of Saturn, the depth of its winds, and the mass and age of its rings constrain its formation and evolution. In the final phase of the Cassini mission, the spacecraft dived between the planet and its innermost ring, at altitudes of 2600 to 3900 kilometers above the cloud tops. During six of these crossings, a radio link with Earth was monitored to determine the gravitational field of the planet and the mass of its rings. We find that Saturn’s gravity deviates from theoretical expectations and requires differential rotation of the atmosphere extending to a depth of at least 9000 kilometers. The total mass of the rings is (1.54 ± 0.49) × 1019 kilograms (0.41 ± 0.13 times that of the moon Mimas), indicating that the rings may have formed 107 to 108 years ago.

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