Research Article

Saturn’s magnetic field revealed by the Cassini Grand Finale

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Science  05 Oct 2018:
Vol. 362, Issue 6410, eaat5434
DOI: 10.1126/science.aat5434

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Cassini's final phase of exploration

The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planet's upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planet's aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planet's upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturn's atmosphere.

Science, this issue p. eaat5434, p. eaat1962, p. eaat2027, p. eaat3185, p. eaat2236, p. eaat2382

Structured Abstract

INTRODUCTION

Starting on 26 April 2017, the Grand Finale phase of the Cassini mission took the spacecraft through the gap between Saturn’s atmosphere and the inner edge of its innermost ring (the D-ring) 22 times, ending with a final plunge into the atmosphere on 15 September 2017. This phase offered an opportunity to investigate Saturn’s internal magnetic field and the electromagnetic environment between the planet and its rings. The internal magnetic field is a diagnostic of interior structure, dynamics, and evolution of the host planet. Rotating convective motion in the highly electrically conducting layer of the planet is thought to maintain the magnetic field through the magnetohydrodynamic (MHD) dynamo process. Saturn’s internal magnetic field is puzzling because of its high symmetry relative to the spin axis, known since the Pioneer 11 flyby. This symmetry prevents an accurate determination of the rotation rate of Saturn’s deep interior and challenges our understanding of the MHD dynamo process because Cowling’s theorem precludes a perfectly axisymmetric magnetic field being maintained through an active dynamo.

RATIONALE

The Cassini fluxgate magnetometer was capable of measuring the magnetic field with a time resolution of 32 vectors per s and up to 44,000 nT, which is about twice the peak field strength encountered during the Grand Finale orbits. The combination of star cameras and gyroscopes onboard Cassini provided the attitude determination required to infer the vector components of the magnetic field. External fields from currents in the magnetosphere were modeled explicitly, orbit by orbit.

RESULTS

Saturn’s magnetic equator, where the magnetic field becomes parallel to the spin axis, is shifted northward from the planetary equator by 2808.5 ± 12 km, confirming the north-south asymmetric nature of Saturn’s magnetic field. After removing the systematic variation with distance from the spin axis, the peak-to-peak “longitudinal” variation in Saturn’s magnetic equator position is <18 km, indicating that the magnetic axis is aligned with the spin axis to within 0.01°. Although structureless in the longitudinal direction, Saturn’s internal magnetic field features variations in the latitudinal direction across many different characteristic length-scales. When expressed in spherical harmonic space, internal axisymmetric magnetic moments of at least degree 9 are needed to describe the latitudinal structures. Because there was incomplete latitudinal coverage during the Grand Finale orbits, which can lead to nonuniqueness in the solution, regularized inversion techniques were used to construct an internal Saturn magnetic field model up to spherical harmonic degree 11. This model matches Cassini measurements and retains minimal internal magnetic energy. An azimuthal field component two orders of magnitude smaller than the radial and meridional components is measured on all periapses (closest approaches to Saturn). The steep slope in this component and magnetic mapping to the inner edge of the D-ring suggests an external origin of this component.

CONCLUSION

Cassini Grand Finale observations confirm an extreme level of axisymmetry of Saturn’s internal magnetic field. This implies the presence of strong zonal flows (differential rotation) and stable stratification surrounding Saturn’s deep dynamo. The rapid latitudinal variations in the field suggest a second shallow dynamo maintained by the background field from the deep dynamo, small-scale helical motion, and deep zonal flows in the semiconducting region closer to the surface. Some of the high-degree magnetic moments could result from strong high-latitude concentrations of magnetic flux within the planet’s deep dynamo. The periapse azimuthal field originates from a strong interhemispherical electric current system flowing along magnetic field lines between Saturn and the inner edge of the D-ring, with strength comparable to that of the high-latitude field-aligned currents (FACs) associated with Saturn’s aurorae.

A meridional view of the results of the Cassini magnetometer observations during the Grand Finale orbits.

Overlain on the spacecraft trajectory is the measured azimuthal field from the first Grand Finale orbit, revealing high-latitude auroral FACs and a low-latitude interhemispherical FAC system. Consistent small-scale axisymmetric internal magnetic field structures originating in the shallow interior are shown as field lines within the planet. A tentative deep stable layer and a deeper dynamo layer, overlying a central core, are shown as dashed semicircles. The A-, B-, C-, and D-rings are labeled, and the magnetic field lines are shown as solid lines. RS is Saturn’s radius, Z is the distance from the planetary equator, ρ is the perpendicular distance from the spin axis, and Bϕ is the azimuthal component of the magnetic field.

Abstract

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.

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