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Abstract:
The magnetometer measurements taken by Cassini have confirmed the unusual character of Saturn’s internal magnetic field known from previous flybys and have revealed additional properties that suggest a rather unique dynamo in this planet. Within measurement uncertainty, the internal magnetic field is completely symmetric with respect to Saturn’s spin axis. The upper limit on the tilt of the magnetic dipole could be reduced from 1 to 0.06 degree. Moreover, only axisymmetric quadrupole and octupole moments are needed to fit the data. The lack of non-axisymmetric field components prevents a reliable determination of the bulk rotation rate of Saturn’s deep interior. Using data from Cassini’s closest approach to Saturn during orbit insertion, the magnetic moments of degrees four and five have been determined. The spatial power spectrum shows a zig-zag pattern with high power in odd spherical harmonic degrees and low power in even degrees. Compared to a simple dipole field, this corresponds to a concentration of magnetic flux towards the rotation poles. The flux concentration becomes progressively more pronounced when the field is continued into the interior. Comparison of the Cassini field model with that based on the Pioneer 11 and Voyager 1 and 2 measurements taken roughly 30 years earlier suggests that the secular variation of Saturn’s field is at least one order of magnitude slower than that of the Earth. A viable explanation for most of the unusual field properties is that a stably stratified and electrically conducting layer, formed by a partial demixing of helium from metallic hydrogen, exists on top of a “standard” dynamo in Saturn’s deep interior. This dynamo, driven by thermal and compositional convection, generates a magnetic field that is moderately asymmetric and time dependent. Rapid time variations and non-axisymmetric field components are filtered out in the stable layer by a skin effect. This model also implies that the top of the active dynamo may be located rather deep in Saturn’s interior and the geometric drop-off of the dipole strength with the radius cubed could explain the unexpectedly low field strength at Saturn’s surface. The stable layer model does not provide an explanation for the magnetic flux concentration towards the poles. Strong differential rotation in the dynamo region can have this effect, but a physical mechanism for such a flow state remains to be explored. From magnetic measurements to be taken during the very close approaches in the Grand Finale of the Cassini mission, we can expect to characterize Saturn’s magnetic field up to at least spherical harmonic degree nine and possibly to detect weak non-axisymmetric field components, which would enable an accurate determination of Saturn’s rotation period.
