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5 - The Mysterious Periodicities of Saturn

Clues to the Rotation Rate of the Planet

Published online by Cambridge University Press:  13 December 2018

By
James F. Carbary ,
Matthew M. Hedman ,
Thomas W. Hill ,
Xianzhe Jia ,
William Kurth ,
Laurent Lamy  and
Gabrielle Provan
Edited by
Kevin H. Baines ,
F. Michael Flasar ,
Norbert Krupp  and
Tom Stallard
Kevin H. Baines
Affiliation:
University of Wisconsin, Madison
F. Michael Flasar
Affiliation:
NASA-Goddard Space Flight Center
Norbert Krupp
Affiliation:
Max-Planck-Institut für Sonnensystemforschung, Göttingen
Tom Stallard
Affiliation:
University of Leicester
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Summary

The rotation rate of a planet is a fundamental parameter, no less than its mass or composition, and planetary investigators require this rate to assess various other phenomena such as planetary wind speeds, internal and atmospheric models, ring dynamics and so forth. Saturn presents a conundrum, however, because none of its various planetary periods indicates the “true” rotation of the planet. Thus, although the planet displays an abundance of periodicities near 10.7 hours, the exact rotation period of Saturn is unknown. In the magnetosphere, “planetary-period oscillations” (PPOs) appear in charged particles, magnetic fields, energetic neutral atoms, radio emissions and motions of the plasma sheet and magnetopause. In Saturn’s rings, the spoke phenomenon can exhibit periodicities near 10.7 hours, and ring phenomena themselves may be related to the interior rotation of the planet. In the high-latitude ionosphere, modulations near this period appear in auroral motions and intensities. In the upper atmosphere, some cloud features rotate near this period, although wind speeds are generally faster, and the well-known polar hexagon rotates with a period close to 10.7 hours. Some of the magnetospheric/ionospheric oscillations differ in the northern and southern hemispheres and their periods do not remain constant, sometimes varying on long time scales of a year or longer and sometimes on much shorter time scales. These variations in the period argue against a cause related to changes interior to Saturn, and because the magnetic and spin axes of Saturn are reported to be axisymmetric (unlike those of any other known planet), Saturn’s periodicities cannot be explained as “wobble” caused by a geometric tilt or by a nondipolar magnetic anomaly. Several models have been proposed to account for the observed periodicities, including rotating atmospheric vortices, periodic plasma releases and a flapping magnetodisk, but none can satisfactorily explain all of Saturn’s periodicities nor their common origin, and none can determine the exact rotation rate of the planet. This chapter reviews Saturn’s periodicities, theories thereof, and how they might be used to determine the elusive rotation rate of the planet.

Type
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Information
Saturn in the 21st Century , pp. 97 - 125
Publisher: Cambridge University Press
Print publication year: 2018

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