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A regular period for Saturn's magnetic field that may track its internal rotation

Nature volume 441, pages 6264 (04 May 2006) | Download Citation



The rotation rate of a planet is one of its fundamental properties. Saturn's rotation, however, is difficult to determine because there is no solid surface from which to time it, and the alternative ‘clock’—the magnetic field—is nearly symmetrically aligned with the rotation axis1,2,3,4,5,6,7. Radio emissions, thought to provide a proxy measure of the rotation of the magnetic field, have yielded estimates of the rotation period between 10 h 39 min 22 s and 10 h 45 min 45 s (refs 8–10). Because the period determined from radio measurements exhibits large time variations, even on timescales of months, it has been uncertain whether the radio-emission periodicity coincides with the inner rotation rate of the planet. Here we report magnetic field measurements that revealed a time-stationary magnetic signal with a period of 10 h 47 min 6 s ± 40 s. The signal appears to be stable in period, amplitude and phase over 14 months of observations, pointing to a close connection with the conductive region inside the planet, although its interpretation as the ‘true’ inner rotation period is still uncertain.

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Results presented here represent one aspect of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research was performed while G.G. held a National Research Council senior research associateship award at JPL. We thank M. E. Burton and J. Wolf for suggestions on the manuscript.

Author information


  1. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA

    • G. Giampieri
    •  & E. J. Smith
  2. The Blackett Laboratory, Imperial College London, London SW7 2AZ, UK

    • M. K. Dougherty
  3. UCLA Institute of Geophysics and Planetary Physics, Los Angeles, California 90025, USA

    • C. T. Russell


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Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

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Correspondence to G. Giampieri.

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