A regular period for Saturn's magnetic field that may track its internal rotation (original) (raw)

Nature volume 441, pages 62–64 (2006)Cite this article

Abstract

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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Acknowledgements

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.

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Authors and Affiliations

  1. Jet Propulsion Laboratory, California Institute of Technology, California, 91109, Pasadena, USA
    G. Giampieri & E. J. Smith
  2. The Blackett Laboratory, Imperial College London, SW7 2AZ, London, UK
    M. K. Dougherty
  3. UCLA Institute of Geophysics and Planetary Physics, California, 90025, Los Angeles, USA
    C. T. Russell

Authors

  1. G. Giampieri
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  2. M. K. Dougherty
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  3. E. J. Smith
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  4. C. T. Russell
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Correspondence toG. Giampieri.

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Giampieri, G., Dougherty, M., Smith, E. et al. A regular period for Saturn's magnetic field that may track its internal rotation.Nature 441, 62–64 (2006). https://doi.org/10.1038/nature04750

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Editorial Summary

A day on Saturn

The absence of a visible solid surface and close alignment of the magnetic field and rotational axis mean that for Saturn, the two best ways of measuring planetary rotation are not available. So Saturn's exact rotation rate is a mystery. The Cassini orbiter provided a detailed map of Saturn's magnetic field, and this has been used to identify an oscillation in the magnetic field with a well-defined period of about 10 hours 47 minutes. The stability of this periodicity over 14 months of observation suggests that it is related to the rotation of Saturn's interior. The nature of the link is not clear, but this is the best estimate yet for Saturn's day length.

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