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Saturn’s rotation period from its atmospheric planetary-wave configuration


The rotation period of a gas giant's magnetic field (called the System III reference frame) is commonly used to infer its bulk rotation1. Saturn's dipole magnetic field is not tilted relative to its rotation axis (unlike Jupiter, Uranus and Neptune), so the surrogate measure of its long-wavelength (kilometric) radiation is currently used to fix the System III rotation period2. The period as measured now by the Cassini spacecraft is up to 7 min longer3 than the value of 10 h 39 min 24 s measured 28 years ago by Voyager2. Here we report a determination of Saturn's rotation period based on an analysis of potential vorticity. The resulting reference frame (which we call System IIIw) rotates with a period of 10 h 34 min 13 ± 20 s. This shifted reference frame is consistent with a pattern of alternating jets on Saturn that is more symmetrical between eastward and westward flow. This suggests that Saturn's winds are much more like those of Jupiter than hitherto believed4.

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Figure 1: Variation of limiting planetary wave angular velocity with latitude for Saturn and Jupiter.
Figure 2: Comparison of cloud-level zonal velocity profiles on Saturn and Jupiter as viewed in System IIIw and System III.


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We are grateful to F. M. Flasar and the Cassini CIRS team for access to the data from which the potential vorticity profiles discussed here were computed. P.L.R. acknowledges support from the UK Science and Technology Facilities Council, T.E.D. acknowledges support from NASA's Planetary Atmospheres and Outer Planet Research Programs, and G.S. acknowledges support from NASA’s Planetary Atmospheres and Planetary Geology and Geophysics programs. We are grateful also to R. Helled for computing values of ρ0 and J2 for Saturn based on System IIIw.

Author Contributions P.L.R. obtained and processed the Cassini data, deriving potential vorticity and ω(α) profiles, and conducted the statistical analysis. T.E.D. made the original suggestion to investigate α(φ) and contributed to the interpretation of the results. G.S. provided additional insights into Saturn’s rotation, and all authors contributed to the text and discussion of results.

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Read, P., Dowling, T. & Schubert, G. Saturn’s rotation period from its atmospheric planetary-wave configuration. Nature 460, 608–610 (2009).

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