Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Letter
  • Published:

Saturn’s fast spin determined from its gravitational field and oblateness

Nature volume 520, pages 202–204 (2015)Cite this article

Subjects

Abstract

The alignment of Saturn’s magnetic pole with its rotation axis precludes the use of magnetic field measurements to determine its rotation period1. The period was previously determined from radio measurements by the Voyager spacecraft to be 10 h 39 min 22.4 s (ref. 2). When the Cassini spacecraft measured a period of 10 h 47 min 6 s, which was additionally found to change between sequential measurements3,4,5, it became clear that the radio period could not be used to determine the bulk planetary rotation period. Estimates based upon Saturn’s measured wind fields have increased the uncertainty even more, giving numbers smaller than the Voyager rotation period, and at present Saturn’s rotation period is thought to be between 10 h 32 min and 10 h 47 min, which is unsatisfactory for such a fundamental property. Here we report a period of 10 h 32 min 45 s ± 46 s, based upon an optimization approach using Saturn’s measured gravitational field and limits on the observed shape and possible internal density profiles. Moreover, even when solely using the constraints from its gravitational field, the rotation period can be inferred with a precision of several minutes. To validate our method, we applied the same procedure to Jupiter and correctly recovered its well-known rotation period.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

Figure 1: An example of the statistical distribution of solutions for Saturn’s rotation period.
Figure 2: Solutions for Saturn’s rotation period.
Figure 3: Solutions for Jupiter’s rotation period.
Figure 4: Solutions for the rotation periods of Saturn and Jupiter when assuming improved gravity data.

Similar content being viewed by others

References

  1. Sterenborg, M. G. & Bloxham, J. Can Cassini magnetic field measurements be used to find the rotation period of Saturn's interior? Geophys. Res. Lett. 37, 11201 (2010)

    Article  ADS  Google Scholar 

  2. Smith, B. A. et al. A new look at the Saturn system: the Voyager 2 images. Science 215, 504–537 (1982)

    Article  CAS  ADS  Google Scholar 

  3. Gurnett, D. A. et al. Radio and plasma wave observations at Saturn from Cassini’s approach and first orbit. Science 307, 1255–1259 (2005)

    Article  CAS  ADS  Google Scholar 

  4. Gurnett, D. A. et al. The variable rotation period of the inner region of Saturn’s plasma disk. Science 316, 442–445 (2007)

    Article  CAS  ADS  Google Scholar 

  5. Giampieri, G., Dougherty, M. K., Smith, E. J. & Russell, C. T. A regular period for Saturn’s magnetic field that may track its internal rotation. Nature 441, 62–64 (2006)

    Article  CAS  ADS  Google Scholar 

  6. Sanchez-Lavega, A., Rojas, J. F. & Sada, P. V. Saturn’s zonal winds at cloud level. Icarus 147, 405–420 (2000)

    Article  ADS  Google Scholar 

  7. Lindal, G. F., Sweetnam, D. N. & Eshleman, V. R. The atmosphere of Saturn—an analysis of the Voyager radio occultation measurements. Astrophys. J. 90, 1136–1146 (1985)

    CAS  ADS  Google Scholar 

  8. Anderson, J. D. & Schubert, G. Saturn’s gravitational field, internal rotation, and interior structure. Science 317, 1384–1387 (2007)

    Article  CAS  ADS  Google Scholar 

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

    Article  CAS  ADS  Google Scholar 

  10. Zharkov, V. N. & Trubitsyn, V. P. Physics of Planetary Interiors 388 (Pachart Publishing House, 1978)

    Google Scholar 

  11. Hubbard, W. B. Planetary Interiors 1–343 (Van Nostrand Reinhold, 1984)

    Google Scholar 

  12. Schubert, G., Anderson, J., Zhang, K., Kong, D. & Helled, R. Shapes and gravitational fields of rotating two-layer Maclaurin ellipsoids: application to planets and satellites. Phys. Earth Planet. Inter. 187, 364–379 (2011)

    Article  ADS  Google Scholar 

  13. Kaspi, Y., Showman, A. P., Hubbard, W. B., Aharonson, O. & Helled, R. Atmospheric confinement of jet streams on Uranus and Neptune. Nature 497, 344–347 (2013)

    Article  CAS  ADS  Google Scholar 

  14. Jacobson, R. A. JUP230 Orbit Solutions http://ssd.jpl.nasa.gov/ (2003)

    Google Scholar 

  15. Jacobson, R. A. et al. The gravity field of the Saturnian system from satellite observations and spacecraft tracking data. Astrophys. J. 132, 2520–2526 (2006)

    Google Scholar 

  16. Hubbard, W. B. et al. Structure of Saturn’s mesosphere from the 28 SGR occultations. Icarus 130, 404–425 (1997)

    Article  ADS  Google Scholar 

  17. Flasar, F., Schinder, P. J., French, R. G., Marouf, E. A. & Kliore, A. J. Saturn’s shape from Cassini radio occultations. AGU Fall Meet. Abstr. B8 (2013)

  18. Helled, R., Schubert, G. & Anderson, J. D. Jupiter and Saturn rotation periods. Planet. Space Sci. 57, 1467–1473 (2009)

    Article  ADS  Google Scholar 

  19. Helled, R. Jupiter’s occultation radii: implications for its internal dynamics. Geophys. Res. Lett. 38, 8204 (2011)

    Article  ADS  Google Scholar 

  20. Helled, R. & Guillot, T. Interior models of Saturn: including the uncertainties in shape and rotation. Astrophys. J. 767, 113 (2013)

    Article  ADS  Google Scholar 

  21. Guillot, T. The interiors of giant planets: Models and outstanding questions. Annu. Rev. Earth Planet. Sci. 33, 493–530 (2005)

    Article  CAS  ADS  Google Scholar 

  22. Fortney, J. J. & Nettelmann, N. The interior structure, composition, and evolution of giant planets. Space Sci. Rev. 152, 423–447 (2010)

    Article  CAS  ADS  Google Scholar 

  23. Dowling, T. E. Saturn’s longitude: rise of the second branch of shear-stability theory and fall of the first. Int. J. Mod. Phys. D 23, 1430006–86 (2014)

    Article  ADS  Google Scholar 

  24. Higgins, C. A., Carr, T. D. & Reyes, F. A new determination of Jupiter’s radio rotation period. Geophys. Res. Lett. 23, 2653–2656 (1996)

    Article  CAS  ADS  Google Scholar 

  25. Porco, C. C. et al. Cassini imaging of Jupiter’s atmosphere, satellites and rings. Science 299, 1541–1547 (2003)

    Article  CAS  ADS  Google Scholar 

  26. Iess, L., Finocchiaro, S. & Racioppa, P. The determination of Jupiter and Saturn gravity fields from radio tracking of the Juno and Cassini spacecraft. AGU Fall Meet. Abstr. B1 (2013)

  27. Finocchiaro, S. & Iess, L. in Spaceflight Mechanics 2010 Vol. 136, 1417–1426 (American Astronautical Society, 2010)

    Google Scholar 

  28. Clairaut, A. C. Traite de la Figure de la Terre, tiree des Principes de l’Hydrostatique (Paris Courcier, 1743)

    MATH  Google Scholar 

  29. Nocedal, J. & Wright, S. J. Conjugate Gradient Methods 102–120 (Springer, 2006)

    Google Scholar 

  30. Carter, J. A. & Winn, J. N. Empirical constraints on the oblateness of an exoplanet. Astrophys. J. 709, 1219–1229 (2010)

    Article  CAS  ADS  Google Scholar 

  31. Kramm, U., Nettelmann, N., Fortney, J. J., Neuhäuser, R. & Redmer, R. Constraining the interior of extrasolar giant planets with the tidal Love number k2 using the example of HAT-P-13b. Astron. Astrophys. 538, A146 (2012)

    Article  ADS  Google Scholar 

Download references

Acknowledgements

We thank G. Schubert, M. Podolak, J. Anderson, L. Bary-Soroker, and the Juno science team for discussions and suggestions. We acknowledge support from the Israel Space Agency under grants 3-11485 (R.H.) and 3-11481 (Y.K.).

Author information

Authors and Affiliations

  1. Department of Geosciences, Raymond & Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, 69978, Israel,

    Ravit Helled

  2. Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot, 76100, Israel,

    Eli Galanti & Yohai Kaspi

Authors
  1. Ravit Helled
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Eli Galanti
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Yohai Kaspi
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

R.H. led the research. R.H. and Y.K. initiated the research and wrote the paper. E.G. designed the optimization approach and executed all the calculations. R.H. computed interior models and defined the parameter space for the figure functions. All authors contributed to the analysis and interpretation of the results.

Corresponding author

Correspondence to Ravit Helled.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 The flattening parameters calculated by the model without constraining the figure functions.

Shown are f, k, h for Saturn with ΔR = 50 km.

Extended Data Figure 2 An example of our statistical optimization model for deriving the rotation period.

The results are shown for a case for which the range of allowed rotation period is between 10 h 24 min and 10 h 54 min. The solution is based on a combination of 2,000 individual sub-cases, each of them representing a case with specific random initial conditions within the defined parameter space. a–c, A scatter plot (similar to Fig. 1a) of the distribution of solutions on the plane of the calculated rotation period Pcalc minus Pvoy versus each of ΔJ2, ΔJ4 and ΔJ6. Each blue dot represents one sub-case converged solution. In a the inner and outer black circles show the first and second standard deviations, respectively. d–j, The distribution of solutions for the figure functions K2, K3, H3, F1, F2 and F3, respectively.

Extended Data Figure 3 Saturn’s calculated rotation period versus the uncertainty in the assumed rotation period and radius.

Pcalc shown (colour scale) as a function of ΔP (h) and ΔR (km). The dashed line presents the transition from the regime where the constraint on the rotation period (above the dashed line) to the regime where the constraint on the shape is more dominant.

Extended Data Figure 4 Radial density profiles for two different interior models for Saturn (top) and Jupiter (bottom).

The black curves correspond to models with very large cores and the blue curves are no-core models in which the density profile is represented by 6th-order polynomials. For the massive-core case we assume a constant core density of ∼1.5 × 104 kg m−3, reaching 20% of the planet’s radius. The density profiles are constrained to match the planetary mass, J2, J4, J6, mean radius, and the atmospheric density and its derivative at 1 bar (see details in refs 8, 13 and 18). We then use the difference in the values of the figure functions in the two limiting cases to limit their values.

Extended Data Figure 5 The calculated flattening parameters when the figure functions are limited by interior models.

a–c, A scatter plot (similar to Fig. 1a) of the distribution of solutions on the plane of the calculated rotation period Pcalc minus Pvoy versus ΔJ2, ΔJ4 and ΔJ6, respectively. Each blue dot represents one sub-case converged solution. The calculated mean radius was set to be within 20 km of Saturn's observed mean radius. In a the inner and outer black circles show the first and second standard deviations, respectively. d–j, The distribution of solutions for the figure functions K2, K3, H3, F1, F2 and F3, respectively.

Extended Data Table 1 The physical properties of Saturn and Jupiter used in the analysis
Full size table
Extended Data Table 2 The calculated figure functions based on interior models of Saturn and Jupiter
Full size table

PowerPoint slides

Source data

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Helled, R., Galanti, E. & Kaspi, Y. Saturn’s fast spin determined from its gravitational field and oblateness. Nature 520, 202–204 (2015). https://doi.org/10.1038/nature14278

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/nature14278

This article is cited by

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing