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An unexpected cooling effect in Saturn’s upper atmosphere

Abstract

The upper atmospheres of the four Solar System giant planets exhibit high temperatures1,2 that cannot be explained by the absorption of sunlight2,3. In the case of Saturn the temperatures predicted by models of solar heating2,4 are 200 K, compared to temperatures of 400 K observed independently in the polar regions5 and at 30° latitude6. This unexplained ‘energy crisis’ represents a major gap in our understanding of these planets’ atmospheres. An important candidate for the source of the missing energy is the magnetosphere1,2,4,7,8,9, which injects energy mostly in the polar regions of the planet. This polar energy input is believed to be sufficient to explain the observed temperatures9, provided that it is efficiently redistributed globally by winds4,8, a process that is not well understood. Here we show, using a numerical model4, that the net effect of the winds driven by the polar energy inputs is not to heat but to cool the low-latitude thermosphere. This surprising result allows us to rule out known polar energy inputs as the solution to the energy crisis at Saturn. There is either an unknown—and large—source of polar energy, or, more probably, some other process heats low latitudes directly.

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Figure 1: Thermal and dynamical structure of the upper atmosphere predicted by our model.
Figure 2: Interpretation of polar dynamics in terms of hydrostatic balance.

References

  1. Atreya, S. K. Atmosphere and Ionospheres of the Outer Planets and their Satellites Ch. 2 (Springer, Heidelberg, 1986)

    Book  Google Scholar 

  2. Yelle, R. V. & Miller, S. in Jupiter: Planet, Satellites and Magnetosphere (eds Bagenal, F., McKinnon, W. & Dowling, T.) 185–218 (Cambridge Univ. Press, Cambridge, UK, 2004)

    Google Scholar 

  3. Strobel, D. F. & Smith, G. R. On the temperature of the jovian thermosphere. J. Atmos. Sci. 30, 718–725 (1973)

    ADS  CAS  Article  Google Scholar 

  4. Mueller-Wodarg, I. C. F., Mendillo, M., Yelle, R. V. & Aylward, A. D. A global circulation model of Saturn’s thermosphere. Icarus 180, 147–160 (2006)

    ADS  Article  Google Scholar 

  5. Melin, H. Comparative Aeronomy of the Upper Atmospheres of the Giant Planets. PhD thesis, Univ. London. (2006)

  6. Smith, G. R. et al. Saturn’s upper atmosphere from the Voyager 2 EUV solar and stellar occultations. J. Geophys. Res. 88, 8667–8679 (1983)

    ADS  CAS  Article  Google Scholar 

  7. Smith, C. G. A., Miller, S. & Aylward, A. D. Magnetospheric energy inputs into the upper atmospheres of the giant planets. Ann. Geophys. 23, 1943–1947 (2005)

    ADS  Article  Google Scholar 

  8. Smith, C. G. A., Aylward, A. D., Miller, S. & Mueller-Wodarg, I. C. F. Polar heating in Saturn’s thermosphere. Ann. Geophys. 23, 2465–2477 (2005)

    ADS  Article  Google Scholar 

  9. Cowley, S. W. H., Bunce, E. J. & O’Rourke, J. M. A simple quantitative model of plasma flows and currents in Saturn’s polar ionosphere. J. Geophys. Res. A18, 5212–5230 (2004)

    ADS  Article  Google Scholar 

  10. Fuller-Rowell, T. J. et al. in STEP Handbook of Ionospheric Models 217–238 (SCOSTEP, Logan, Utah, 1996)

    Google Scholar 

  11. Moses, J. I. et al. Photochemistry of Saturn’s atmosphere. I. Hydrocarbon chemistry and comparisons with ISO observations. Icarus 143, 244–298 (2000)

    ADS  CAS  Article  Google Scholar 

  12. Atreya, S. K. Eddy mixing coefficient on Saturn. Planet. Space Sci. 30, 849–854 (1982)

    ADS  CAS  Article  Google Scholar 

  13. Davis, L. J. & Smith, E. J. A model of Saturn’s magnetic field based on all available data. J. Geophys. Res. 95, 15257–15261 (1990)

    ADS  Article  Google Scholar 

  14. Moore, L. E., Mendillo, M., Mueller-Wodarg, I. C. F. & Murr, D. L. Modeling of global variations and ring shadowing in Saturn’s ionosphere. Icarus 172, 503–520 (2004)

    ADS  CAS  Article  Google Scholar 

  15. Richardson, J. D. Thermal ions and Saturn—Plasma parameters and implications. J. Geophys. Res. 91, 1381–1389 (1986)

    ADS  CAS  Article  Google Scholar 

  16. Stallard, T. S., Miller, S., Trafton, L. M., Geballe, T. R. & Joseph, R. D. Ion winds in Saturn’s southern auroral/polar region. Icarus 167, 204–211 (2004)

    ADS  CAS  Article  Google Scholar 

  17. Clarke, J. T. et al. Morphological differences between Saturn’s ultraviolet aurorae and those of Earth and Jupiter. Nature 433, 717–719 (2005)

    ADS  CAS  Article  Google Scholar 

  18. Codrescu, M. V., Fuller-Rowell, T. J. & Foster, J. C. On the importance of E-field variability for Joule heating in the high-latitude thermosphere. Geophys. Res. Lett. 22, 2393–2396 (1995)

    ADS  Article  Google Scholar 

  19. Young, L. A., Yelle, R. V., Young, R., Seiff, A. & Kirk, D. B. Gravity waves in Jupiter’s thermosphere. Science 276, 108–111 (1997)

    ADS  CAS  Article  Google Scholar 

  20. Matcheva, K. I. & Strobel, D. F. Heating of Jupiter’s thermosphere by dissipation of gravity waves due to molecular viscosity and heat conduction. Icarus 140, 328–340 (1999)

    ADS  Article  Google Scholar 

  21. Hickey, M. P., Walterscheid, R. L. & Schubert, G. Gravity wave heating and cooling in Jupiter’s thermosphere. Icarus 148, 266–281 (2000)

    ADS  Article  Google Scholar 

  22. Hickey, M. P., Schubert, G. & Walterscheid, R. L. Gravity wave heating and cooling in Saturn’s thermosphere. Eos 86 (Suppl. 18), abstr. SA24A–06. (2005)

  23. Festou, M. C. & Atreya, S. K. Voyager ultraviolet stellar occultation measurements of the composition and thermal profiles of the Saturnian upper atmosphere. Geophys. Res. Lett. 9, 1147–1150 (1982)

    ADS  CAS  Article  Google Scholar 

  24. Atreya, S. K., Waite, J. H., Donahue, T. M., Nagy, A. F. & McConnell, J. C. in Saturn (eds Gehrels, T. & Matthews, M. S.) 239–277 (Univ. Arizona Press, Tucson, Arizona, 1984)

    Google Scholar 

  25. Smith, G. R. & Hunten, D. M. Study of planetary atmospheres by absorptive occultations. Rev. Geophys. 28, 117–143 (1990)

    ADS  Article  Google Scholar 

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Acknowledgements

The simulations described in this study were performed using the HiPerSPACE facility at UCL, funded by the UK Particle Physics and Astronomy Research Council (PPARC). C.G.A.S. acknowledges receipt of a CASE studentship funded by PPARC and Sun Microsystems Ltd.

Author Contributions The thermosphere modelling was carried out by C.G.A.S., A.D.A., G.H.M. and S.M. L.E.M. provided the ionosphere model.

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Correspondence to A. D. Aylward.

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Supplementary Information

This file contains Supplementary Methods, Supplementary Discussion, Supplementary Notes, Supplementary Figures 1-11 with legends and Supplementary Table 1. The file contains further description of the model and discussion of our results, including a detailed description of our numerical thermosphere model, descriptions of our models of ionospheric conductivity and magnetospheric plasma flow, details of our formulation of Joule heating and ion drag, analysis of the detailed force balances calculated by the model, description and discussion of a sensitivity study examining the effect of an increased ionospheric conductivity and a discussion of possible circumstances in which the observed cooling effect may be mitigated, or even reversed. (PDF 1416 kb)

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Smith, C., Aylward, A., Millward, G. et al. An unexpected cooling effect in Saturn’s upper atmosphere. Nature 445, 399–401 (2007). https://doi.org/10.1038/nature05518

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