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An unexpected cooling effect in Saturn’s upper atmosphere


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.


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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).

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