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Layered convection as the origin of Saturn’s luminosity anomaly

Abstract

As the giant planets of our Solar System continue to cool and contract, they radiate more energy than they receive from the Sun. A giant planet’s cooling rate, luminosity and temperature at a given age can be determined using the first and second principles of thermodynamics. Measurements of Saturn’s infrared luminosity, however, reveal that Saturn is significantly brighter than predicted for its age1,2. This excess luminosity has been attributed to the immiscibility of helium in Saturn’s hydrogen-rich envelope, which leads to rains of helium-rich droplets3,4,5,6,7,8. Existing calculations of Saturn’s evolution, however, suggest that the energy released by helium rains might be insufficient to resolve the luminosity puzzle9. Here we demonstrate, using semi-analytical models of planetary thermal evolution, that the cooling of Saturn’s interior is significantly slower in the presence of layered convection generated—like in Earth’s oceans—by a compositional gradient. We find that layered convection can explain Saturn’s present luminosity for a wide range of initial energy configurations without invoking any additional energy source. Our findings suggest that the interior structure, composition and thermal evolution of giant planets in our Solar System and beyond may be more complex than the conventional approximation of giant planets as homogeneous adiabatic bodies.

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Figure 1: Evolution of the total internal energy () with time for various Saturn models.
Figure 2: Cooling sequences of Saturn models with layered convection.
Figure 3: Impact of the size of the layered convection zone on Saturn’s cooling sequence.
Figure 4: Past luminosity of Saturn’s model with layered convection.

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Acknowledgements

J.L. thanks J. Fortney for making his atmospheric grids available to us in electronic format. The research leading to these results has received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013 Grant Agreement no. 247060). J.L. acknowledges financial support from the DIM ACAV.

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J.L. carried out analytical calculations, developed the model and performed the numerical simulations. G.C. suggested the idea and carried out analytical calculations. J.L and G.C. wrote the manuscript.

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Correspondence to Jérémy Leconte.

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The authors declare no competing financial interests.

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Leconte, J., Chabrier, G. Layered convection as the origin of Saturn’s luminosity anomaly. Nature Geosci 6, 347–350 (2013). https://doi.org/10.1038/ngeo1791

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