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Deep winds beneath Saturn’s upper clouds from a seasonal long-lived planetary-scale storm

Abstract

Convective storms occur regularly in Saturn’s atmosphere1,2,3,4. Huge storms known as Great White Spots, which are ten times larger than the regular storms, are rarer and occur about once per Saturnian year (29.5 Earth years). Current models propose that the outbreak of a Great White Spot is due to moist convection induced by water5,6. However, the generation of the global disturbance and its effect on Saturn’s permanent winds1,7 have hitherto been unconstrained8 by data, because there was insufficient spatial resolution and temporal sampling9,10,11 to infer the dynamics of Saturn’s weather layer (the layer in the troposphere where the cloud forms). Theoretically, it has been suggested that this phenomenon is seasonally controlled5,9,10. Here we report observations of a storm at northern latitudes in the peak of a weak westward jet during the beginning of northern springtime, in accord with the seasonal cycle but earlier than expected. The storm head moved faster than the jet, was active during the two-month observation period, and triggered a planetary-scale disturbance that circled Saturn but did not significantly alter the ambient zonal winds. Numerical simulations of the phenomenon show that, as on Jupiter12, Saturn’s winds extend without decay deep down into the weather layer, at least to the water-cloud base at pressures of 10–12 bar, which is much deeper than solar radiation penetrates.

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Figure 1: Initial growth of the Great White Spot.
Figure 2: Expansion of the storm clouds and the planetary-scale disturbance.
Figure 3: Zonal winds from motions of the disturbance clouds.
Figure 4: Models of the GWS planetary-scale disturbance.

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Acknowledgements

A.S.-L., T.d.R.-G., R.H., J.L., J.M.G.-F., E.G.-M. and J.F.S.-R. are supported by the Spanish MICIIN, by FEDER and by Gobierno Vasco. We thank S. Pérez-Hoyos for initial support of this study and M. Alises and A. Guijarro for taking the Calar Alto Observatory images (CAHA and MPG/CSIC). E.G.-M. used computing facilities at CESCA (Barcelona) supported by MICIIN. L.N.F. is supported by a Glasstone fellowship at the University of Oxford. The International Outer Planet Watch (IOPW) Team and other individual contributors listed in Supplementary Information provided most of the images used for tracking in this study; these images were complemented in some cases with images taken from contributors to the ALPO Japan (Association of Lunar and Planetary Observers) database (http://alpo-j.asahikawa-med.ac.jp/indexE.htm).

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A.S.-L. coordinated the study and performed the motion and wind measurements and the interpretation. T.d.R.-G. performed the photometric measurements and filter calibrations. R.H. measured the storm growth rate and with J. Legarreta coordinated the IOPW database. J.M.G.-F. prepared the map projections and image search. J.F.S.-R. performed the radiative transfer calculations. E.G.-M. and J. Legarreta performed the EPIC simulations. F.C. and J. Lecacheux provided the Pic-du-Midi photometric images. L.N.F. provided data on the thermal structure of the storm. D.B.-N. provided the photometric images obtained at Calar Alto Observatory, and D.P. provided photometric images at selected wavelengths. All these authors discussed the results and commented on the manuscript. Contributors to the IOPW-PVOL database are listed at the end of this Letter.

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Correspondence to A. Sánchez-Lavega.

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The file contains Supplementary Table 1, Supplementary Figures 1-3 with legends and additional references. (PDF 533 kb)

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Sánchez-Lavega, A., Río-Gaztelurrutia, T., Hueso, R. et al. Deep winds beneath Saturn’s upper clouds from a seasonal long-lived planetary-scale storm. Nature 475, 71–74 (2011). https://doi.org/10.1038/nature10203

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