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A complex storm system in Saturn’s north polar atmosphere in 2018

Abstract

Saturn’s convective storms usually fall in two categories. One consists of mid-sized storms 2,000 km wide, appearing as irregular bright cloud systems that evolve rapidly, on scales of a few days. The other includes the Great White Spots, planetary-scale giant storms ten times larger than the mid-sized ones, which disturb a full latitude band, enduring several months, and have been observed only seven times since 1876. Here we report a new intermediate type, observed in 2018 in the north polar region. Four large storms with east–west lengths 4,000–8,000 km (the first one lasting longer than 200 days) formed sequentially in close latitudes, experiencing mutual encounters and leading to zonal disturbances affecting a full latitude band 8,000 km wide, during at least eight months. Dynamical simulations indicate that each storm required energies around ten times larger than mid-sized storms but 100 times smaller than those necessary for a Great White Spot. This event occurred at about the same latitude and season as the Great White Spot in 1960, in close correspondence with the cycle of approximately 60 years hypothesized for equatorial Great White Spots.

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Fig. 1: The 2018 complex north polar storm system and disturbances.
Fig. 2: Storm motions from 29 March to 29 October 2018.
Fig. 3: Convective onset in a compact cyclone.
Fig. 4: Vertical cloud structure and particle imaginary refractive index.
Fig. 5: Numerical simulations of the disturbances generated by the storm outbreaks.
Fig. 6: Seasonal insolation at the top of Saturn’s atmosphere and convective events.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author on reasonable request. This work relies on images that can be downloaded from the following sources (see Supplementary Information for further details): ALPO-Japan (http://alpo-j.asahikawa-med.ac.jp/Latest/Saturn.html); PVOL2 database (http://pvol2.ehu.eus/pvol2/); HST-OPAL programme (https://archive.stsci.edu/prepds/opal/); and Cassini ISS images at NASA Planetary Data System (https://pds-imaging.jpl.nasa.gov/volumes/iss.html). PlanetCam images are available from the corresponding author.

Code availability

The shallow water model code30 is available from E.G.-M. (enrique.garcia.melendo@upc.edu) on request. The radiative transfer code NEMESIS (http://users.ox.ac.uk-/atmp0035/nemesis.html) is available on request from P.Irwin (patrick.irwin@physics.ox.ac.uk). The EPIC numerical model31 is an open-code funded by NASA; see details: http://surveygizmoresponseuploads.s3.amazonaws.com/fileuploads/15647/4054745/254-fd0a70105de25e281834d7f5dcc5451c_DowlingTimothyE.pdf.

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Acknowledgements

This work has been supported by the Spanish project AYA2015-65041-P (MINECO/FEDER, UE) and Grupos Gobierno Vasco IT-366-19. A list of the sources for the images used in this paper can be found in the Supplementary Information. This work used data acquired from the NASA/ESA HST Space Telescope, associated with OPAL programme (principal investigator: Simon, GO13937), and archived by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract no. NAS 5-26555. All OPAL maps are available at https://doi.org/10.17909/T9G593, and M.H.W. and A.A.S. acknowledge financial support from his programme. M.H.W. through a grant from the Space Telescope Science Institute, which is operated by AURA under NASA contract NAS 5-26555.

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Contributions

A.S.-L. directed the work, made the features tracking measurements, retrieved the winds and interpreted the results. E.G.-M., M.S. and J.L. performed the shallow water and EPIC numerical simulations. T.d.R.-G. performed the Cassini image analysis of the storm precursor. R.H., J.M.G.-F., T.B., M.D. contributed to the analysis of ground-based observations. J.F.S.-R. and S.P.-H. performed the radiative transfer analysis. A.A.S. and M.H.W. performed the HST observations and helped in their analysis. K.M.S., J.J.B. and J.L.G. mapped and analyzed Cassini ISS images. U.D. and S.E. designed the ISS observation sequences. All authors discussed the results and contributed to preparing the manuscript.

Corresponding authors

Correspondence to A. Sánchez-Lavega or E. García-Melendo.

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Peer review information Nature Astronomy thanks Cheng Li and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Sánchez-Lavega, A., García-Melendo, E., Legarreta, J. et al. A complex storm system in Saturn’s north polar atmosphere in 2018. Nat Astron 4, 180–187 (2020). https://doi.org/10.1038/s41550-019-0914-9

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