A giant thunderstorm on Saturn

Journal name:
Nature
Volume:
475,
Pages:
75–77
Date published:
DOI:
doi:10.1038/nature10205
Received
Accepted
Published online

Lightning discharges in Saturn’s atmosphere emit radio waves1 with intensities about 10,000 times stronger than those of their terrestrial counterparts2. These radio waves are the characteristic features of lightning from thunderstorms on Saturn, which last for days to months2. Convective storms about 2,000 kilometres in size have been observed in recent years at planetocentric latitude 35° south3, 4, 5 (corresponding to a planetographic latitude of 41° south). Here we report observations of a giant thunderstorm at planetocentric latitude 35° north that reached a latitudinal extension of 10,000 kilometres—comparable in size to a ‘Great White Spot’6, 7—about three weeks after it started in early December 2010. The visible plume consists of high-altitude clouds that overshoot the outermost ammonia cloud layer owing to strong vertical convection, as is typical for thunderstorms. The flash rates of this storm are about an order of magnitude higher than previous ones, and peak rates larger than ten per second were recorded. This main storm developed an elongated eastward tail with additional but weaker storm cells that wrapped around the whole planet by February 2011. Unlike storms on Earth, the total power of this storm is comparable to Saturn’s total emitted power. The appearance of such storms in the northern hemisphere could be related to the change of seasons7, given that Saturn experienced vernal equinox in August 2009.

At a glance

Figures

  1. Time-frequency spectrogram of the SED episode on 12 December 2010.
    Figure 1: Time-frequency spectrogram of the SED episode on 12 December 2010.

    The colour-coded intensity (with 30% background division) of the radio emissions is plotted as a function of spacecraft event time (SCET) over 6h and frequency from 500kHz to 16MHz on a logarithmic scale. Cassini coordinates (distance to Saturn’s centre in units of Saturn’s radius, RS, and SLS west longitude in degrees, ‘Long.’) are indicated on the abscissa. Cassini was in the equatorial plane at a local time of ~18.6h. The RPWS instrument sweeps in frequency, and it detects the broadband SEDs at whatever frequency (above the ionospheric cut-off) it happens to be tuned to at the time of the flash. This SED episode shows such a high flash rate that the receiver sweep rate of ~28 frequency channels per second (35.2ms per channel) can no longer resolve the single SEDs. Flash rates of 5–10 SEDs per second can lead to a temporal superposition of SEDs that normally extend over several frequency channels. At the edges of the episode, where the rate is lower, one can see the individual SED bursts. The continuous emission below 800kHz is Saturn kilometric radiation.

  2. Saturn/'s lightning activity and Cassini/'s distance to Saturn as a function of time for December 2010.
    Figure 2: Saturn’s lightning activity and Cassini’s distance to Saturn as a function of time for December 2010.

    a, The number of SED pixels as a function of day in December 2010. Owing to the high flash rates, the single SEDs sometimes cannot be counted. Instead, we give the number of SED pixels, which is the same as the number of single time–frequency measurements. Previous lightning storms from 35° S (ref. 5) showed SED numbers more than an order of magnitude smaller for similar distances. b, The distance of Cassini to Saturn’s centre in units of Saturn radii (RS); these data help interpretation of a, as the signal intensities of SEDs decrease with distance squared. The peak in SED numbers on 20/21 December coincides with Cassini’s closest approach to Saturn.

  3. Images of Saturn with the storm.
    Figure 3: Images of Saturn with the storm.

    a, Image taken on 13 December 2010 with an 11-inch Schmidt-Cassegrain telescope in the Philippines (by C.G.). The centre of the storm was located at longitude ~262° W and latitude ~34° N. At that time, the latitudinal size of the storm had already grown from 1,300km to ~6,800km (~7°). b, Image taken on 22 December 2010 with a 16-inch Newtonian telescope in Australia (by A.W.). It shows that the plume had grown to a latitudinal and longitudinal extent of ~9,000km and ~15,000km, respectively. The centre was at longitude ~283° W. c, Image taken on 24 December 2010 with the wide-angle camera of Cassini. Here the storm’s latitudinal and longitudinal extent is ~10,000km and ~17,000km, respectively, with an eastward tail that extends much further. The storm centre was at longitude ~288° W. Image credits: C.G., A.W., NASA/JPL/SSI.

  4. False-colour views showing the height of the storm clouds.
    Figure 4: False-colour views showing the height of the storm clouds.

    Images with three filters sensitive to different amounts of absorption by methane gas were superposed to make these colour mosaics. The filtered image at 889nm is projected as blue, and sees only the highest clouds. The filtered image at 727nm is projected as green, and sees high and intermediate clouds but not the lowest clouds. The filtered image at 750nm is projected as red, and sees clouds at all levels. a, b, Magnified views at full resolution of the areas indicated by white brackets in c. d, Image showing the storm ~11h after image c was taken. The images were taken by Cassini’s narrow angle camera on 26 February 2011 from a distance of 2.4×106km. Image credit: NASA/JPL/SSI.

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Author information

Affiliations

  1. Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, A-8042 Graz, Austria

    • G. Fischer
  2. Department of Physics and Astronomy, The University of Iowa, 203 Van Allen Hall, Iowa City, Iowa 52242, USA

    • W. S. Kurth &
    • D. A. Gurnett
  3. Observatoire de Paris-Meudon, 5 Place Jules Janssen, 92195 Meudon Cedex, France

    • P. Zarka
  4. Geological and Planetary Sciences, 150-21, California Institute of Technology, Pasadena, California 91125, USA

    • U. A. Dyudina,
    • A. P. Ingersoll &
    • S. P. Ewald
  5. Cassini Imaging Central Laboratory for Operations, Space Science Institute, 4750 Walnut Street, Boulder, Colorado 80301, USA

    • C. C. Porco
  6. 82 Merryville Drive, Murrumbateman, 2582 New South Wales, Australia

    • A. Wesley
  7. Physics Department, University of San Carlos, Nasipit, Talamban, 6000 Cebu City, Philippines

    • C. Go
  8. Commission des observations planétaires, Société Astronomique de France, 2 rue de l'Ardèche, 31170 Tournefeuilee, France

    • M. Delcroix

Contributions

G.F. analysed Cassini RPWS data and wrote the paper. W.S.K., D.A.G. and P.Z. helped in this analysis. U.A.D., A.P.I., S.P.E. and C.C.P. analysed the Cassini ISS image and calculated the energy of the storm. A.W. and C.G. imaged Saturn from the ground, and M.D. measured the size and drift of the storm from several images. All authors discussed the results and commented on the manuscript.

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The authors declare no competing financial interests.

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