Stormy Weather Saturn’s Raging Superstorm 20 May 2012 sky & telescope A mysterious Great White Spot erupted on Saturn in late 2010, only the sixth such storm in recorded history. agustín sánchez-lavega NASA / JPL-CALTECH / SPACE SCIENCE INSTITUTE
GCP-UPV / EHU In the august 1989 S&T I wrote about the most spectacular meteorological phenomenon in Saturn’s atmosphere: a Great White Spot (GWS). Only four GWS events were known at that time, occurring in 1876, 1903, 1933, and 1960. Information on them w<strong>as</strong> rather sparse, just a few photos and drawings. But the apparent regularity of <strong>this</strong> phenomenon, with one GWS per Saturn year (29.46 Earth years), encouraged me to predict that a new one would soon break out. Sure enough, the fi fth storm erupted in September 1990, in the planet’s equatorial zone, just like the 1876 and 1933 events. Professional <strong>as</strong>tronomers imaged the storm with new CCD camer<strong>as</strong> on ground-b<strong>as</strong>ed telescopes and with the still-uncorrected optics on the Hubble Space Telescope. This fi fth storm bolstered our view that the GWS phenomenon is se<strong>as</strong>onal and concentrated in the summer of Saturn’s northern hemisphere. According to <strong>this</strong> periodicity, the next event should have fl ared up around 2020. But to our great surprise, in early December 2010 amateur <strong>as</strong>tronomers around the world obtained images of the outbreak of a sixth GWS, again in the northern hemisphere, but at mid-latitudes <strong>as</strong> w<strong>as</strong> the c<strong>as</strong>e in 1903. The storm erupted in the northern hemisphere’s early spring, about 10 years before summer. This time, <strong>as</strong>tronomers were equipped with better <strong>as</strong>tronomical detectors in the visible and infrared, both from the ground and from NASA’s C<strong>as</strong>sini spacecraft SATURN STORMS Left: C<strong>as</strong>sini took <strong>this</strong> image of the storm’s tail on February 25, 2011. Right: All six observed Great White Spots broke out in the northern hemisphere. The white dots on <strong>this</strong> C<strong>as</strong>sini image show the latitudes where each storm erupted. Below left: Spanish <strong>as</strong>tronomer Josep Com<strong>as</strong> I Solà drew <strong>this</strong> illustration of the 1903 storm, which erupted at a similar latitude <strong>as</strong> the 2010 GWS. Below right: A GWS broke out near Saturn’s equator in September 1990. In November 1990, before corrective optics were installed, the Hubble Space Telescope took <strong>this</strong> image after the storm had wrapped around the planet. North is up in all images in <strong>this</strong> article. in orbit around Saturn. Amateur <strong>as</strong>tronomers using advanced imaging techniques provided daily coverage that gave scientists their most detailed view yet of a storm’s evolution. Papers later published in the prestigious journals Science and Nature included the contributions of several dozen amateur <strong>as</strong>tronomers, among them S&T imaging editor Sean Walker. Discovery by Amateurs Amateur <strong>as</strong>tronomers Sadegh Gomizadegh in Iran and Teruaki Kumamori in Japan provided the fi rst clear evidence of the 2010 GWS outbreak in images taken on December 8th and 9th, respectively. But a reanalysis of earlier Saturn images showed that in a December 5th picture from Japanese observer Toshihiko Ikemura, a very small white spot w<strong>as</strong> located at the same position (latitude 37.7° north) <strong>as</strong> the eruption. This date coincides with C<strong>as</strong>sini’s fi rst detection of the spot with two instruments. Its Radio Pl<strong>as</strong>ma and Wave Science (RPWS) experiment captured powerful radio emissions from intense lightning activity coming from the spot’s position, and in its routine study of the planet, the Imaging Science Subsystem (ISS) caught a 1,000-kmwide bright spot. From humble beginnings, the GWS quickly swelled to a complex phenomenon of planetary scale. December 10th images from Anthony Wesley in Australia and 1960 2010 1903 1990 1933 1876 SkyandTelescope.com May 2012 21 NASA / STSCI NASA / JPL-CALTECH / SPACE SCIENCE INSTITUTE
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