Just as regions of our planet have monsoon season, or tornado season, so too does Saturn have its own stormy season.
Once every Saturn year or so—which corresponds to roughly 30 Earth years—a giant, churning storm works its way through the clouds of Saturn's northern hemisphere, sometimes encircling the entire planet like a belt. Lasting a few dozen days or more, these storms have been documented as far back as 1876.
The sixth giant Saturnian storm on record arrived a bit early, kicking off in late 2010, just 20 years after the previous storm. The timing proved fortuitous for planetary astronomers, who currently have a dedicated orbiter called Cassini stationed at the ringed planet. And Cassini's ringside seat, so to speak, has afforded the NASA spacecraft quite a show.
A new study summarizing Cassini observations of the giant Saturnian storm adds to a growing body of evidence demonstrating that this was no ordinary outburst. The 2010 storm reached well into 2011—at roughly 200 days in duration, it is the longest such storm on record. It produced an unprecedented vortex that could just about swallow planet Earth. And it persisted until the head of the storm advanced all the way around the planet to rear-end the slower-moving vortex; their collision appears to have terminated much of the storm's action.
Cassini recorded the storm in great detail, both with its cameras and with its Radio and Plasma Wave Science instrument, which detected electrostatic pulses from lightning strikes within the clouds. Kunio Sayanagi, an assistant professor of planetary science at Hampton University, and his colleagues describe those observations in a study that will appear in the journal Icarus.
Sayanagi and his co-authors report that the storm, sometimes called the "Great White Spot," began on December 5, 2010, and lasted until June 20, 2011, although the endpoint of the storm is somewhat ambiguous. Nevertheless, the storm's duration solidly surpasses the great Saturnian storm of 1903, which raged for 150 days.
The latest storm spread steadily across Saturn in a band that eventually encircled the planet at 33 degrees north latitude. At the front of the storm was a fast-moving bright feature, sparkling with lightning activity, called the "head," trailed by a giant cyclonic vortex that also took shape in December and finally a "tail" of turbulent clouds. By January, the researchers report, the vortex had grown to a massive whorl 12,000 kilometers wide—roughly the diameter of Earth. That ranks as the largest vortex ever recorded in Saturn's troposphere, the study's authors note, although they point out that a more recent vortex detected in the Saturnian stratosphere (a higher layer of the atmosphere) is even larger—some 50,000 kilometers across.
The two vortices may well have been spun up by the same storm, "most likely as a result of a 'planetary burp'—a warm mass ascended from depth and curled around on itself in the atmospheric layer," Sayanagi says. "It seems that the vortex sheared apart vertically into two components, the tropospheric vortex we saw in visible [light], and the stratospheric vortex" that other researchers documented in infrared radiation.
By June of 2011, the fast-moving head of the storm had raced around the planet to essentially lap the tropospheric vortex, leading to a collision that effectively ended the storm. Lightning strikes became intermittent, and the bright clouds making up the head disappeared. Based on past superstorms, however, the researchers predict that the aftermath of the Great White Spot will continue to disturb Saturn's atmosphere for years—maybe even a decade—to come.