What caused Saturn’s strange spell of storms in 2018?
Researchers have uncovered a new category of giant storm on Saturn’s surface.
Saturn in 2008, during a relatively quiet spell. Every 29 years, a giant storm erupts on the planet's surface, appearing as a vibrant white splotch. Image Credit: NASA / JPL / Space Science Institute
With its unmatched entourage of rings and moons, Saturn is no stranger to bravado.
Roughly every 29 years—the time it takes Saturn to circle the sun—the ringed planet spits out an absolute doozy of a storm. These so-called Great White Spots, named for the way they mottle Saturn’s surface with cream-colored whorls, can balloon into blobs big enough to swallow Earth whole.
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Now, researchers have uncovered a new kind of giant Saturnian storm unlike anything observed before, according to a paper published today in Nature Astronomy. More petite than Great White Spots but just as long-lived, the newcomers could hold tantalizing clues to how atmospheres behave both near and far from home.
“This adds an important piece to the giant puzzle...that is the global picture of Saturn’s atmosphere,” says Cassini Project Scientist Linda Spilker, who was not involved in the study. “By learning more about [Saturn’s storms], we can maybe understand more of the weather on our own planet as well.”
Since they were first observed in 1876, Great White Spots have popped up on the ringed planet in a fairly predictable pattern. That all changed in December of 2010, when NASA’s Cassini spacecraft watched Saturn debut its most recent Great White Spot a whopping 10 years early. For months, clouds and lightning roiled across Saturn’s northern hemisphere, wreathing it in a thick, pearly tail that outshone even the planet’s luminous rings. Peaking at 12,000 miles wide, it was the largest Saturnian tempest ever documented.
Since then, researchers like Agustín Sánchez Lavega of the University of the Basque Country and Enrique García Melendo of the Polytechnic University of Catalonia have been keeping close tabs on Saturn’s surface. Regularly scheduled programming should have served up a Great White Spot closer to now—and scientists were unsure whether the events of late 2010 had thrown off Saturn’s typical tempestuous timetable.
So when Brazilian amateur astronomer Maciel Bassani Sparrenberger spied a brilliant white blemish blooming near Saturn’s north pole on March 29, 2018, Sánchez Lavega sprang into action. Curious to see how long it would take the new storm to fizzle out, he posted an urgent request online for Saturn enthusiasts around the globe to turn their telescopes stormward. Over the next several months, a steady stream of images poured in to a database run by the University of the Basque Country.
A series of images of Saturn's northern hemisphere. Names in parentheses indicate the observer. a, April 1 (D. Peach). b, May 26 (A. Casely). c, June 6 (HST-OPALprogram). d, June 23 (T. Barry). e, June 28 (D. Peach). f, June 30 (D. P. Milika and P. Nicholas). g, July 11 (B. Macdonald). h, August 8 (T. Barry). i, August 16 (F. Silva-Correa). j, August 18 (D. Peach). k, August 19 (T. Barry). l, September 16 (B. Macdonald). Image Credit: Sánchez Lavega and García Melendo et al., Nature Astronomy, 2019
Within days, the March storm—now dubbed White Spot 1—had swollen into a stripe some 2,500 miles across, twice the size of Earth’s most massive typhoons. Two months later, as it still churned in Saturn’s northern hemisphere, White Spot 1 was joined by a second storm (White Spot 2). By August, two more storms (White Spots 3 and 4) had joined the initial pair, each erupting a bit closer to Saturn’s pole. The fourth gale even appeared to brush up against the mysterious six-sided jet stream (a fast-flowing current of air) that crowns the planet.
This is the first time scientists have observed multiple cyclones at different latitudes on Saturn, Sánchez Lavega explains via email. The four storms are also unusually northerly compared to Great White Spots, he says, which tend to blossom closer to the equator. And though they arose at different times, the quartet emerged in close enough proximity that a couple of them occasionally collided, sprouting a chain of bright orbs that encircled the planet’s pole like a pearl necklace.
Maxing out at 5,000 miles across, the White Spots were big, but nowhere near the size of their Great counterparts. They were at least as long lived, though, with White Spots 1 and 2 enduring for 214 and 157 days, respectively, before flaming out last fall. This mish-mash of longevity and size sets the storms apart from both rare, gargantuan Great White Spots and the more frequent “mid-size” storms, small-scale squalls that flare up on Saturn’s flanks for just a few days at a time.
Rather, White Spots seem to be in a league of their own. They’re what Sánchez Lavega and García Melendo call intermediates: the previously underappreciated middle children in Saturn’s clan of climate-related phenomena.
An image of Saturn taken on May 20, 2018 by astronomer Damian Peach, showing a White Spot blooming near the planet's north pole. Image Credit: Damian Peach
As on Earth, storms can brew on Saturn when warm, water-laden air rises rapidly through the atmosphere and cools, condensing its moisture into tempestuous clouds. This could explain what’s happening with the White Spots, too, the team’s models suggest, though it’s still not clear what triggered the recent storms, or why they lasted so long.
“A hundred to 200 days is a long time,” Spilker says. “There has to be a continuous source of energy [input], or they wouldn’t last as long.”
Our hurricanes are driven by the sun’s warmth, but solar energy is scarcer on planets like Saturn, whose orbit is almost 10 times that of Earth’s. (Though Great White Spots do tend to take place when the planet’s northern hemisphere tilts most toward our solar system’s star.) It’s possible some of Saturn’s storm-sparking heat could be internal, leftover from planetary formation.
Either way, the White Spots’ timing is intriguing, Spilker says. While 2010’s early-arriving cataclysm broke form, the events of 2018 seem on track for Saturn’s 29-year storm cycle. It seems the planet somehow stores up its energy for decades at a time before releasing it all at once, Spilker says. If that’s the case, perhaps the recent Great preemptively depleted Saturn’s cache of atmospheric ammo, undermining the more-on-schedule White Spots of last year.
Saturn's Great Northern Storm, which bloomed on the planet in December of 2010. The storm lasted for several months, developing a whirling tail that stretched across Saturn's northern hemisphere. Image Credit: NASA/JPL-Caltech/Space Science Institute
That’s just a theory to mull over for now, García Melendo says. The team needs more data before a link between the pair can actually be established—and it may be that the intensities of the two storm systems are actually totally unrelated. Additionally, researchers still don’t know if intermediately-sized White Spots are a common fixture on the ringed planet or a one-off event.
Hopefully, those answers are forthcoming, García Melendo says. Saturn will be serving up cyclones for years to come. Cassini—the intrepid probe that glimpsed 2010’s record-breaking storm up close—is long gone (RIP). But even in the post-Cassini era, plenty of eyes right here on Earth remain trained on the ringed planet and its enigmatic atmosphere. “Citizen science is essential to survey possible future outbreaks,” Sánchez Lavega says. “The next storm discovery will surely be done by an amateur.”
“I’m excited to see that Earth-based telescopes are picking up where Cassini left off,” Spilker says, “and continuing the legacy that Cassini got started.”
