Earth’s northern and southern lights—the result of a rendezvous between magnetic fields, energized particles from the Sun, and our planet’s atmospheric admixture—are wondrous spectacles. But Earth doesn’t hold a monopoly on auroras. They exist on other worlds with magnetic fields, including Saturn, whose auroral glow shimmers in the infrared and ultraviolet.
Now, as revealed by a recent study published in the journal Geophysical Research Letters, scientists have discovered an aurora on that ringed world that is unlike any other. Like Earth’s, Saturn’s northern lights are fueled by a shower of energized particles from the heavens. But some of its auroras only make an appearance when screaming winds shoot across the north pole—a bit like a gust of air stirring up a cosmic bonfire.
“To my knowledge, [this is the] first time an aurora driven by atmospheric winds has been detected,” says Rosie Johnson, a space physics researcher at Aberystwyth University in Wales who is not involved with the study. “It’s a really great result!”
It also happens to be a revelation that came about while scientists puzzled over a seemingly innocuous question: Why can’t we work out how long a day lasts on Saturn? As it turns out, it only took 40 years, a spacecraft with a death wish, ice volcanoes, and a telescope atop a Hawaiian mountain to find out.
Earth makes it easy to measure how long a day lasts: 24 hours. That’s because our planet is covered in readily identifiable, fixed landmarks. All an extraterrestrial viewer needs to do is tag one of those, wait for it to rotate out of sight and then return to view, and voilà: That’s how long it takes Earth to make one complete rotation on its axis.
You can’t do this for worlds where the surfaces are obfuscated by thick gaseous veils, like Jupiter, Saturn, Uranus, and Neptune. Fortunately, they all have magnetic fields rooted to their geologic hearts, shields that protect their atmospheres from being stripped away by the solar wind. These magnetic fields have charged particles scooting up and down them, emitting radio pulses as they go. As planets rotate, so do their magnetic fields, which take this radio pulse signal along for the ride.
Think of these planets like radio “lighthouses”—when they make one full rotation, so does the radio beam sweeping from them. A distant observer can “see” a bright radio signal spinning around in the darkness. “You can do this for Uranus and Neptune. It’s also been done for the Earth. It works,” says James O’Donoghue, a planetary astronomer at the Japan Aerospace Exploration Agency and coauthor of the new study.
Not so for Saturn.
Ever since the two Voyager probes took a close look at Saturn in the early 1980s, various spacecraft have tried measuring the spin of its radio lighthouse to determine the length of a Saturnian day. But every time it has been measured, the length of a day seems to change, with values ranging between 10.5 Earth hours to 10.9 Earth hours. The Cassini spacecraft, which entered Saturn’s orbit in 2004 and stayed there until 2017, learned more about this resplendent gas giant than any other mechanical visitor—but it still couldn’t work out how long a day was. “It just found more problems,” says O’Donoghue.
What did become clear during its tenure, though, was that Saturn appeared to have three different radio lighthouses. The bulk of the planet had one, but its north and south poles each possessed their own, spinning at different rates. That must have been why the length of Saturn’s day appeared to keep changing.
But why does Saturn have multiple lighthouses? “A lot of people had theories. It was one of those late-night pub discussions, you know,” says Tom Stallard, a planetary astronomer at the University of Leicester and a co-author of the new study. Some people thought it had something to do with the way the planet’s magnetic field was being generated. Others wondered if the answer was hiding within Saturn’s tempestuous atmosphere.
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Resolving this riddle seemed necessary if scientists wanted to determine the length of a Saturnian day. But in 2019, planetary scientists had an epiphany while examining another of the gas giant’s features: its rings. Whenever the planet’s concealed innards twitch, convulse, and rotate, the planet’s gravity field shifts. This tugs at the icy particles within Saturn’s rings, creating fine ripples and waves. That year, scientists decoded these undulations, revealing, finally, the length of a day on Saturn: 10 hours, 33 minutes, and 38 seconds.
But the mystery of the origins of Saturn’s many radio lighthouses remained frustratingly unsolved. In the summer of 2017, with its rocket propellant almost entirely spent, Cassini was ordered to plunge into Saturn’s atmosphere so as not to risk crashing into and contaminating one of its potentially life-harboring moons. When it burned up in the Saturnian skies on September 15, 2017, the last great hope of cracking the case appeared poised to vanish with it.
Except—hope wasn’t entirely lost. During the probe’s final months, there was a very narrow window of opportunity. Stallard and his colleagues reasoned that if Saturn’s multiple radio lighthouses could be explained by something weird happening in the upper atmosphere, they would need to pick a radio lighthouse, track its behavior, and then compare that with matching observations of its atmosphere, hoping to see a sign that the two were interlocked in a strange dance. The fated-to-die probe, they thought, could provide those contemporaneous atmospheric observations on the final arc of its journey.Now they were in a race against time.
Stallard applied for time at the Keck Observatory, a pair of 300-ton telescopes sitting atop the 13,800-foot-high peak of Hawai‘i’s Mauna Kea, a dormant volcano. By peering at Saturn’s north pole in infrared during the summer of 2017, he could track the movements of hydrogen ions in its skies, essentially permitting him to see which way the winds were blowing up there.
He didn’t have long. “Cassini was about to crash,” says Stallard. While Cassini took one last look at the north pole’s lighthouse, Stallard waited at the observatory. Between June and August, whenever Saturn could be seen as a patch of fuzzy light in Earth’s sky, he aimed Keck at its north pole and soaked up the data.
“Making measurements of the upper atmosphere is horribly hard,” he says, but everything worked out. “We got every single one of our nights with good weather, which is kind of miraculous, really.” By the time Cassini was no more in mid-September, the team had the data they were hoping for—and, after a few years of analysis, in late 2021 they found their answer to why Saturn has three radio lighthouses. “It was ridiculous,” says Stallard, but “what we saw was a very definite answer.”
Plasma, a soup of charged particles, falls from the stars and flows down to Saturn’s magnetic poles. This plasma follows the paths of field lines, invisible magnetic filaments stretching out of the poles. As it does so, radio pulses are emitted. That was already known.
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But the team discovered that high-altitude, low-density winds, moving at speeds of up to 6,700 miles per hour, rush over the north pole. On either side of this prevailing wind current are two vortices, two swirls spinning in opposite directions. This twin-cell wind system itself fully rotates. The entire thing resembles a fairground ride from hell.
The mechanisms of this polar perturbation are not yet fully understood. Its effects, however, were clear to see. This powerful northern maelstrom grabs the magnetic field lines diving into the north pole, bends them out of shape, and spins them about. That means the north pole’s radio lighthouse is swiveling around differently than the one for the bulk of the planet—and it explains why, if you try to measure the length of a day on Saturn using its rotating magnetic field, you get a range of answers.
Much about Saturn’s skies remains puzzling. But in recent decades, painstaking work has begun to unravel some of its peculiar features. The identification of these vortexes “adds another piece in the puzzle,” says Zarah Brown, a planetary atmosphere researcher at the University of Arizona who was not involved with the study.
“It’s nice to have a solution,” says O’Donoghue. It’s also nice to discover that this errant radio lighthouse is accompanied by a novel type of aurora.
The fundamentals of auroral cookery are similar for many planets. Earth, for example, is bombarded by expulsions of the Sun’s magnetic field and its plasma. When this plasma falls down Earth’s magnetic field lines, they bounce off gas particles in the upper atmosphere above the north and south poles. The electrons attached to these gas particles get excited and jump up, ultimately releasing energy and creating an auroral glow.
The same thing happens on Saturn. But being so far from the Sun, it doesn’t receive much solar plasma. Instead, most of its plasma comes from icy volcanism on Enceladus, a gelid moon that erupts water-ice slush from deep crevasses around its south pole. Much of this cryovolcanic matter falls into orbit around the moon itself. Some of it drifts into space, bathes in sunlight, gets energized, and becomes a plasma. It is subsequently swept up by Saturn’s magnetic field lines, where it pings off its plentiful hydrogen and creates an auroral glow.
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“You get used to a lot of things when you do science,” says O’Donoghue. But Saturn’s ultraviolet and infrared auroras fueled by ice volcanism? “That’s one of the things I’ve never quite got over.”
The locations of Saturn’s auroras are dictated by where the magnetic field lines go. But as the team’s work has revealed, it’s not so straightforward at the north pole. Up there, that twin-cell tempest warps these field lines, pulling them through the upper atmosphere. Any low-hanging plasma attached to them is dragged through high-flying hydrogen gas clouds, creating plenty of new plasma-hydrogen collisions and creating another auroral glow around the north pole. Together, Saturn’s “classic”-style aurora and this wind-driven aurora combine to form a beautiful, bespoke iridescence: a halo-shaped outer ring that encircles various bright auroral patches within.
So far, Saturn’s auroral mashup is unique. But could it be found on other worlds? “With the amount of exoplanets in the universe, I’m going to say, ‘Definitely yes!’” says Johnson. There may also be hidden wind-driven auroras closer to Saturn’s shores—on Jupiter and, perhaps, even on Earth, just on smaller scales.It’s difficult to say for now whether a wind-driven aurora does more than just look cool; if it can influence planets in a way we are yet to comprehend. “I don’t know how important it is. It’s just that, at Saturn, it’s so important that the aurora is 50 percent generated by [the wind],” says Stallard. “We should probably think about it more as a community.”
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