A New Type of Aurora Found on Saturn Resolves a Planetary Mystery

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.

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p class=”paywall”>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.