North of the Earth and the southern lights — the result of a meeting point between the magnetic field, the energetic particles from the Sun, and the additive in our planet’s atmosphere — are wondrous spectacles. But Earth is not exclusive to the aurora. They exist on other worlds with magnetic fields, including Saturn, where the aurora lights sparkle in infrared and ultraviolet light.
Now, as revealed by a recent study published in the journal Geophysical research letter, scientists have discovered an aurora on the ring-shaped world unlike any other. Like Earth’s, Saturn’s northern lights are powered by a shower of energetic particles from the sky. But some of its auroras appear only when howling winds shoot across the north pole — a bit like a gust of wind stirring up cosmic flames.
“To my knowledge, [this is the] for the first time an aurora caused by atmospheric winds has been detected,” Rosie Johnson, a space physics researcher at Aberystwyth University in Wales, who was not involved in the study. “That’s a really great result!”
It’s also a revelation that comes as scientists are baffled by a seemingly innocuous question: Why can’t we figure out how long a day on Saturn lasts? As it turns out, it took only 40 years for a spaceship carrying a death wish, an ice volcano, and a telescope atop Mount Hawaii to be discovered.
Earth made it easily measure how long a day lasts: 24 hours. That’s because our planet is covered with fixed, identifiable landmarks. All extraterrestrial viewers need to do is tag one of those, wait for it to go out of view and then come back to watch, and voilà: That’s how long it takes the Earth to make one rotation complete on its axis.
You can’t do this for worlds where the surfaces are obscured by thick layers of gas, like Jupiter, Saturn, Uranus and Neptune. Fortunately, they all have magnetic fields that originate from their geological hearts, a shield that protects their atmospheres from being stripped by the solar wind. These magnetic fields have charged particles moving up and down them, emitting radio pulses as they move. As the planets rotate, so does their magnetic field, and their magnetic field carries this radio pulse signal for transit.
Think of these planets like radio “lighthouses” — as they make one complete rotation, so does the beam of radio waves sweeping from them. A distant observer can “see” a bright radio signal circling in the dark. “You can do this for Uranus and Neptune. It was also made for the Earth. It works,” says James O’Donoghuea planetary astronomer at the Japan Aerospace Exploration Agency and a co-author of the new study.
Not for Saturn.
Since the two Voyager probes closely observed Saturn in the early 1980s, various spacecraft have attempted to measure the rotation of its radio lighthouse to determine the length of a Saturn day. But every time it is measured, the length of a day seems to change, with values ranging from 10.5 Earth hours to 10.9 Earth hours. The Cassini spacecraft, which entered Saturn’s orbit in 2004 and stayed there until 2017, has learned more about this dazzling gas giant than any other mechanical visitor — but it still doesn’t know. How long is a day? “It just discovered more problems,” says O’Donoghue.
What has become clear during its tenure, however, is that Saturn appears to have three different radio beacons. Most planets have one, but its north and south poles each have them, rotating at different speeds. That must be why the length of Saturn’s day seems to be constantly changing.
But why does Saturn have so many lighthouses? “A lot of people had theories. It’s one of those late-night pub discussions, you know,” Tom Stallard, a planetary astronomer at the University of Leicester and a co-author of the new study. Some think it has something to do with the way the planet’s magnetic field is generated. Others wonder if the answer lies in Saturn’s hot atmosphere.
