Saturn’s polar winds baffle NASA, power strong auroras

Powerful winds at Saturn’s north pole are powering its aurora borealis and skewing scientists’ measurements, astronomers have found.

The winds, which flow through two huge vortices at the planet’s north pole, carry electrically charged ions that drive the planet’s magnetic field. The previously mysterious phenomenon prevented scientists from measuring the length of a day on Saturn for decades.

Since clouds move on their own, looking at a gaseous planet from afar won’t tell you how fast it is spinning. Instead, scientists measure the magnetic field, which is rooted deep in the planet’s core. But when NASA’s Cassini spacecraft reached Saturn in 2004, scientists were surprised to find that the planet’s magnetic field radio pulse rate had changed since NASA’s Voyager 2 mission had flown past it in nineteen eighty one.

“The magnetic field and the resulting radio emissions generally act like a beacon,” meaning the radio bursts come in regular pulses, James O’Donoghue, planetary scientist at the Japan Aerospace Exploration Agency, told Insider. and co-author of the new study.

animation of rotating twin vortexes at saturn's north pole

An animation shows how two vortices spin around Saturn’s north pole, driving electrical currents in the upper atmosphere that produce auroras and drive the planet’s magnetic field.

James O’Donoghue/JAXA and Tom Stallard/University of Leicester



“For Saturn, this rotation period has oddly drifted over time, confusing everyone,” he added. “Even stranger, the planet’s rotational period drifted independently north and south.”

It was unlikely that Saturn would rotate at different speeds in different seasons, or that its northern half would rotate at a different speed than its southern half. But something had to be lurking in the magnetic field between Saturn’s surface and NASA’s spacecraft.

During nightly discussions, scientists tossed around theories. Perhaps Saturn’s magnetic field slowed as it passed through erupting lava and gases from volcanoes on its moon Enceladus. Or perhaps the thick methane atmosphere of the moon Titan – or Saturn’s rings themselves – could be the culprit.

All of these theories have been proven wrong. According to the new study published in the February issue of Geophysical Research Letters, electrically charged wind was powering Saturn’s aurora and driving its magnetic field along it.

Saturn has the first wind-driven aurora borealis

Saturn

NASA’s Hubble Space Telescope captured this image of Saturn on July 4, 2020.

NASA, ESA, A. Simon (Goddard Space Flight Center), MH Wong (University of California, Berkeley) and the OPAL team



Scientists have finally figured out how to measure Saturn’s rotation without the help of its magnetic field. They studied slight changes in the gravitational pull the planet exerts on its rings. In 2019, they determined that Saturn rotates every 10 hours, 33 minutes, and 38 seconds.

But the mystery of the changing magnetic field remains.

While scientists had previously speculated that a strong double-vortex polar weather pattern could be behind Saturn’s moving magnetic lines, the new study is the first evidence to support the idea.

To test the theory, researchers from the University of Leicester studied Saturn’s auroras from Hawaii’s Keck Observatory throughout the summer of 2017, using infrared light. Most planets, including Earth, get their auroras from electrically charged particles in space – either from the sun or from nearby lunar volcanoes. These particles are captured by the planet’s magnetic field and funneled towards the poles, where they interact with gases in the atmosphere to create colorful dancing lights.

In the case of Saturn, winds from the upper atmosphere, charged with electrically charged ions, create part of this interaction. The researchers tracked the movements of a hydrogen ion in the planet’s upper atmosphere and found that it moved through twin vortices, just as weather theory predicted.

“This new document finally shows what the bug was,” O’Donoghue said.

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