Astronomers have discovered a new way for Saturn to get aurorae, the light shows created by charged particles slamming into a planet's atmosphere. The high-altitude winds swirling around the planet are responsible for this phenomenon and it’s not all down to magnetic interactions like here on Earth.
A paper, published in Geophysical Research Letters, is the first reporting of this phenomenon. And it actually helps explain a mystery that has surrounded the ringed planet for decades: the length of its day.
Usually, planetary rotation is measured using magnetic fields. If the magnetic field and rotation axis of a planet are not aligned, there will be pulses of radio waves that can be used to track this. Saturn’s, unfortunately, has a great alignment so the method required high precision. To the surprise of the scientists, these radio pulses appeared to be varied across the decades, across the seasons, across the planet’s hemispheres.
The rotation of the planet couldn’t be varying that much on such a short time scale, so something else must be happening to explain the peculiar radio pulses. And researchers are confident that it's the high-winds and their newly discovered aurorae.
“This study represents the first detection of the fundamental driver, situated in the upper atmosphere of the planet, which goes on to generate both the observed planetary periodicities and aurorae," lead author Ph.D. researcher Nahid Chowdhury, a member of the Planetary Science Group within the University of Leicester, said in a statement.
"It’s absolutely thrilling to be able to provide an answer to one of the longest standing questions in our field. This is likely to initiate some rethinking about how local atmospheric weather effects on a planet impact the creation of aurorae, not just in our own solar system but farther afield too.”
The international team used the Keck Observatory’s Near-Infrared Spectrograph to track the flow of Saturn’s ionosphere. They measured winds in this region of Saturn’s atmosphere and discovered they can reach between 0.3 and 3 kilometers per second (670 to 6700 miles per hour) in speed. These fast winds are driven by the energy from a deeper layer, the thermosphere. And these strong flows affect the magnetic field lines and generate aurorae.
“Our study, by conclusively determining the origin of the mysterious variability in radio pulses, eliminates much of the confusion into Saturn’s bulk rotation rate and the length of the day on Saturn,” said Kevin Baines, a JPL-Caltech-based co-author of the study and a member of the Cassini Science Team.
The rotation rate for Saturn was estimated in 2019 and it had to rely on something other than the radio pulses. Scientists used data from the Cassini mission to look at gravity-induced perturbation in the rings of Saturn and found the periodicity. A day on Saturn is 10 hours, 33 minutes, and 38 seconds long, they confirmed.