Saturn Has Its Own Northern Lights—Scientists Finally Know How They Happen

Space scientists have discovered the secret mechanism that is driving massive light shows called planetary aurorae around Saturn and is making the length of its days difficult to measure.

The international team of researchers found that some of the aurorae—known on Earth as the northern and southern lights—on Saturn are driven by swirling winds within its own atmosphere.

This makes Saturn unique as around other observed planets, including our own, aurorae are exclusively formed by currents from magnetospheres flowing into the planet's atmospheres.

That process is caused by charged particles striking the magnetosphere from the sun, as happens here on Earth, or by volcanic material hitting the magnetosphere from an orbiting moon, as happens on the solar system's gas giants Jupiter and Saturn itself.

Ph.D. researcher at the Planetary Science Group within the School of Physics and Astronomy at Leicester University, Nahid Chowdhury, told Newsweek: "At Earth, the aurorae are the result of the interaction of neutral components of the atmosphere such as oxygen, nitrogen, etc... with energetically charged solar particles. These interactions release energy which we see in the form of aurorae.

"At Saturn, the aurorae are generated as a result of similar interactions but the key players involved in setting off these interactions and where they originate from are very different. For instance, the energetically charged particles at Saturn mostly take the form of charged ions which are produced by ice geysers on the surface of the icy moon Enceladus. These go on to interact with neutral atoms such as hydrogen in Saturn's upper atmosphere."

Chowdhury, who is one of the authors of a paper published in the journal Geophysical Research Letters which documents the team's findings, continued by explaining that while researchers have known that interactions with material from Enceladus is a strong driver for the aurorae since the measurements from the NASA Cassini mission only about half the aurorae are generated in that way.

He added: "This new result allows us to explain the other half. The fact that the aurorae caused by Saturn's weather is just as bright as those produced from sources outside the planet is fairly startling!"

Not only does the discovery challenge what scientists know about planetary aurorae, but it could also solve the long-standing mystery of why it is so hard to measure a day on Saturn.

When it first arrived at Saturn in July 2004, NASA's Cassini probe attempted to measure the bulk rotation rate of the planet, the quantity that determines the length of a planet's day.

The spacecraft did this by tracking pulses of radio emission from the atmosphere of the ringed gas giant. To their great surprise scientists checking those measurements discovered that the rate of pulses appeared to have changed in the 23 years that had passed since the Voyager 2 craft buzzed the planet.

While Voyager measured Saturn's day at 10 hours, 45 minutes and 45 seconds, measurements from Cassini suggested the ringed planet's day was 10 hours, 39 minutes and 24 seconds long, a difference of around six minutes.

Chowdhury said in a press release: "Our understanding of the physics of planetary interiors tells us the true rotation rate of the planet can't change this quickly, so something unique and strange must be happening at Saturn."

Chowdhury, told Newsweek: "Several theories have been proposed for explaining why Saturn behaves like this and our work provides evidence that these changes in the internal rotation rate at Saturn are ultimately driven by a unique mechanism found in the planet's upper atmosphere.

"We have provided a solution to the decades-long mystery regarding time variations within the planetary environment at Saturn."

Associate professor in Planetary Astronomy at the University of Leicester, Tom Stallard, explains some of the other suggested mechanisms point to the volcanic moon Enceladus as the cause, or interactions with the thick atmosphere of the moon Titan, or even the effect of Saturn's bright rings. It wasn't until recently that researchers started to look at the gas giant's upper atmosphere as a possible culprit for these mysterious changes in Saturn's rotation rate.

Chowdhury continued: "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."

In order to solve this decades-long mystery the team, which also included researchers from NASA's Jet Propulsion Laboratory, the Japan Aerospace Exploration Agency and universities like Boston, Imperial College London, and Leicester, measured infrared emissions from Saturn's upper atmosphere.

With this data, collected in 2017 using the W.M. Keck Observatory in Hawai'i, the scientists were able to map flows in Saturn's ionosphere, located beneath the magnetosphere over the course of a month.

When the resultant map was measured against the known pulse of Saturn's radio aurorae, it showed that a significant proportion of the planet's aurorae are generated by the swirling weather pattern in its atmosphere.

The team concluded that they are also responsible for variations in Saturn's rate of rotation, thus solving the mystery of its varying day lengths.

Chowdhury concluded: "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."

Update 02/09/22, 9:16 a.m. ET: This article was updated to include animation of Saturn and comments from Nahid Chowdhury.

Saturn an aurora at its southern pole
An image collected by NASA's Cassini probe showing an aurorae at Saturn's southern pole. Researchers have discovered that the planet's aurorae are caused by high-altitude winds. Cassini, VIMS Team, University of Arizona, University of Leicester, JPL, ASI/NASA