For years, Saturn appeared to be doing something impossible.
Measurements suggested the giant planet's rotation rate was changing over time, as if Saturn were somehow speeding up or slowing down. That puzzling result left scientists searching for answers. Now, researchers using the James Webb Space Telescope (JWST) say they have finally solved the mystery.
The new findings, published in the Journal of Geophysical Research: Space Physics, reveal that Saturn's spectacular northern lights are at the heart of the phenomenon. The study shows that the planet's aurora drives a powerful cycle involving heat, winds, and electrical currents that can make Saturn appear to spin at different speeds depending on how it is measured.
The puzzle dates back decades, but it gained renewed attention after observations from NASA's Cassini spacecraft in 2004 suggested that Saturn's rotation rate was gradually changing.
That result was difficult to explain because planets do not simply alter their spin rates on short timescales.
In 2021, a team led by Professor Tom Stallard of Northumbria University proposed a different explanation. Their research showed that Saturn's rotation was not actually changing. Instead, electrical signals linked to the planet's aurora were being affected by winds in Saturn's upper atmosphere. Those winds generated electrical currents that altered the auroral signal scientists were using to estimate the planet's rotation.
While that study explained the misleading measurements, one major question remained unanswered: What was driving those atmospheric winds?
James Webb Maps Saturn's Aurora
To investigate, Stallard and colleagues from institutions across the United Kingdom and United States turned to the James Webb Space Telescope.
The team observed Saturn's northern auroral region continuously for an entire Saturnian day. The observations provided a level of detail that previous instruments could not achieve.
Researchers focused on infrared light emitted by a molecule known as trihydrogen cation. This molecule forms in Saturn's upper atmosphere and serves as a natural indicator of temperature. By analyzing its glow, the team created the most detailed maps ever produced of temperatures and charged particle densities within Saturn's auroral region.
The improvement in accuracy was dramatic. Earlier measurements carried uncertainties of roughly 50 degrees Celsius, making it difficult to detect subtle changes. JWST's observations were about ten times more precise, allowing scientists to identify localized patterns of heating and cooling for the first time.
The new data closely matched predictions from computer models developed more than a decade ago. However, the models only worked if the source of the atmospheric heating was located exactly where the strongest auroral particles enter Saturn's atmosphere.
The results indicate that Saturn's aurora is doing far more than creating a dazzling light show.
Energy deposited by the aurora heats specific regions of the atmosphere. That heating generates winds, which then create electrical currents. Those currents help power the aurora itself, which continues heating the atmosphere and sustaining the entire cycle.
Lead researcher Professor Tom Stallard said: "What we are seeing is essentially a planetary heat pump. Saturn's aurora heats its atmosphere, the atmosphere drives winds, the winds produce currents that power the aurora, and so it goes on. The system feeds itself.
"For decades, we knew something strange was happening with Saturn's apparent rotation rate, but we could not explain it. We then showed it was being driven by atmospheric winds, but we still did not know why those winds existed. These new observations, made possible by JWST, finally give us the evidence we needed to close that loop."
The discovery may have significance far beyond a single planet.
Researchers found evidence that Saturn's atmosphere and magnetosphere are closely connected. The magnetosphere is the vast region of space shaped by the planet's magnetic field. Activity in the atmosphere appears to influence conditions in the magnetosphere, while the magnetosphere feeds energy back into the atmosphere.
This ongoing exchange could help explain why the process remains stable over long periods.
According to the researchers, similar interactions may occur on other planets as well.
Professor Stallard added: "This result changes how we think about planetary atmospheres more generally. If a planet's atmospheric conditions can drive currents out into the surrounding space environment, then understanding what is happening in the stratospheres of other worlds may reveal interactions we have not yet even imagined."
The James Webb Space Telescope is the world's premier space science observatory. The telescope is designed to study objects throughout the solar system, investigate planets orbiting distant stars, and explore the origins and evolution of the universe. Webb is an international project led by NASA in partnership with ESA (European Space Agency) and CSA (Canadian Space Agency).
The study was conducted by researchers from Northumbria University together with collaborators from Boston University, the University of Leicester, Aberystwyth University, the University of Reading, Imperial College London, Lancaster University, and Johns Hopkins University Applied Physics Laboratory. Funding for the research was provided by the Science and Technology Facilities Council (STFC).
