Saturn’s decades-long spin mystery solved

   Decades-Long Saturn’s spin mystery finally solved by Astronomers

A decade-long mystery about apparent changes in Saturn’s rotation has finally been solved, thanks to observations from the most powerful space telescope ever built. Scientists at Northumbria University have discovered why Saturn appears to spin at a different speed depending on how such is measured, and it has nothing to do with the actual rotation of the planet, but rather with its aurora. Scientists discovered that Saturn’s changing “rotation rate” was never caused by the planet speeding up or slowing down, but by powerful winds high in its atmosphere. Webb’s unprecedented observations revealed that Saturn’s northern lights actively heat the atmosphere, creating winds which generate electrical currents, and then power the aurora all over again in a self-sustaining cycle. The offset was observed from where the currents flow into and out of the planet, but ultimately, the winds generated by this temperature offset are what drive those currents. 

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 study reveals for the first time complex patterns of heat and electrically charged particles in Saturn's aurora, showing that the entire system is driven by a self-sustaining feedback loop powered by the planet's own northern lights. The mystery dates back to 2004, when NASA's Cassini spacecraft in 2004 suggested the planet's rotation rate was gradually changing, which should not have been possible, because planets cannot simply speed up or slow down their spin. The new findings 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 which 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. The result was difficult to explain because planets do not simply alter their spin rates on short timescales. In 2021, astronomer professor Tom Stallard of Northumbria University and colleagues found that the apparent changes in Saturn's rotation were being driven by winds in the planet's upper atmosphere, which were producing electrical currents which created the misleading auroral signal. However, this discovery left one key question unanswered: what was causing those atmospheric winds? New research by Professor Stallard and colleagues across the UK and US has now provided the first direct evidence of the answer. 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. This 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 which 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 team observed Saturn's northern auroral region, the equivalent of Earth's northern lights, continuously for a full Saturnian day. They then analyzed the infrared glow from a molecule called trihydrogen cation, which forms in Saturn's upper atmosphere and acts as a natural thermometer, producing the first high-resolution maps of both temperature and particle density across Saturn's auroral region. Compared to earlier data, the improvement was extraordinary. Previous measurements had errors of around 50 degrees Celsius, roughly on a par with the differences the scientists were trying to detect, and were produced by combining broad regions of the hot polar aurora.

To investigate, Stallard and colleagues from institutions across the UK and US turned to the James Webb Space Telescope. The observations provided a level of detail that previous instruments could not achieve. The improvement in accuracy was dramatic. JWST's observations were about ten times more precise, allowing scientists to identify localized patterns of heating and cooling for the first time. 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."

What the team found matched long-standing predictions, but only when the heat was concentrated exactly where the aurora enters the atmosphere. This led to a crucial discovery: Saturn’s aurora actively heats its atmosphere in specific regions. The heat generates winds, which then produce electrical currents. Those currents power the aurora itself, creating a continuous feedback loop. “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," said Stallard. “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 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). The study also points to a deeper link between Saturn’s atmosphere and its magnetosphere, the vast region of space shaped by the planet's magnetic field, which could explain why the effect remains stable over long periods.