Scientists solve decades-long mystery about why Saturn appears to change its spin

Researchers at Northumbria University have used the most powerful space telescope ever built to answer one of the longest-standing puzzles in planetary science – why does Saturn appear to spin at a different speed depending on how you measure it?

The findings, published in the Journal of Geophysical Research: Space Physics,reveal for the first time the complex patterns of heat and electrically charged particles in Saturn's aurora, and show that the entire system is driven by a self-sustaining feedback loop powered by the planet's own northern lights.

Saturn has puzzled scientists for many years. Measurements taken by NASA's Cassini spacecraft in 2004 suggested the planet's rotation rate was slowly changing over time – yet this should not have been possible, as a planet cannot simply speed up or slow down its spin.

In 2021, a study led by Tom Stallard, Professor of Planetary Astronomy at Northumbria University, showed that the mystery did not actually involve Saturn's rotation at all. Instead, the apparent changes were being driven by winds in the planet's upper atmosphere, which were producing electrical currents that created the misleading auroral signal.

However, the findings raised a further question for the research team – if atmospheric winds were responsible for the effect, what was causing those winds?

New research by Professor Stallard and colleagues across the UK and US has now provided the first direct evidence of the answer.

Using the James Webb Space Telescope (JWST), the team observed Saturn's northern auroral region – the equivalent of Earth's northern lights – continuously for a full Saturnian day, capturing detailed measurements that were simply not possible with any previous instrument.

By analysing the infrared glow from a molecule called trihydrogen cation, which forms in Saturn's upper atmosphere and acts as a natural thermometer, the researchers were able to produce the first high-resolution maps of both temperature and particle density across Saturn's auroral region.

The level of detail 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. The new JWST data was ten times more accurate than previous measurements, allowing the team to map fine details of heating and cooling across Saturn's auroral region for the very first time.

What the team found was that these temperature and density patterns match remarkably well with predictions made by computer models more than a decade ago, but only if the source of heat is placed exactly where the main auroral emissions enter the atmosphere.

This means Saturn's aurora is not just a visual display – it is actively heating the atmosphere in a specific location. That localised heating drives winds, which in turn generate the electrical currents responsible for the aurora. The aurora then heats the atmosphere again, sustaining the whole 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 findings also have broader implications. The research suggests that what happens in Saturn's atmosphere directly influences conditions in its surrounding magnetosphere – the vast region of space shaped by the planet's magnetic field – which in turn feeds energy back into the system. This two-way relationship between atmosphere and magnetosphere may help explain why the effect is so stable and long-lasting.

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. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).

The study was carried out by researchers from Northumbria University, alongside 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. The research was supported by the Science and Technology Facilities Council (STFC).

Visit the Northumbria University Research Portal to find out more about Professor Tom Stallard’s work.

The paper JWST/NIRSpec reveals the atmospheric driver of Saturn's variable magnetospheric rotation rate, was published in Journal of Geophysical Research: Space Physics on 3 March 2026 (DOI 10.1029/2025GL118553).

Ends

Media descriptions:

Here, we show the asymmetric temperatures, density and intensity of the auroral ionosphere revealed in this recent research, as it was seen from JWST.

We have combined spectral imagery taken on the same day, 29 November 2024, by the JWST NIRSPEC and JWST NIRCAM instruments.

The NIRSPEC data was taken under programme GO-5308, PI: Moore, co-PI: Stallard, Melin, and were processed into these final data products by T. Stallard.

The three-color NIRCAM image of Saturn were taken under programme DD-9219, PI: Garcia Marin, and were processed into the final three-color image by Melina Thévenot (https://bsky.app/profile/melina-iras07572.bsky.social).

Image/movie credit:NASA/ESA/CSA, Tom Stallard (Northumbria University), Melina Thévenot, Macarena Garcia Marin (STScI/ESA).

• context_saturns_temperatures_movie.mov shows the asymmetric temperature structure revealed in the paper, as it was observed from JWST. These are offset 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
• context_saturns_h3p_density_movie.mov shows the asymmetric density structure, revealing where the auroral current was preferentially flowing into (as darker) and out of (as brighter) the planet. These are offset from the temperature peaks, but ultimately drive that temperature asymmetry
• context_saturns_h3p_emission_movie.mov shows the auroral brightening, as has previously been observed from both Earth and in orbit around Saturn

The following images are three frames taken from the same movies at the same time, showing how these three asymmetric features are related:

• context_saturn_asymmetric_densities.png
• context_saturn_asymmetric_temperatures.png
• context_saturn_asymmetric_intensity.png

Data movies:

Here, we show the asymmetric temperatures, density and intensity of the auroral ionosphere as viewed from above the auroral region, rotating to highlight how interconnected these different parameters are:

• data_parameters.mov shows these three parameters (top row) and the different from the median values at each latitude (bottom row) - here, red is higher and blue lower, revealing that not only the brighter regions but also weaker regions follow very similar patterns, driven by and driving the planetary-period currents flowing into and out of the planet.