JWST Solves Saturn's Decades-Long Rotation Mystery, Revealing a Self-Sustaining Auroral Heat Pump in the Upper Atmosphere

Overview

Astronomers say they have resolved a decades-long puzzle over why Saturn appears to change its rotation rate, concluding that the planet’s spin was never actually shifting. Using the James Webb Space Telescope (JWST), a team led by Professor Tom Stallard of Northumbria University found that the apparent change is produced by winds high in Saturn’s atmosphere, which are themselves driven by the planet’s aurora in a self-sustaining cycle, according to Northumbria University.

What We Know

The mystery dates to 2004, when NASA’s Cassini spacecraft returned measurements suggesting Saturn’s rotation rate was gradually changing, as reported by ScienceDaily. In 2021, Stallard’s team proposed that the rotation was not actually changing and that atmospheric winds were instead distorting the auroral signals used to estimate the planet’s spin, according to ScienceDaily.

The new JWST observations supply the evidence the earlier work lacked. “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,” Stallard said, according to Phys.org.

Stallard described the mechanism as a 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,” he said, according to Newsweek.

To capture the data, the team used JWST’s NIRSpec instrument to observe Saturn’s northern auroral region continuously for a full Saturnian day, according to Northumbria University. They analyzed the infrared glow from trihydrogen cation (H3+), a molecule that forms in Saturn’s upper atmosphere and acts as a natural thermometer, allowing the researchers to map both temperature and particle density across the auroral zone, according to Northumbria University.

The JWST measurements were roughly ten times more accurate than previous data, which had carried uncertainties of around 50 degrees Celsius, according to Newsweek. That precision let the researchers resolve specific patterns of heating across the region for the first time, as reported by ScienceDaily.

The findings were published in the Journal of Geophysical Research: Space Physics, according to Phys.org. The work drew on collaborators from Boston University, the University of Leicester, Aberystwyth University, the University of Reading, Imperial College London, Lancaster University, and the Johns Hopkins University Applied Physics Laboratory, according to Northumbria University.

Why It Matters

The finding reframes a long-standing problem: the auroral signals scientists had used to estimate Saturn’s spin were being distorted by atmospheric winds rather than tracking the planet’s true rotation, according to ScienceDaily.

Stallard suggested the result has implications beyond Saturn. “This result changes how we think about planetary atmospheres more generally,” he said, according to SciTechDaily.

What We Don’t Know

The published observations focus on Saturn’s northern auroral region, which the team monitored over a single Saturnian day, according to SciTechDaily. Stallard framed whether similar atmosphere-driven currents shape conditions on other worlds as an open question for future study, according to SciTechDaily.