Astronomers Map Uranus’s Upper Atmosphere for the First Time Revealing Key Insights on Temperature and Magnetic Interactions

In a groundbreaking achievement, an international team of astronomers has successfully mapped the vertical structure of Uranus’s upper atmosphere for the first time, shedding light on the distribution of temperature and charged particles at various altitudes. Using the advanced capabilities of the James Webb Space Telescope’s NIRSpec instrument, the researchers conducted observations of Uranus during nearly an entire rotation, capturing the subtle luminous emissions from molecules situated high above the planet’s cloud cover. These findings provide new insights into the complex processes governing energy distribution in the atmospheres of ice giant planets.

The study, spearheaded by Paola Tiranti from Northumbria University in the UK, charted the temperature and ion density within Uranus’s atmosphere, extending up to 5,000 kilometers above the cloud tops. This specific region, known as the ionosphere, is critical as it becomes ionized and interacts significantly with the planet’s magnetic field. The data obtained affords the most comprehensive view to date of where auroras manifest on Uranus, how they’re shaped by the planet’s uniquely tilted magnetic field, and how its atmosphere has been cooling over the past thirty years. The research revealed that temperatures peak between 3,000 and 4,000 kilometers, while ion density reaches its highest levels at around 1,000 kilometers, indicating distinct longitudinal variations that correlate with the magnetic field’s intricate structure.

“This is the first time we’ve been able to visualize Uranus’s upper atmosphere in three dimensions,” stated Tiranti. She noted the importance of Webb’s sensitivity in uncovering how energy ascends through the atmosphere, while also illuminating the effects of the planet’s peculiarly arranged magnetic field. The observations confirm a continued cooling trend in Uranus’s upper atmosphere that began in the early 1990s, with an average temperature recorded at approximately 426 kelvins (about 150 degrees Celsius), which is notably lower than prior measurements taken by ground-based instruments or past space missions.

Additionally, the researchers identified two prominent auroral bands close to Uranus’s magnetic poles, alongside a noticeable decrease in emissions and ion density in an area situated between the two bands. This particular feature is thought to be associated with transitions in the magnetic field lines, reminiscent of similar dark zones observed at Jupiter, where its magnetic field geometry dictates the movement of charged particles in the upper atmosphere.

Tiranti emphasized the peculiarity of Uranus’s magnetosphere, highlighting its tilt and positional offset from the planet’s rotation axis, which causes its auroras to traverse the surface in unpredictable patterns. The data collected by Webb reveals not only the reach of these effects into the atmosphere but also assists scientists in understanding the energy balance of ice giants. This research serves as an essential milestone toward better characterizing larger planets beyond our own solar system.

The study’s findings, derived from data collected during the JWST General Observer program, were documented in the recent issue of Geophysical Research Letters. The observations were conducted on January 19, 2025, over a 15-hour period, paving the way for new explorations of the mysteries surrounding Uranus and its atmospheric dynamics.