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Two bright auroral bands were detected near Uranus's magnetic poles, together with reduced emission and ion density in part of the region between the two bands (a feature likely linked to transitions in magnetic field lines). Credit: ESA/Webb, NASA, CSA, STScI, P. Tiranti, H. Melin, M. Zamani
For the first time, an international team of astronomers have mapped the vertical structure of Uranus's upper atmosphere, uncovering how temperature and charged particles vary with height across the planet. Using Webb's NIRSpec instrument, the team observed Uranus for nearly a full rotation, detecting the faint glow from molecules high above the clouds.
These unique data provide the most detailed portrait yet of where the planet's auroras form, how they are influenced by its unusually tilted magnetic field, and how Uranus's atmosphere has continued to cool over the past three decades. The results, published in Geophysical Research Letters, offer a new window into how ice-giant planets distribute energy in their upper layers.
Led by Paola Tiranti of Northumbria University in the United Kingdom, the study mapped out the temperature and density of ions in the atmosphere extending up to 5,000 kilometers above Uranus's cloud tops, a region called the ionosphere where the atmosphere becomes ionized and interacts strongly with the planet's magnetic field. The measurements show that temperatures peak between 3,000 and 4,000 kilometers, while ion densities reach their maximum around 1,000 kilometers, revealing clear longitudinal variations linked to the complex geometry of the magnetic field.
"This is the first time we've been able to see Uranus's upper atmosphere in three dimensions," said Tiranti. "With Webb's sensitivity, we can trace how energy moves upward through the planet's atmosphere and even see the influence of its lopsided magnetic field."
This timelapse video is believed to be the only dataset to date that has continuously observed a full rotation of Uranus by a single telescope, which was facilitated by Webb's uniquely positioned orbit at L2 that observed the planet for approximately 17 hours. Credit: ESA/Webb, NASA, CSA, STScI, P. Tiranti, H. Melin, M. Zamani (ESA/Webb)
Webb's data confirm that Uranus's upper atmosphere is still cooling, extending a trend that began in the early 1990s. The team measured an average temperature of around 426 kelvins (about 150°C), lower than values recorded by ground-based telescopes or previous spacecraft.
Two bright auroral bands were detected near Uranus's magnetic poles, together with a distinct depletion in emission and ion density in part of the region between two bands (a feature likely linked to transitions in magnetic field lines). Similar darkened regions have been seen at Jupiter, where the geometry of the magnetic field there controls how charged particles travel through the upper atmosphere.
"Uranus's magnetosphere is one of the strangest in the solar system," added Tiranti. "It's tilted and offset from the planet's rotation axis, which means its auroras sweep across the surface in complex ways. Webb has now shown us how deeply those effects reach into the atmosphere. By revealing Uranus's vertical structure in such detail, Webb is helping us understand the energy balance of the ice giants. This is a crucial step towards characterizing giant planets beyond our solar system."
The study is based on data from JWST General Observer program 5073 (PI: H. Melin of Northumbria University in the United Kingdom), which used NIRSpec's Integral Field Unit on 19 January 2025 to observe Uranus for 15 hours.
Publication details Paola I. Tiranti et al, JWST Discovers the Vertical Structure of Uranus' Ionosphere, Geophysical Research Letters (2026). DOI: 10.1029/2025gl119304 Journal information: Geophysical Research Letters
— Source: Phys.org (https://phys.org/news/2026-02-webb-mysterious-upper-atmosphere-uranus.html)
