Using the James Webb Space Telescope, operated by NASA with ESA and CSA, lead author Paola Tiranti and colleagues observed Uranus for nearly a full rotation and detected faint infrared emissions from molecules located as high as 5,000 kilometers above the visible cloud tops.
The new observations deliver the most detailed picture so far of where auroras form at Uranus and how energy is transported vertically through its upper atmosphere and ionosphere.
The study, published in Geophysical Research Letters, also confirms that Uranus upper atmosphere has continued to cool over the past three decades, extending a long-standing and puzzling trend in the ice giant's climate.
Auroras occur when charged particles are funneled along a planet's magnetic field lines and collide with atoms and molecules in the upper atmosphere, releasing energy as light across a range of wavelengths.
By using Webb's Near-Infrared Spectrograph in its Integral Field Unit mode, the team mapped the temperature and density of ions in Uranus ionosphere, the region where the atmosphere becomes ionized and is strongly coupled to the magnetic field.
They found that temperatures in the upper atmosphere peak between roughly 3,000 and 4,000 kilometers above the cloud tops, while ion densities reach their maximum closer to 1,000 kilometers altitude.
"This is the first time we've been able to see Uranus upper atmosphere in three dimensions. 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," said Tiranti.
Uranus magnetosphere ranks among the most unusual in the Solar System. Unlike Earth, where the magnetic field is closely aligned with the rotation axis, Uranus field is tilted by nearly 60 degrees and offset from the planet's center, causing its auroras to sweep across the planet in complex patterns.
Webb data revealed two bright auroral bands near the planet's magnetic poles, separated by a narrow region where emissions and ion densities are depleted, a structure the team links to the way magnetic field lines channel charged particles through the atmosphere.
Similar darkened regions have been identified at Jupiter, where the geometry of the magnetic field determines how particles flow and where they precipitate into the upper atmosphere.
The Webb measurements show that Uranus upper atmosphere is still cooling, with an average temperature of about 426 kelvins, or roughly 150 degrees Celsius, which is lower than temperatures recorded by previous spacecraft and ground-based observations.
Explaining why the upper atmosphere is cooling, despite Uranus distance from the Sun and internal heat sources, could offer important clues to how ice giant planets control their energy budgets and atmospheric temperatures.
"By revealing Uranus vertical structure in such detail, Webb is helping us understand the energy balance of the ice giants. This is a crucial step towards characterising giant planets beyond our Solar System," Tiranti said.
The results come from JWST General Observer program 5073, led by Dr Henrik Melin of Northumbria University, which used Webb's Integral Field Unit to monitor Uranus for about 15 hours on 19 January 2025.
Research Report:JWST Discovers the Vertical Structure of Uranus' Ionosphere
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