For the first time, astronomers have created a three-dimensional map of Uranus’s upper atmosphere, revealing how temperature and charged particles vary with altitude across the planet. Using the NASA/ESA/CSA James Webb Space Telescope, an international team observed the ice giant for nearly a full rotation, detecting the faint glow of molecules far above its cloud tops.
Led by Paola Tiranti of Northumbria University in the United Kingdom, the team employed Webb’s NIRSPEC instrument to probe the planet’s ionosphere—a region extending up to 5,000 kilometers above the surface where the atmosphere becomes ionized and interacts with the magnetic field. The results, published in Geophysical Research Letters, provide the most detailed portrait yet of where Uranus’s auroras form and how its uniquely tilted magnetic field shapes them.
The data reveal that temperatures peak between 3,000 and 4,000 kilometers, while ion densities reach their maximum around 1,000 kilometers. Clear longitudinal variations linked to the planet’s complex magnetic field geometry also emerged from the observations.
“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 atmosphere and even see the influence of its lopsided magnetic field.”
Webb’s measurements confirm that Uranus’s upper atmosphere continues a cooling trend first observed in the 1990s. The team recorded an average temperature of approximately 426 kelvins (around 150°C)—lower than values reported by ground-based telescopes or previous spacecraft.
The telescope detected two bright auroral bands near Uranus’s magnetic poles, accompanied by a distinct depletion in emission and ion density in the region between them. This darkened area, likely caused by transitions in magnetic field lines, resembles similar features observed at Jupiter, where magnetic geometry controls how charged particles travel through the upper atmosphere.
“Uranus’s magnetosphere is one of the strangest in the Solar System,” Tiranti explained. “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.”
The observations, part of JWST General Observer programme 5073 led by H. Melin, took place on 19 January 2025 with Webb’s NIRSpec instrument observing Uranus continuously for 15 hours. By revealing the planet’s vertical structure in such detail, Webb is helping scientists understand the energy balance of ice giants—knowledge that could prove crucial for characterizing similar exoplanets beyond our Solar System.
