Webb Telescope Delivers First 3D Map of Uranus’s Atmosphere, Reveals Cooling Trend
For the first time, astronomers have created a three-dimensional map of Uranus’s upper atmosphere, providing unprecedented insight into the ice giant’s temperature, ion density, and the behavior of its auroras. The breakthrough, achieved using the James Webb Space Telescope’s Near-Infrared Spectrograph (NIRSpec), reveals how energy moves through the planet’s atmosphere and is influenced by its uniquely tilted magnetic field.
The research, led by Paola Tiranti of Northumbria University in the United Kingdom, involved observing Uranus for nearly a full rotation. By detecting faint emissions from molecules high above the cloud tops – extending up to 5,000 kilometers – the team was able to map the vertical structure of the planet’s ionosphere, a region where the atmosphere becomes ionized and interacts strongly with the magnetic field. The findings were published in the journal Geophysical Research Letters.
Mapping the Ionosphere and Aurora Formation
The observations pinpointed temperature peaks between 3,000 and 4,000 kilometers above the cloud tops, while ion densities reached their maximum around 1,000 kilometers. Crucially, the data revealed longitudinal variations in these measurements, directly linked to the complex geometry of Uranus’s magnetic field. This detailed mapping provides the clearest picture yet of where the planet’s auroras form and how they are shaped.
“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.”
Continuing Atmospheric Cooling
The data also confirms a long-observed trend: Uranus’s upper atmosphere continues to cool. Researchers measured an average temperature of approximately 426 kelvins (around 150 degrees Celsius), lower than previous measurements taken by ground-based telescopes and earlier spacecraft missions. This cooling trend was initially identified in the early 1990s, and the new Webb observations provide further evidence that it is ongoing.
A Unique Magnetosphere and Auroral Activity
Uranus possesses a highly unusual magnetosphere, tilted and offset from the planet’s rotational axis. This configuration results in complex auroral displays that sweep across the planet’s surface in ways unlike those seen on Earth or other planets in our solar system. Webb’s observations have now revealed how deeply these magnetic field effects penetrate into the atmosphere.
The telescope detected two bright auroral bands near the planet’s magnetic poles. Between these bands, the team identified a region with reduced emissions and fewer ions, a feature believed to be linked to transitions in the magnetic field lines. Similar darker regions have been observed on Jupiter, where the magnetic field guides the movement of charged particles in the upper atmosphere.
“Uranus’s magnetosphere is one of the strangest in the Solar System,” Tiranti added. “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.”
Implications for Understanding Ice Giants and Exoplanets
The findings have significant implications for our understanding of ice giant planets, both within our solar system, and beyond. Uranus and Neptune represent a distinct class of planets, and studying their atmospheric dynamics and energy balance is crucial for characterizing similar exoplanets discovered orbiting other stars. The detailed data provided by Webb will serve as a benchmark for future observations of these distant worlds.
“This is a crucial step towards characterizing giant planets beyond our Solar System,” Tiranti explained.
Webb Telescope and International Collaboration
The observations were made possible by the James Webb Space Telescope, a joint mission of NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA). The data utilized JWST’s General Observer program 5073, with H. Melin of Northumbria University as the Principal Investigator. The team used NIRSpec’s Integral Field Unit to observe Uranus continuously for 15 hours on .
ESA provided the launch service for Webb using the Ariane 5 rocket and oversaw modifications required for the mission. The agency also contributed the NIRSpec instrument and 50 percent of the Mid-Infrared Instrument (MIRI), developed in partnership with JPL and the University of Arizona.
