New research reveals the vertical structure of Uranus' ionosphere, discovers auroras influenced by the planet's unusual tilted magnetic field, and confirms that its upper atmosphere has been cooling since the 1990s.
Astronomers have created the first detailed map showing the layered structure of Uranus' atmosphere from the bottom up. The international team tracked how temperatures and electrically charged particles change with altitude, providing a clearer picture of conditions high above the planet's clouds.
To do this, they used the Webb Space Telescope and its NIRSpec instrument, observing Uranus for nearly an entire orbit and picking up faint molecular emissions that glow high in the atmosphere. The findings provide new insights into how ice giant planets flow and balance energy in their outermost layers.
These sweeping images show a complete orbit of Uranus as captured by the Webb Space Telescope. Thanks to its orbit at the L2 point, Webb was able to observe the planet continuously for about 17 hours.
Credit: ESA/Webb, NASA, CSA, STScI, P. Tiranti, H. Melin, M. Zamani (ESA/Webb)
Exploring the ionosphere and magnetic field of Uranus
The study was led by Paula Tiranti of Northumbria University in the UK. The scientists measured temperature and ion density at altitudes up to 5000 km above the clouds, in a region called the ionosphere where the atmosphere is ionized and there is a strong interaction between it and the magnetic field.
The data provide the most comprehensive picture yet of the origin of Uranus' auroras and how they are shaped by its unusually tilted magnetic field. The results also confirm that the planet's upper atmosphere has continued to cool over the past three decades. Temperatures were found to be highest at altitudes between 3,000 and 4,000 km above the clouds, while ion densities were highest at an altitude of about 1,000 km. The team also identified clear variations with longitude, reflecting the complex structure of Uranus' magnetic field.
"This is the first time we've been able to see Uranus' upper atmosphere in 3D," said Paula. "With Webb's sensitivity, we can track how energy moves up through the planet's atmosphere and even see the effect of its tilted magnetic field."
Long-term cooling of Uranus' atmosphere
Measurements from the Webb show that Uranus' upper atmosphere is still cooling, continuing a pattern first identified in the early 1990s. The researchers calculated an average temperature of about 426 degrees Kelvin (about 150 degrees Celsius), which is lower than temperatures previously reported by ground-based telescopes or previous space missions.
Bands of auroras and an unusual magnetosphere
Webb observed two bright bands of light near Uranus' magnetic poles. Between these two bands, the team detected a marked decrease in emission and ion density (a phenomenon likely related to changes in magnetic field lines). Similar darker regions have also been observed on Jupiter, where the shape of its magnetic field directs the movement of charged particles through its upper atmosphere.
"Uranus' magnetosphere is one of the most peculiar in the solar system," added Paula. "It is tilted and offset relative to the planet's axis of rotation, meaning its auroras move across Uranus in complex ways. Webb has now shown us how deep into the atmosphere these effects reach. By revealing the vertical structure of Uranus, Webb helps us understand the energy balance of the ice giant. This is a crucial step in characterizing giant planets beyond our solar system."
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