An international team of researchers used Webb’s Near-Infrared Spectrograph (NIRSpec) instrument to map the temperature and density of ions found about 3,000 miles (5,000 kilometers) above Uranus’s cloud tops. The findings, published in Geophysical Research Letters, reveal how the planet’s eccentric magnetic field affects its auroras and provide new insights into how energy behaves in the ice giants’ upper layers.
“This is the first time we have been able to see the upper atmosphere of Uranus in three dimensions,” Paola Tiranti, a PhD student at Northumbria University in the UK and lead author of the study, said in a statement. “With Webb’s sensitivity, we can trace how energy moves upward through the planet’s atmosphere and even see the effects of its unbalanced magnetic field.”
giant freak
Uranus has an unusual magnetic field. The planet’s rotation axis is tilted by more than 90 degrees, causing the planet to rotate on its side. The tilt of the magnetic axis is also large, being about 60 degrees off its rotation axis. According to NASA, its lateral magnetic field gives Uranus a more variable magnetosphere, meaning its auroras spread across the surface in more complex ways.
Recent Webb observations detected two bright auroral bands near Uranus’s magnetic poles, as well as a significant reduction in ion density and emission in the region between the bands. This is likely due to how magnetic field lines guide charged particles through Uranus’s atmosphere.
It was the Hubble Space Telescope that first observed auroras on Uranus in 2012. Since then, scientists have tried to understand how the planet’s peculiar magnetosphere affects its auroral displays.
“Uranus’ magnetosphere is one of the strangest in the solar system,” Tiranti said. “Webb has now shown us how deep into the atmosphere those effects reach.”
cold world
NASA’s Voyager 2 spacecraft made the first close flyby of Uranus on January 24, 1986. The mission revealed a pale blue world that is much cooler than its neighboring planets. At that time, the temperature in Uranus’s upper atmosphere dropped below -353 Fahrenheit (-214 Celsius).
Webb’s recent data also confirmed that Uranus’s upper atmosphere is still cooling. Based on data collected by the Voyager 2 flyby, Uranus’s upper atmosphere was undergoing a long-term cooling trend, with temperatures falling over time. The team measured temperatures about 302 Fahrenheit (150 Celsius) lower than those recorded during previous attempts.
“By revealing the vertical structure of Uranus in such detail, Webb is helping us understand the energy balance of the ice giants,” Tiranti said. “This is an important step toward characterizing giant planets beyond our solar system.”
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