Uranus, the seventh planet from the Sun, has only been visited once by NASA’s Voyager 2 spacecraft, which performed a flyby of the ice giant in 1986. It came within just tens of thousands of miles of the planet’s cloud tops, where it appeared as a surprisingly “drab” light-blue orb in the blackness of space, over a billion miles from Earth.
Now, thanks to recent observations by NASA’s groundbreaking James Webb Space Telescope, we’re getting an unprecedented peek inside the layers of its upper atmosphere.
The observatory’s Near-Infrared Spectrograph (NIRSpec) instrument observed Uranus for almost a full rotation just over a year ago, showing how the planet’s ionosphere, a thin layer in the planet’s upper atmosphere that’s ionized by solar radiation, is interacting with its magnetic field.
It’s the most detailed picture of the planet’s atmosphere yet, demonstrating where auroras form on its surface. It also sheds light on the planet’s unusually tilted magnetic field. Uranus is the only planet whose equator is almost at a right angle to its orbit, an astonishing tilt of 97.77 degrees. Its magnetic axis, on the other hand, has a large tilt relative to its rotation axis, making its magnetosphere a significant outlier compared to other planets.
“This is the first time we’ve been able to see Uranus’s upper atmosphere in three dimensions,” said Northumbria University PhD student Paola Tiranti, lead author of a new paper published in the journal Geophysical Research Letters, in a statement. “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.”
The latest findings also support existing theories that Uranus’ upper atmosphere is still cooling, a trend that was first observed in the early 1990s, when near-infrared observations began.
Thanks to the planet’s unusual tilt, auroras act in a surprisingly different way. As on Earth, Uranian auroras are the result of charged particles from the Sun colliding with atmospheric gases, causing dancing colors to appear in the night sky. But while they’re most commonly seen around our world’s north and south poles, the situation is drastically different on Uranus.
“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.”
In the case of Uranus, auroras appear as glowing patches of orange and red light that extend past the edges visible in JWST observations.
“These auroral detections are hugely important because they are a direct manifestation of the planet’s internal magnetic field,” JWST interdisciplinary scientist Heidi Hammel, who was not involved in the study, told Scientific American. “We really have no other way to probing the magnetic field remotely without a spacecraft in situ.”
The findings could inform future visits to the distant ice giant. But when we’ll have a chance to get another, closer look four decades after Voyager 2’s flyby remains as uncertain as ever. Tight budgets have endangered interplanetary missions as of late, and it remains to be seen whether a trip to Uranus will still be in the cards in the coming years.
“Webb has now shown us how deeply those effects reach into the atmosphere,” Tiranti said in the statement. “By revealing Uranus’s 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.”
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