JWST Reveals 3D View of Uranus’s Unusual Atmosphere & Auroras
- Its magnetic field tilts by nearly 60 degrees and sits off-center, resulting in auroras that don’t gather in neat rings.
- Using the James Webb Space Telescope (JWST), an international team led by Paola Tiranti, a PhD student at Northumbria University, has produced the first three-dimensional view of Uranus’...
- The team observed Uranus for 15.4 hours on January 19, 2025, nearly a full rotation of the planet, which takes approximately 17.2 hours.
Uranus doesn’t behave like an ordinary planet. Its magnetic field tilts by nearly 60 degrees and sits off-center, resulting in auroras that don’t gather in neat rings. Instead, charged particles spark auroras that sweep across the ice giant in complicated paths, brightening and thinning depending on how the magnetic field funnels energy into the upper atmosphere.
That odd geometry now has a new kind of map. Using the James Webb Space Telescope (JWST), an international team led by Paola Tiranti, a PhD student at Northumbria University, has produced the first three-dimensional view of Uranus’ upper atmosphere. The study, published in Geophysical Research Letters, tracks faint infrared emission from molecules as high as about 5,000 kilometers above the cloud tops.
A Night-Long Watch on a Rotating Planet
The team observed Uranus for 15.4 hours on , nearly a full rotation of the planet, which takes approximately 17.2 hours. The data came from JWST General Observer program 5073, led by Dr. Henrik Melin of Northumbria University, utilizing Webb’s Near-Infrared Spectrograph Integral Field Unit.
The sensitivity of JWST is crucial here. Uranus’ infrared glow is faint, and previous Earth-based observations have struggled to discern vertical structure at the planet’s limb. This time, the team analyzed the signal in 350-kilometer altitude steps from 475 kilometers to 5,025 kilometers, retrieving local temperatures and ion densities in the ionosphere – the region where the atmosphere becomes ionized and strongly couples with the magnetic field.
They focused on emission from H3+, a molecular ion commonly used to remotely probe temperature and density in giant-planet ionospheres.
“Here’s 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.”
Heat High Up, Ions Lower Down
The new profiles reveal that Uranus’ upper atmosphere doesn’t peak in temperature at the same altitude as its ion density. Across the globe, the team found temperatures rising from about 419 K at 475 kilometers to a peak of 470 K at roughly 3,625 kilometers, then decreasing with height. In simpler terms, the warmest layer sits between approximately 3,000 and 4,000 kilometers above the cloud tops.
Ion densities peaked much lower, just above 1,000 kilometers, specifically at 1,175 kilometers. These two peaks are significant because they provide clues about how energy is deposited, and redistributed. The team also compared total infrared emission to temperature and density at different altitudes. In the 3,000 to 4,000 kilometer band, emission correlated more strongly with temperature than with density, suggesting thermal processes dominate the glow at those heights. Lower down, around the density peak, the pattern reversed.
Two Bright Bands and a Dim Gap
Webb’s data also revealed structure in Uranus’ auroras that corresponds to the planet’s unusual magnetic field. The observations show two bright auroral bands near the magnetic poles, along with a distinct region between them where emission and ion density drop. The team links this depleted zone to how magnetic field lines guide charged particles through the atmosphere. Similar darker regions have been observed at Jupiter, where magnetic geometry also controls particle flow.
The study notes that the bright emission regions extended over large longitude ranges, around 50 degrees, even though other observations, including from JWST and the Hubble Space Telescope, have often shown compact, spot-like brightenings.
The coverage of the observations is a limitation. The data included latitudes between 25°N and 25°S, and the authors acknowledge that the southern aurora is only partially sampled in their current geometry.
The Long Cooling Trend Still Holds
One result in the paper isn’t directly related to auroras. Uranus’ upper atmosphere has been cooling for decades, and Webb’s measurements extend that trend. The team reports a column-weighted temperature of 426 K, cooler than values reported in earlier ground-based work, and slightly higher than a JWST disk-averaged temperature of 415 K, which the authors attribute to differences in the parts of the planet emphasized in each measurement.
The reason for the cooling remains an open question. The study notes that the decline has been attributed to reduced solar wind power, though this explanation is still debated. The authors also point to limitations in how the infrared signal behaves at very low densities. At high altitudes, where H3+ becomes sparse, non-local thermodynamic equilibrium effects may influence the retrieved profiles. They also state that their profiles become unreliable above 5,025 kilometers, with typical uncertainties remaining under 10% up to that altitude.