Webb for the first time captured a “vertical cross-section” of Uranus’s ionosphere: a 3D map reveals where auroras are born and why the planet is still cooling
Scientists have, for the first time, managed to create a detailed map of the layers of Uranus’s upper atmosphere, not only “across the surface” but also by altitude — from the tops of the clouds to thousands of kilometers above them. The observations were carried out with the James Webb Space Telescope, whose sensitivity enabled the detection of an extremely faint glow from molecules high above the clouds, where the atmosphere transitions into the ionosphere and directly “connects” with the planet’s unusual magnetic field.
This result is important for two reasons. First, it provides the clearest picture to date of where and how Uranus’s auroras form and how their shape and position change due to a magnetic field that is tilted and offset relative to the rotation axis. Second, the measurements confirm that Uranus’s upper atmosphere continues to cool — a trend that has puzzled scientists since the early 1990s.
What exactly was measured and why this is unprecedented
The ionosphere is a layer of the atmosphere in which solar radiation and charged particles create ions and electrons, so the gas is no longer “neutral.” It is precisely in this region that intense energy exchange occurs: charges are “controlled” by the magnetic field, and heating and cooling depend on a combination of solar radiation, the influx of particles from the magnetosphere, and the dynamics of the atmosphere below.
Until now, researchers had a fragmented picture of Uranus — comparable to knowing what the shimmering in the sky looks like, but not knowing at what exact altitude it happens and how “thick” the zone is in which energy is released. The new Webb data enabled the creation of vertical profiles of temperature and ion density: how hot it is and how many charged particles there are at different altitudes, and how these quantities change with longitude as the planet rotates.
How Webb observed Uranus: almost an entire “day” in a single run
The observation was conducted on January 19, 2025, and lasted about 15 hours, covering almost a full rotation of Uranus. The key tool was NIRSpec, Webb’s near-infrared spectrograph, used in Integral Field Unit (IFU) mode. This approach provides both a spectrum and spatial information at the same time, so it is possible to distinguish where in the atmosphere the glow of certain molecules appears, and where it is absent.
The scientific team was led by Paola Tiranti from Northumbria University in the United Kingdom. At the center of the analysis was a faint infrared glow above the clouds, which arises when excited particles and molecules in the ionosphere return to lower energy states and emit light at specific wavelengths. It is precisely this “signature” that enables an estimate of ion temperature and density.
Key results: where it is hottest, where there are the most ions
The measurements show a clear vertical structure extending up to approximately 5,000 kilometers above the cloud tops. The temperature in the upper layers reaches a peak between 3,000 and 4,000 kilometers in altitude, while the ion density reaches a maximum around 1,000 kilometers. An important detail is that pronounced changes depending on longitude were also observed — in other words, the same layer is not identical everywhere around the planet, but is “modulated” by the complex geometry of the magnetic field.
The average temperature of the upper atmosphere from Webb’s measurements is about 426 kelvin, that is, approximately 150 degrees Celsius. This value is lower than earlier estimates obtained by ground-based telescopes and older observations from space, which further strengthens the conclusion that Uranus’s upper atmosphere continues to cool.
Auroras in two bands and a “dark zone” between them: a trace of magnetic geometry
In Webb’s data, two bright auroral bands stand out near the magnetic poles. Such structures arise when charged particles, guided by magnetic field lines, plunge into the atmosphere and release energy. But what is missing is just as interesting: between those two bands, a region of reduced emission and lower ion density was recorded — a kind of “dark zone” in which the glow is suppressed.
Similar darker regions have also been observed on Jupiter, where it is known that the geometry of magnetic field lines determines the trajectories of charged particles, so the aurora does not distribute evenly. On Uranus, this effect is additionally emphasized because the magnetic field is extremely “asymmetric”: the magnetic axis is tilted by almost 60 degrees relative to the rotation axis, and the center of the magnetic field is offset from the center of the planet by about one third of the radius. Because of this, the auroral zones do not behave like a stable ring around the poles, but during rotation they “spill over” across different geographic regions and profoundly affect the distribution of energy in the atmosphere.
Why Uranus’s cooling is a big mystery
Uranus is the coldest of the giant planets in the Solar System when talking about the temperatures we “see” in the atmosphere, but the upper layers — the thermosphere and ionosphere — should be sensitive to energy input. An additional problem is that Uranus is far from the Sun, so the influx of solar radiation is weaker than at Jupiter or Saturn, yet despite that the upper atmosphere shows complex changes that are not easy to explain solely by distance from the Sun.
The cooling trend tracked since the early 1990s has sparked debates about how energy is transferred upward from the lower layers, how efficiently the atmosphere “cools” by radiating into space, and what role seasonal changes, the solar wind, and internal processes in the magnetosphere play. Webb’s results do not close the question, but for the first time they provide a sufficiently detailed “3D context” so that hypotheses can be tested without relying on individual, unconnected measurements.
The bigger picture: a lesson about ice giants and worlds beyond the Solar System
Uranus and Neptune are often called ice giants, and their atmospheric and magnetic processes are key to understanding a class of planets that is probably common in the galaxy. In exoplanet observations, especially of those similar in size and mass to Uranus, scientists often have only an “integrated” signal — an average over the whole planet. That is why any improvement in understanding how energy is distributed in the upper layers of an atmosphere is directly useful for interpreting data about distant worlds as well.
This is precisely where Webb’s measurements have additional value: they show how local maxima of temperature and ion density can appear in the ionosphere, how auroral structures can organize into multiple bands, and how magnetic asymmetry leaves a visible signature not only on the “light show,” but also in the vertical structure of the atmosphere itself.
What’s next: more rotations, comparisons, and the search for a mechanism
The obtained results are based on observations from the JWST General Observer program 5073, in which the principal investigator is Henrik Melin. The scientific community now has a reference dataset that can be compared with future Webb measurements, as well as with data from other observatories. It will be especially important to track how the signals change over time: does the position of the auroral bands change depending on solar conditions, does the “dark zone” always appear in the same place in the magnetic geometry, and does the cooling continue at the same rate.
For Uranus, a planet that historically has had few visits and even fewer long-term observing campaigns, such data represent a kind of “mini mission” from afar. Instead of a brief flyby, we get continuous observing that reveals how the atmosphere behaves as the planet rotates and how the magnetic field reshapes the ionosphere through altitude and longitude. In this way, step by step, the mechanism is assembled that could explain how ice giants balance energy in the extreme conditions of the outer Solar System.
Sources:- ESA/Webb – scientific release on 3D mapping of Uranus’s ionosphere, results on temperature, ion density, and auroral bands (link)- Northumbria University Research Portal – bibliographic data and paper status in Geophysical Research Letters (DOI: 10.1029/2025GL119304) (link)- NASA Science – basic facts about Uranus’s magnetic field, its tilt and offset from the planet’s center (link)- ESA/Webb – description of the NIRSpec instrument and integral field (IFU) capabilities in spectroscopy (link)
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