Webb Captures Io’s and Europa’s Auroral Footprints in Jupiter’s Atmosphere

The NASA/ESA/CSA James Webb Space Telescope performed a clockwise scan around Jupiter’s entire limb, chasing the auroras as they circled toward our view. This dynamic phenomenon is the result of charged particles traveling along magnetic field lines, crashing into the planet’s ionosphere and causing it to glow. During the analysis, Webb’s Near-Infrared Spectrograph (NIRSpec) captured an extraordinary aspect of Jupiter’s auroras, known as auroral fingerprints, which are light emission patterns produced as a result of the interaction between Jupiter’s Galilean moons and the space environment surrounding the giant planet. Using NIRSpec data, planetary scientists measured the physical properties of the auroral imprints of Jupiter’s two innermost Galilean moons, Io and Europa, including local temperature and ionospheric density, in the near infrared. They discovered a never-before-seen low-temperature structure, centered on Io’s emitting bright spot, possessing extremely high densities; this is likely due to extreme changes in the flow of electrons crashing into the upper atmosphere.

Webb captured the auroral imprints of Io and Europa, providing spectral measurements for the first time and revealing extreme changes in the physical properties of Io’s auroral imprint, likely related to electrons crashing into the upper part of Jupiter’s atmosphere. Image credit: NASA / ESA / CSA / Webb / NIRCam / Jupiter ERS Team / Judy Schmidt / Katie L. Knowles, University of Northumbria.

“These emissions have been measured before in ultraviolet and infrared wavelengths, but only with their brightness,” said lead author Katie Knowles, a Ph.D. student at Northumbria University.

“For the first time, we were able to describe the physical properties of auroral imprints – upper atmospheric temperature and ion density, which has never been reported before.”

Unlike Earth’s northern lights, which are primarily powered by the solar wind, Jupiter’s northern lights include the impact of its four large Galilean moons – Io, Europa, Ganymede and Callisto – which create their own “mini aurora” on the planet.

Jupiter’s powerful magnetic field rotates about once every 10 hours with the planet itself, carrying charged particles with it.

But its moons orbit much more slowly: Io, the innermost moon, takes about 42.5 hours to complete an orbit.

“The moons constantly interact with the magnetic field and plasma surrounding the planet, and this interaction leads to highly energetic particles traveling along the magnetic field lines and then crashing into the planet’s atmosphere, creating the auroral imprints that correspond to where the moons orbit Jupiter,” Knowles said.

“The aurora of Jupiter is the most powerful and constant of all the auroras in the solar system.”

“What we observe with Webb gives us an unprecedented window into how Jupiter’s moons directly affect the upper part of the planet’s atmosphere.”

During a 22-hour observing window that took place in September 2023, Webb scanned the outline of Jupiter, tracking the northern lights as they circled toward our view.

It was during this observation that they also happened to capture the imprints of the aurora.

However, the imprints created by Io and Europa did not have the expected characteristics of Jupiter’s main aurora, which is relatively hot and contains a lot of material.

Instead, in a single snapshot, they discovered a cold spot in Io’s auroral imprint that recorded temperatures far lower than expected with extraordinarily high densities.

Io is the most active volcanic body in our solar system, with its volcanoes ejecting around 1,000 kilograms of material into space every second, fueling the dense plasma surrounding Jupiter.

This material ionizes and forms a donut-shaped cloud around Jupiter called the Io plasma torus.

As Io moves through this environment, it generates powerful electrical currents that create the brightest spots in Jupiter’s northern lights.

The researchers found that these auroral imprints contain densities of trihydrogen cations three times higher than those found in Jupiter’s main auroras, with some regions showing density variations of up to 45 times within a single small area.

“We saw extreme variability in temperature and density within Io’s auroral fingerprint, which occurred on a scale of minutes,” Knowles said.

“This tells us that the stream of high-energy electrons crashing into Jupiter’s atmosphere is evolving incredibly quickly.”

“The cold spot recorded temperatures of just 538 K (265 degrees Celsius or 509 degrees Fahrenheit) compared to 766 K (493 degrees Celsius or 919 degrees Fahrenheit) in the rest of Jupiter’s auroras.”

“The cold spot also contained material three times denser than Jupiter’s main aurora.”

The findings could extend far beyond Jupiter and open questions about other planetary systems.

Saturn’s moon Enceladus also creates an auroral imprint on the planet, and scientists wonder whether similar phenomena occur there.

“This work opens entirely new avenues for studying not only Jupiter and its other Galilean moons, but potentially other giant planets and their lunar systems,” Knowles said.

“We see Jupiter’s atmosphere reacting in real time to its moons, which gives us insight into processes occurring throughout our solar system and perhaps further afield.”

“We only saw this phenomenon in one of our five shots, which leaves us with questions. How often does this happen? Does it turn on and off? How does it change under different conditions?”

The study appears in the journal Geophysical research letters.

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Katie L. Knowles and others. 2026. Short-term variability of Jupiter satellite footprints identified by JWST. Geophysical research letters 53 (5): e2025GL118553; doi: 10.1029/2025GL118553