NASA/ESA/CSA’s James Webb Space Telescope has meticulously scanned Jupiter’s circumference, documenting the mesmerizing aurora as it came into view. This dynamic spectacle arises from charged particles traveling along magnetic field lines and colliding with the planet’s ionosphere, creating a stunning glow. Utilizing Webb’s Near Infrared Spectrometer (NIRSpec), researchers captured an intriguing feature of Jupiter’s aurora, known as an auroral footprint. These bright luminescent patterns result from interactions between Jupiter’s Galilean moons—Io, Europa, Ganymede, and Callisto—and the surrounding cosmic environment. Planetary scientists leveraged NIRSpec data to analyze the physical characteristics of the auroral footprints of Jupiter’s innermost moons, Io and Europa, measuring local temperature and ionospheric density in near-infrared light. They uncovered a previously unseen low-temperature structure centered around Io’s bright spots, characterized by an exceptionally high density, likely caused by significant electron flow impacting the upper atmosphere.

“Previously, these emissions were measured in ultraviolet and infrared wavelengths solely by their brightness,” stated lead author Dr. Katie Knowles, a student at Northumbria University.

“For the first time, we can describe the physical properties of an auroral footprint: the upper atmosphere’s temperature and ion density, which have never been documented before.”

Unlike Earth’s auroras, which primarily result from solar wind, Jupiter’s auroras are influenced by its four major Galilean moons, which generate their own “mini auroras.”

Jupiter’s immense magnetic field rotates every 10 hours, channeling charged particles. In contrast, its moons orbit much more slowly; for instance, Io takes approximately 42.5 hours to complete one orbit.

“The moons continuously interact with the planet’s magnetic field and plasma, driving high-energy particles down magnetic field lines into the atmosphere, forming auroral footprints that trace their orbits around Jupiter,” Knowles explained.

“Jupiter’s auroras are the most potent and persistent within the solar system.”

“Our observations with Webb offer an unprecedented glimpse into how Jupiter’s moons directly affect the upper atmosphere.”

During a 22-hour observation span in September 2023, Webb meticulously scanned around Jupiter’s edge, tracking auroras as they appeared.

Interestingly, they captured auroral footprints originating from Io and Europa, which did not exhibit the typical characteristics of Jupiter’s main auroras, which are generally hotter and denser.

Instead, researchers discovered a cold spot within Io’s auroral footprint that exhibited significantly lower temperatures and unusually high density compared to typical expectations.

Io is notably the most volcanically active celestial body in the solar system, ejecting approximately 1,000 kilograms of material into space every second, thus replenishing the dense plasma enveloping Jupiter.

This ejected material becomes ionized, forming a toroidal cloud around Jupiter known as the Ioplasma torus.

As Io moves through this complex environment, it generates powerful electrical currents that contribute to the brightest regions in Jupiter’s auroras.

The team found that these auroral footprints contained trihydrogen cation densities three times greater than those present in Jupiter’s primary auroras, with some localized areas experiencing density fluctuations of up to 45 times.

“We observed rapid fluctuations in both temperature and density within Io’s auroral footprint occurring within mere minutes,” Knowles noted.

“This indicates that the flow of high-energy electrons impacting Jupiter’s atmosphere is changing at an incredibly fast pace.”

The recorded temperature at the cold spot was only 538 degrees Celsius (265 degrees Fahrenheit), compared to 766 K (493 degrees Celsius or 919 degrees Fahrenheit) in the surrounding aurora.

This cold spot also contained three times the density of material found in Jupiter’s main aurora.

This discovery could have implications extending well beyond Jupiter, posing intriguing questions about other planetary systems.

Saturn’s moon Enceladus similarly generates auroral footprints on Earth, leading scientists to suspect that comparable phenomena may occur there too.

“This research opens up new avenues for studying not only Jupiter and its Galilean moons but also other giant planets and their satellite systems,” Knowles remarked.

“We are witnessing Jupiter’s atmosphere responding to its moons in real-time, providing insights into processes that may occur throughout our solar system and beyond.”

“This phenomenon was only observed in one of five snapshots, prompting questions: how frequently does this occur? Does it vary? How does it change under different conditions?”

The study is published in the journal Geophysical Research Letters.

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Katie L. Knowles et al. 2026. Short-term fluctuations in Jupiter’s moon footprint discovered by JWST. Geophysical Research Letters 53 (5): e2025GL118553; doi: 10.1029/2025GL118553