Ganymede’s Auroral Patches Reveal Shared Physics with Earth’s Aurorae

Scientists from the United States, Europe and China used the ultraviolet spectrograph (UVS) aboard NASA’s Juno spacecraft to map detailed spot structures in Ganymede’s aurora that parallel those seen on Earth. Their results indicate that the interaction between magnetic fields and charged particles could provide a universal driver for auroral lights, with implications for understanding magnetospheres across the solar system.

Artist’s impression of auroras on Jupiter’s moon Ganymede. Image credit: NASA / ESA / G. Bacon, STScI / J. Saur, University of Cologne.

Ganymede, the only moon known to have its own intrinsic magnetic field, maintains a miniature magnetosphere integrated with that of Jupiter.

Its auroras originate primarily from oxygen emissions at wavelengths of 130.4 and 135.6 nm, excited by rushing electrons.

In new research, researcher Philippe Gusbin of the University of Liège and his colleagues analyzed ultraviolet observations of Ganymede recorded on June 7, 2021 by the Juno space probe.

They identified several auroral spots on the main hemisphere downstream of the Moon.

The patches have typical sizes of around 50 km and luminosities reaching around 200 Rayleigh.

“Aurora are also observed on Ganymede and are caused by the precipitation of electrons in its thin oxygen atmosphere,” Gusbin explained.

“Observations of Ganymede’s aurora before Juno were limited by the spatial resolution of ground-based observations, and they could not resolve the small-scale structures typical of planetary auroras.”

The morphology and scale of the features resemble auroral “beads” observed on Earth before magnetospheric substorms and on Jupiter during so-called dawn storms.

The apparent absence of similar spots in the Southern Hemisphere is attributed to the viewing geometry, although asymmetries related to Ganymede’s position in Jupiter’s plasma disk cannot be excluded.

“The ‘pearls’ have been observed in the auroras of Earth and Jupiter, where they are linked to substorms and dawn storms, large-scale rearrangements of the magnetosphere that release enormous amounts of energy and produce intense auroral activity,” said Dr Alessandro Moirano, a postdoctoral researcher at the University of Liège and the National Institute of Astrophysics in Rome.

The results suggest that comparable physical mechanisms can operate in magnetospheres despite large differences in scale and environment.

“Juno’s close observations of Ganymede lasted less than 15 minutes and the probe will never fly over Ganymede again. Therefore, we do not know how common these spots are or how they change over time,” said Dr Bertrand Bonfond, an astrophysicist at the University of Liège.

“Fortunately, ESA’s Juice (Jupiter Icy Moons Explorer) mission is currently en route to Jupiter, where it will arrive in 2031, and will carry out dedicated observations of Ganymede.”

“The spacecraft is equipped with an ultraviolet spectrograph similar to that of Juno: this will allow us to collect observations over longer periods, follow the evolution of the Ganymede auroras and, hopefully, uncover new mysteries.”

An article on the results was published in the journal Astronomy and astrophysics.

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A. Moirano and others. 2026. High spatial resolution ultraviolet observations by Juno of Ganymede auroral spots. Constraints on the magnetospheric source region. A&A 706, L16; doi: 10.1051/0004-6361/202558379