Jupiter showcases the most brilliant and magnificent auroras in the solar system. Near its poles, these shimmering lights provide insight into how its moons and planets engage with the solar wind propelled by Jupiter’s magnetic field. In contrast to Earth’s auroras, the largest of Jupiter’s moons generates its own unique auroral signature within the planet’s atmosphere. The auroras linked to these moons, referred to as satellite footprints, illustrate the interactions of each moon with its immediate spatial environment.

Prior to NASA’s Juno Mission, three of Jupiter’s largest moons—Io, Europa, and Ganymede—were known to produce distinct auroral signatures.

However, the farthest moon, Callisto, remained an enigma.

Despite numerous attempts using the NASA/ESA Hubble Space Telescope, Callisto’s footprints were faint and difficult to detect, often overshadowed by the bright Main Auroral Oval, the region where auroras are prominently observed.

NASA’s Juno Mission has been in orbit around Jupiter since 2016, providing an unprecedented close-up view of these polar light displays.

To capture Callisto’s footprint, the main auroral oval needs to be bypassed while imaging the polar regions.

Additionally, to incorporate it into the suite of instruments analyzing the fields and particles within Juno’s payload, the spacecraft’s path must cross the magnetic field line linking Callisto to Jupiter.

These necessary conditions coincidentally occurred during Juno’s 22nd orbit of the giant planet in September 2019, leading to the discovery of Callisto’s Auroral Footprint and offering samples of the magnetic fields related to particle populations, electromagnetic waves, and interactions.

Jupiter’s magnetic field extends far beyond its largest moon, forming a vast area (magnetosphere) where solar wind flows from the sun.

Just like solar storms on Earth can push the Northern Lights to lower latitudes, Jupiter’s auroras are also influenced by solar activity.

In September 2019, a significant and dense solar stream impacted Jupiter’s magnetosphere, causing the auroral ellipse to shift towards the equator, revealing a faint yet distinct feature associated with Callisto.

This finding confirms that all four Galilean moons leave their imprint on Jupiter’s atmosphere, with Callisto’s footprints closely resembling those of its inner companions, thus completing the family portrait marked by Galilean Moon Auroras.

“Our observations substantiate the electrodynamic coupling between Callisto and Jupiter,” stated Dr. Jonas Lavia, a researcher at Astrophysics-Planetology and CNRS, along with colleagues.

“This combination will undergo further examination by NASA’s JUICE mission, which was successfully launched in April 2023. This mission will facilitate repeated explorations of Callisto and its local environment, enhancing our understanding of the magnetospheric interactions between Callisto and Jupiter.”

“Reported in situ and remote observations complete the family portrait of the footprints of Galilean Moon Auroras, addressing a long-standing question about whether Callisto’s electromagnetic interactions differ fundamentally from the inner three Galilean satellites.”

“The observed similarities in both the auroral structure and the in situ characteristics of electrons point to the universal physical mechanisms at play in the magnetospheric interaction of moons and stars, akin to other binary systems accessible within the solar system and beyond.”

The team’s paper was published this week in the journal Nature Communications.

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J. Ravia et al. 2025. in situ Remote observation of Callisto’s UV footprint by Juno spacecraft. Nat Commun 16, 7791; doi:10.1038/s41467-025-62520-4