Jupiter hosts the brightest and most spectacular aurorae in the Solar System. Near its poles, these shimmering lights offer a glimpse into how the planet interacts with the solar wind and moons swept by Jupiter’s magnetic field. Unlike Earth’s northern lights, the largest moons of Jupiter create their own auroral signatures in the planet’s atmosphere. These moon-induced aurorae, known as satellite footprints, reveal how each moon interacts with its local space environment.
Juno capturing the marks on Jupiter of all four Galilean moons; the aurorae related to each are labeled Io, Eur (for Europa), Gan (for Ganymede), and Cal (for Callisto). Image credit: NASA / JPL-Caltech / SwRI / UVS Team / MSSS / Gill / Jónsson / Perry / Hue / Rabia.
Before NASA’s Juno mission, three of Jupiter’s four largest moons, known as Galilean moons — Io, Europa, and Ganymede — were shown to produce these distinct auroral signatures.
But Callisto, the most distant of the Galilean moons, remained a mystery.
Despite multiple attempts using the NASA/ESA Hubble Space Telescope, Callisto’s footprint had proven elusive, both because it is faint and because it most often lies atop the brighter main auroral oval, the region where aurorae are displayed.
NASA’s Juno mission, orbiting Jupiter since 2016, offers unprecedented close-up views of these polar light shows.
But to image Callisto’s footprint, the main auroral oval needs to move aside while the polar region is being imaged.
And to bring to bear Juno’s arsenal of instruments studying fields and particles, the spacecraft’s trajectory must carry it across the magnetic field line linking Callisto and Jupiter.
These two events serendipitously occurred during Juno’s 22nd orbit of the giant planet, in September 2019, revealing Callisto’s auroral footprint and providing a sample of the particle population, electromagnetic waves, and magnetic fields associated with the interaction.
Jupiter’s magnetic field extends far beyond its major moons, carving out a vast region (magnetosphere) enveloped by, and buffeted by, the solar wind streaming from our Sun.
Just as solar storms on Earth push the northern lights to more southern latitudes, Jupiter’s aurorae are also affected by our Sun’s activity.
In September 2019, a massive, high-density solar stream buffeted Jupiter’s magnetosphere, briefly revealing — as the auroral oval moved toward Jupiter’s equator — a faint but distinct signature associated with Callisto.
This discovery finally confirms that all four Galilean moons leave their mark on Jupiter’s atmosphere, and that Callisto’s footprints are sustained much like those of its siblings, completing the family portrait of the Galilean moon auroral signatures.
“Our observations confirm the electrodynamic coupling between Callisto and Jupiter,” said Dr. Jonas Rabia, a researcher at the Institut de Recherche en Astrophysique et Planétologie and CNRS, and colleagues.
“This coupling will be further analyzed by NASA’s JUICE mission, successfully launched in April 2023, which will repeatedly visit Callisto and its local environment, enabling a better characterization of the interaction of Callisto with Jupiter’s magnetosphere.”
“The reported in situ and remote observations complete the family portrait of Galilean moon auroral footprints, and resolve the longstanding mystery as to whether Callisto’s electromagnetic interaction is fundamentally different from the inner three Galilean satellites.”
“The observed similarities, both in the auroral structure and the electron in situ properties, point towards a universal physical mechanism at work for moon-planet and planet-star magnetospheric interactions, relevant for other binary systems that are only accessible remotely, in the Solar System and beyond.”
The team’s paper was published this week in the journal Nature Communications.
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J. Rabia et al. 2025. In situ and remote observations of the ultraviolet footprint of the moon Callisto by the Juno spacecraft. Nat Commun 16, 7791; doi: 10.1038/s41467-025-62520-4